Streamlines, A Field Flow Visualization Utility

David I. Brown
NCAR, P.O. Box 3000
Boulder, Colorado 80307-3000

Table of Contents

1.0 PREFACE

The streamline field flow plotting utility described here was originally written in the early 1970's and has previously been known as STRMLN. Over the years, a number of enhancements have been made, but the basic capabilities and interface have not changed much. Late in 1992 a more extensive revision was undertaken. Both the interface and functionality of the package underwent a major upgrade. There were several goals: first, make the package work more like other utilities in the NCAR Graphics toolkit suite; second, make it easier to use in combination with the other utilities; third, allow easier access to standard coordinate system mappings, including especially the EZMAP transformations; and fourth, improve the information content of the final plot. Most of these goals have been at least partially realized in the current version of the package, although, as usual, not all the envisioned improvements could be accomplished in the time allotted to the task. Since it might be confusing to retain the name of a now obsolete entry point as the utility name, and moreover, as it now seems unwise to share the package name with any entry point name, the new name for the NCAR Graphics streamline plotting utility is simply Streamlines.

2.0 INTRODUCTION

This document describes the NCAR Graphics utility, known as Streamlines, designed to allow a user to plot a streamline representation of field flow given two arrays containing the components of the field vectors on a uniform grid in a user-defined coordinate system.

In the past, subroutines within the STRMLN utility were assigned names meaningful to the programmer, but there was no comprehensive naming convention in place. Beginning with NCAR Graphics Version 3.2, however, all new routines, whether user-entry points or not, are six characters long and begin with the key letters, 'ST'. The old entry points are still supported through a compatibility version of the STRMLN subroutine, and some, though not all, of the features provided by the new routines are available through these entry points by appropriately setting the compatibility mode internal parameter, CPM.

This section is intended to give an overall view of Streamlines and selected aspects of its design; it covers some details, but, in general, one should refer to the sections SUBROUTINES and PARAMETERS for detailed descriptions of subroutines and parameters mentioned. (Parameters are mentioned by name; all the names are of the form XXX, where XXX is a three-character mnemonic.) The section ERROR MESSAGES describes error messages written by Streamlines. The section EXAMPLES describes the examples available for Streamlines. It is assumed that the reader is familiar with NCAR Graphics in general and has some knowledge of (or can find and read the documentation for) the utilities AREAS and EZMAP.

Note that currently a Streamlines plot gives information about the instantaneous directional characteristics of the field flow, but cannot show variations in flow intensity. For the future, enhancements are planned to allow Streamlines to represent field intensity using either a graduated color palette, or perhaps variable line width or line spacing.

2.1 Streamlines User Entry Point Routines

The code to draw a streamline flow field plot (unless using the obsolete entries points discussed at the end of this section) must include a call to the routine STINIT followed at some point by a call to STREAM. Before and after the STINIT call you may use several internal parameter setting routines to alter the behavior of the streamline plotting code. You must set some of these parameters before the call to STINIT in order to have any (or the correct) effect. In other cases, you can safely set the parameter between the call to STINIT and the call to STREAM. However, there is currently little benefit in delaying any of the parameter setting calls. You are encouraged, therefore, to make all parameter setting calls before calling STINIT.

You may be wondering why the Streamlines utility requires two calls where formerly only a call to STRMLN was required. The answer is twofold. First, the interface to Streamlines is now almost identical to the new interface to the companion utility, Vectors, so that if you know one of the utilities you will have less to learn and remember to use the other effectively. Second, future enhancements to the Streamline utility are planned that will allow you to use parameter getting routines to obtain information about the target data sets after the STINIT call; then, using parameter setting routines, you will be able to adjust the behavior of the STREAM routine to best fit the requirements of the data.

All the internal parameters have default values; only those which are to have values different from the default need to be set. The user may call routines to set the values of parameters as follows:

In general, once a parameter is given a value by a call to one of these routines, it retains that value until a similar call resets it. Thus, many of the parameter-setting calls do not need to be repeated for each new streamliner field flow plot.

The user may retrieve the value of an internal parameter at any time by calling one of the parameter getting routines:

The core functionality of the Streamlines utility is contained in the two routines that are required in order to create a streamline field flow plot, STINIT and STREAM.

Streamlines contains four user-modifiable routines that are not invoked directly by the user, but by Streamlines itself. Three of the routines, STUMXY, STIMXY, and STUMTA, are provided for the user to implement custom mappings from data to user coordinate space. The fourth, STUMSL, draws streamline segments when the parameter, MSK, is set to enable masking to an area map. The default versions of these routines contain simple code that may handle a basic case, but little more. These routines are as follows:

Two Streamlines routines are supplied to provide compatibility with pre-Version 3.2 codes. You may choose from a variety of compatibility options by setting the compatibility mode parameter CPM appropriately; this facility offers considerable flexibility in making a transition to the new version of the utility.

2.2 Using Streamlines

In order to use Streamlines, you must make the following sequence of calls:

  1. Optionally call parameter-setting routines.

  2. Call STINIT to initialize the streamline flow field plot.

  3. Call STREAM to draw the streamlines.

  4. Optionally call PERIM(1,0,1,0) if you want to place a border around the plot.

  5. Call FRAME to advance the plotter frame.

Assuming that step 1 is null and that all default parameter values are used, the resulting plot will look similar to what would have been drawn by STRMLNs.

2.3 Coordinate Systems in Streamlines

In order to understand the process of transforming the data contained in the vector field component input arrays into streamlines following particular paths on an output plot, it would be helpful to differentiate unambiguously each stage of the transformation pipeline within the context of the coordinate systems defined by NCAR Graphics. In this discussion, the following terms are used:

Note that all reliance on the integer metacode coordinate system has been eliminated from the Streamline utility as of Version 3.2.

Given the default values of the parameters XC1, XCM, YC1 and YCN, the mapping from grid to data coordinate space is an identity transformation. That is, the data coordinate space extends from (1.0,1.0) to (REAL(M), REAL(N)). However, you may assign any desired boundaries to the data coordinate system by setting XC1, XCM, YC1 and YCN to values of your choice. For satisfactory results these choices need to be consistent with the user coordinate boundaries established by the SET call and the mapping established by the MAP parameter. In general, the location of an array element A(I,J) in data coordinate space is given as:

Xdata = XC1 + (I - 1)*(XCM - XCM)/(M - 1)
Ydata = YC1 + (J - 1)*(YCN - YC1)/(N - 1)
where (XC1,YC1) is the lower boundary of the data coordinate space and (XCM,YCN) is the upper boundary.

2.4 Coordinate Space Mapping

If the parameter MAP has values between 0 and 2 Streamlines handles the mapping from data space to user and NDC space internally, using routines STMPXY, STIMXY, and STMPTA. If MAP has any other value the user-modifiable routines STUMXY, STUIXY, and STUMTA are called to perform a mapping defined by the user. Note that Streamlines uses three routines to perform mappings similar to those accomplished in one routine by other utilities in NCAR Graphics. This is because in order to handle non-linear mappings properly, Streamlines must map points both in both directions of the transformation pipeline, and in addition must map the directional angle from grid to NDC space at each incremental point of the streamline. New points on the streamline are generated using a constant reference increment size in NDC space, avoiding the discontinuities that can occur from using grid coordinate based increments that can vary greatly in size after applying a non-linear mapping.

The mappings handled internally by Streamlines are as follows:

When a user-defined mapping is required, you must modify the user-definable mapping routines, STUMXY, STUIXY, and STUMTA, to respond in a consistent way to the MAP code you choose to identify your mapping. In the standard distribution, the default versions of these routines are all located in the single source file, 'stumxy.f'. If the Streamlines routines are loaded from a binary library, you can usually replace the library version just by compiling the routines and explicitly linking its object with the NCAR Graphics library.

2.5 How Streamlines Are Constructed

If you are creating your own mapping routines, it may be helpful to have some idea of how streamlines are created. The steps of the iterative streamline construction process may be summarized as follows:

  1. Find the next grid box eligible for starting a streamline, and generate the normalized interpolated vector components for the point at the center of this box.

  2. Convert the grid coordinate point to data coordinates and call STMPXY to map the point to user coordinates. Then use the SPPS routines CUFX and CUFY to find the point in NDC space.

  3. Given the coordinates of the point in data, user, and NDC space, and also the normalized interpolated vector component values, call STMPTA to determine the streamline directional angle at the point relative to NDC space.

  4. Using the directional angle, the normalized device point coordinates, and the NDC differential magnitude value (indirectly specified by the internal parameter, DFM) generate the next point of the streamline in NDC space.

  5. Convert this point back to user coordinates using the SPPS calls, CFUX and CFUY. Then call STIMXY to map the user coordinates back to data coordinates. Finally convert the data coordinates to grid coordinates and generate the normalized interpolated vector component values for this new point.

  6. Repeat steps three through five until a streamline termination condition is met. Go back to the original starting point of the stream and perform the same steps in the other direction to complete the streamline. Then start over at step one until no eligible streamline starting grid boxes remain.

In contrast, when Streamlines is emulating the pre-Version 3.2 mapping method, the steps are somewhat different. Here is a summary:

  1. Find the next grid box eligible for starting a streamline, and generate the normalized interpolated vector components for the point at the center of this box.

  2. Call FX and FY to convert the point in grid coordinates into user coordinates. (Any transitional mapping from grid to data coordinates must be handled within these functions.) Then use the SPPS routines CUFX and CUFY to find the point in NDC space.

  3. Use the function ATAN2 applied directly to the normalized interpolated vector components to determine the streamline directional angle. (This is used only to establish the direction of the directional arrowheads)

  4. Add the interpolated components, which are normalized to represented a fractional distance in grid coordinate space, to the grid coordinates of the point to generate a new point. Generate the normalized interpolated vector components of this new point.

  5. Repeat steps two through four until a streamline termination condition is met. Go back to the original starting point of the stream and perform the same steps in the other direction to complete the streamline. Then start over at step one until no eligible streamline starting grid boxes remain.

Clearly, somewhat less processing is involved when the older mapping technique is used, so if execution time is a primary concern, and the mapping is linear (or you can afford to ignore the non-linearities), you may prefer to use the older mapping method. You can do this, even when using the Version 3.2 interface, simply by setting the compatibility mode parameter, CPM, to a positive value.

When the mapping is non-linear, the Version 3.2 approach has two primary advantages: (1) the directional arrows always point in the correct direction, and (2) the likelihood of discontinuities appearing in the streamline as the result of grid box stretching and distortion over the transformation is greatly reduced. If discontinuities do appear they can generally be eliminated simply by setting the differential magnitude parameter, DFM, to a suitably small value. Note, however, that Streamlines still chooses stream starting points and places the directional arrowheads relative to the grid coordinate system. Over non-linear mappings, therefore, the arrowheads and the streams may be crowded in some areas of the plot and sparse in others. Future versions of the Streamlines utility plan to address these problems.

2.6 The SET Call

The mapping into the plotter frame depends on the boundaries of the user coordinate space and the viewport defined in NDC space. If the user coordinates are not log-scaled and neither axis is mirrored user coordinates and GKS world coordinates are the same. In this case, and it is possible to define the user/world coordinate boundaries (also known as the window) and map them to the viewport using GKS calls. However, this document only discusses the SPPS routine, SET, since it accomplishes all the same goals and works under all conditions recognized by NCAR Graphics.

A call to the SPPS routine SET has the form

      CALL SET (VPL,VPR,VPB,VPT,WDL,WDR,WDB,WDT,LL)
All arguments are REAL except for LL, which is an INTEGER. The first four arguments must all be between 0 and 1, inclusive; they define a rectangular area on the plotter frame known as the "viewport". The next four arguments define the boundaries of the user coordinate space (the "window"). The final argument indicates whether the mapping of user coordinates into the viewport is to be linear or logarithmic in X and Y. See the documentation of the SPPS utility for further details.

By default, Streamlines calls SET. However, by setting the parameter SET to zero, you may prevent Streamlines from doing making the call. In this case, however, you must do your own SET call or depend on some other utility (such as EZMAP) to have done it.

If Streamlines calls SET, it always uses LL = 1, requesting a linear-linear mapping from the window to the viewport. The viewport is positioned as specified by the current values of the parameters VPL, VPR, VPB, VPT, and VPS. The first four of these specify the boundaries of a "viewport area", in which the viewport is to be centered and made as large as possible; the final one determines how the shape of the viewport is to be chosen. You could, for example, render several plots within a single plotter frame by setting these parameters to define separate non-overlapping areas for each plot.

By default, the user coordinate boundaries are set equal to the data coordinate boundaries, which, in turn, default to the grid coordinate boundaries (1: M, 1: N) unless the parameters XC1, XCM, YC1, YCM have non-default values. You may override this behavior by setting parameters WDL, WDR, WDB, and WDT to non-default values, explicitly specifying the values used to define the window in the SET call. Normally you would modify the window setting parameters only when the parameter MAP has a non-default value, but Streamlines is still expected to perform the SET call. Whereas the viewport area may be set to any part of the plotter frame, without fear of losing the plot, you must be careful setting WDL, WDR, WDB, and WDT, or the region of interest is not likely to match up with the viewport.

When MAP is set to 1, specifying an EZMAP projection, you should give the SET parameter a value of 0, since EZMAP must take charge of the SET call to perform the requested map transformation. If you want to adjust the viewport size or position in this case, do it using the EZMAP call MAPPOS.

When MAP is set to 2, specifying a polar coordinate mapping, you may choose either to leave the SET parameter defaulted and set the user coordinate boundary parameters WDL, WDR, WDB, and WDT appropriately, or else to set the SET parameter to zero and make your own call to the SET routine.

Note that, as long as the parameter MAP is zero, a SET call performed by Streamlines will define the window in such a way as to be consistent with the ranges of the X and Y coordinates it generates. If MAP is set non-zero, then you will probably need to set XC1, XCM, YC1, and YCN to transform the grid coordinates into the range of data coordinates expected by Streamlines mapping routine. The call to SET must specify user coordinate boundaries commensurate with the range generated by the data coordinate to user coordinate transformation; if Streamlines is to do the call, then you will need to set WDL, WDR, WDB, and WDT to accomplish this.

2.7 Masked Streamline Overlays Using an Area Map

At times you may need to overlay a streamline flow field over a plot created by another NCAR Graphics utility, such as Conpack. There may be certain areas, such as the boxes containing contour level labels, where you do not want the streamlines to appear. Beginning with Version 3.2, the Streamlines utility supports streamline masking based on the contents of a area map previously established using routines from the Areas utility. Most of the information required to use the area map routines is documented elsewhere, either in the descriptions of the Conpack utilities that work with area maps or in the actual documentation of the Areas utility itself. You create the area map and the underlying plot just as you normally would. Then, in order to create the streamline overlay, you would need to take the following steps:

EXTERNAL STUMSL

2.8 Special Values

If the data at a particular grid point is unavailable or unreliable, a "special" value, typically a value outside the possible range of valid data, may be assigned to the array element associated with the grid point in question. Since Streamlines has two vector field component arrays as input, it uses several parameters to control processing of special value data points. Each of the two input arrays may have its own separately defined special value. The parameters USV and VSV are used to set the special values for the U and V vector field component arrays, respectively.

The parameter SVF controls special value processing. When SVF is set to its default value, 0, Streamlines assumes that the data contains no special values. If the data actually does contain special values, the resulting plots hopefully will look strange enough that you will realize that special values need to be taken into account. When SVF has any other value, special value processing is enabled. In this case, Streamlines overrides the interpolation method flag and insists on using the 4-point bi-linear interpolation method. This is because the other available interpolation method generally requires 16 surrounding good data points to determine the vector component values at any point in the streamline. If the 16 point method were used with special values present, not only would 32 special value checks, rather than 8, need to be performed at each step of the streamline construction, but also Streamlines would be required to omit the streamlines from a larger portion of the field plot, because only one special value out of the 32 array elements checked would be enough to cause termination of the streamline.

As it is, Streamlines checks the U and V data arrays at each of the four corner points of the grid box, each time a streamline under construction enters a new grid cell. If any of the eight values checked is a special value, then the streamline is terminated and a new one begun if possible.

2.9 Supplementary Text and Plot Information

2.10 Streamlines' Grid Window

Note that the grid window for Streamlines is not the same as the grid window for AUTOGRAPH, since the former is expressed with respect to the plotter frame and the latter is expressed with respect to the AUTOGRAPH "graph window". There is an easy conversion from one to the other, however. If AGFL, AGFR, AGFB, and AGFT specify the AUTOGRAPH graph window and AGDL, AGDR, AGDB, and AGDT specify the AUTOGRAPH grid window and CGDL, CGDR, CGDB, and CGDT specify the Streamline grid window, then the following relationships ensure coincidence of the AUTOGRAPH and Streamlines' grid windows:

CGDL=AGFL+AGDL*(AGFR-AGFL)
CGDR=AGFL+AGDR*(AGFR-AGFL)
CGDB=AGFB+AGDB*(AGFT-AGFB)
CGDT=AGFB+AGDT*(AGFT-AGFB)
Note what happens when AGFL and AGFB are 0's and AGFR and AGFT are 1's, which is the default situation; in that case CGDL=AGDL, CGDR=AGDR, CGDB=AGDB, and CGDT=AGDT.

2.11 GKS Considerations

Certain assumptions are made by Streamlines about the state of GKS, as follows:

  1. Like all the utilities in NCAR Graphics, Streamlines assumes that GKS has been opened and that the desired workstations have been opened and activated. The statement

          CALL OPNGKS
    calls the SPPS routine OPNGKS, the GKS equivalent of which is

          CALL GOPKS (6,0)
      CALL GOPWK (1,2,1)
      CALL GACWK (1)
    creating a single metacode workstation associated with FORTRAN unit 2.

  2. Similarly, at the end of one's program, the workstations must be deactivated and closed and then GKS must be closed. The statement

          CALL CLSGKS
    calls the SPPS routine CLSGKS, the GKS equivalent of which is

          CALL GDAWK (1)
      CALL GCLWK (1)
      CALL GCLKS
  3. It is assumed that the aspect source flags for various quantities are set to "individual". (NCAR GKS does this by default, but other packages may not.) To make sure that all the aspect source flags are set correctly, use the following code:

          DIMENSION IASF(13)
      ...
      DATA IASF / 13*1 /
      ...
      CALL GSASF (IASF)
  4. Streamlines uses the GKS polyline color index set by the most recent call to GSPLCI for rendering the streamlines.

3.0 SUBROUTINES

All Streamlines routines added for NCAR Graphics Version 3.2 and later have six-character names beginning with the letters 'ST. Current user entry points and user modifiable routines are described in detail in alphabetical order in this section. Obsolete routines appear in the section following entitled OBSOLETE SUBROUTINES.

3.1 STGETC (CNM,CVL)

This routine is used to get the current value of a parameter of type CHARACTER

3.1.1 Usage

Use the statement 

      CALL STGETC (CNM,CVL)
at any time to retrieve in CVL the current character value of the parameter whose name is CNM.

3.1.2 Arguments

CNM (CHARACTER, input) is the name of a parameter whose character value is to be retrieved. Only the first three characters of CNM are examined. The three characters may either be entirely upper or entirely lower case; mixed case is not recognized. It is recommended that the rest of the character string be used to improve the readability of the code. For example, instead of just 'ZFT', use 'ZFT - Zero Field Text String'.

CVL (CHARACTER, output) is a variable in which the value of the parameter specified by CNM is to be returned.

3.2 STGETI (CNM,IVL)

This routine is used to get the current value of a parameter of type INTEGER.

3.2.1 Usage

Use the statement 

      CALL STGETI (CNM,IVL)
at any time to retrieve in IVL the current integer value of the parameter whose name is CNM.

3.2.2 Arguments

CNM (CHARACTER, input) is the name of a parameter whose integer value is to be retrieved. Only the first three characters of CNM are examined. The three characters must either be entirely upper or entirely lower case; mixed case is not recognized. It is recommended that the rest of the character string be used to improve the readability of the code. For example, instead of just 'PLR', use 'PLR - Polar Input Mode'.

IVL (INTEGER, output) is a variable in which the value of the parameter specified by CNM is to be returned.

3.3 STGETR (CNM,RVL)

This routine is used to get the current value of a parameter of type REAL.

3.3.1 Usage

Use the statement 

      CALL STGETR (CNM,RVL)
at any time to retrieve in RVL the current real value of the parameter whose name is CNM.

3.3.2 Arguments

CNM (CHARACTER, input) is the name of a parameter whose real value is to be retrieved. Only the first three characters of CNM are examined. The three characters must either be entirely upper or entirely lower case; mixed case is not recognized. It is recommended that the rest of the character string be used to improve the readability of the code. For example, instead of just 'ARL', use 'ARL - Arrowhead Length'.

RVL (REAL, output) is a variable in which the value of the parameter specified by CNM is to be returned.

3.4 STINIT (U,LU,V,LV,P,LP,M,N,WRK,LW)

STINIT performs initialization tasks required before STREAM may be called to create a streamline field flow plot. Information about the input data arrays is copied into internal common block variables and the coordinate system mappings and boundaries are established.

3.4.1 Usage

Call STINIT before the first invocation of STREAM, and again any time you modify the contents of the input data arrays. You may precede an STINIT call with any number of calls to the parameter setting routines, STSETC, STSETR and STSETI. Although it is possible to call STSETC, STSETI or STSETR between calls to STINIT and STREAM, changing the setting of many of Streamline's parameters at this point will result in incorrect behavior. When in doubt, set parameters before the STINIT call.

Set up the two vector component arrays prior to calling STINIT. To permit multiple purpose use of the array space, the STINIT argument list includes both the actual size and an assumed size for the first dimension of each input array. Due to FORTRAN array ordering conventions, only the assumed size needs to be specified for the second dimension. (Note: when using the C bindings, mentally exchange all references to first and second dimensions in this discussion.) The arguments LU and LV contain the actual size of the first dimensions of U and V, respectively. Since the grid locations for each of the data arrays are assumed to coincide, a single argument, M, represents the assumed size of the first dimension for both arrays. Similarly, the argument, N, is the assumed size of the second dimension. The only requirement for the actual second dimension size is that it be greater than or equal to N for each array.

The P and LP arguments are intended for future enhancement of the Streamlines utility. They are not currently used or examined in any way. For maximum assurance of compatibility with future changes to the code, give the LP argument the INTEGER value 0.

WRK is an array used to store normalized values of the U and V vector components while the streamlines are calculated. It also holds bit flags indicating whether a grid location is eligible for starting a streamline or placing a directional arrowhead. Its assumed size is specified by the argument LW. LW must have a value greater or equal to 2*M*N.

3.4.2 Arguments

U (REAL 2-dimensional array, dimensioned LU xn: n >= N, input): By default, assumed to contain the first dimensional Cartesian components of the vector field. However, if PLR is non-zero, it is treated as containing the vector magnitudes.

LU (INTEGER, input): Actual value of the first dimension of array U.

V (REAL 2-dimensional array, dimensioned LV x n: n >= N, input): By default, assumed to contain the second dimensional Cartesian components of the vector field. However, if PLR is non-zero, it is treated as containing the vector angles.

LV (INTEGER, input): Actual value of the first dimension of array V

P (REAL, input, DUMMY): This argument is currently ignored and may be assigned a dummy value.

LP (INTEGER, input, DUMMY): The first dimension of array P. Not currently used, this argument should be assigned the integer value 0.

M (INTEGER, input): Number of contiguous elements along the first dimensional axis containing data to be processed in each of the arrays, U and V.

N (INTEGER, input): Number of contiguous elements along the second dimensional axis containing data to be processed in each of the arrays, U and V.

WRK (REAL, array dimensioned n: n >= LW, input/output): Work array used to store normalized values of the U and V vector components while the streamlines are calculated. It also holds bit flags indicating whether a grid location is eligible for starting a streamline or placing a directional arrowhead.

LW (INTEGER, input): Assumed size of the array WRK. Currently it must greater or equal to 2*M*N.

3.5 STREAM (U,V,P, IAM,STUMSL,WRK)

This routine plots a streamline representation of field flow data, based on conditions established by STINIT and the current values of a set of user-modifiable internal parameters associated with the Streamlines utility.

3.5.1 Usage

A call to STINIT must precede the first call to STREAM, and is again required any time the vector array data changes or any of the coordinate space control parameters are modified. However, you may modify certain rendering control parameters between multiple invocations of STREAM. See the section, PARAMETERS, for more information.

Arguments to STINIT establish the sizes of the vector component arrays and the WRK array. STINIT places these values into common block variables where they become available to STREAM. Therefore no size arguments need appear in the STREAM argument list. When area masking is not enabled, the third, fourth and fifth arguments to STREAM may all have dummy values and the invocation would be something like:

      CALL STREAM(U,V,IDM,IDM,IDM,WRK)
where IDM is an arbitrary variable that need not have been initialized. The masking capability supported by Streamlines allows you to overlay vector fields on top of graphics produced by other utilities, such as CONPACK contour plots, without encroaching on areas, like label boxes, that should appear in the foreground. Normally the area map should be generated and used in the normal way (as described in the AREAS and CONPACK documentation) before calling any routines in the Streamlines utility. When the parameter MSK has a non-zero value, masking is enabled and a previously created area map must be passed to STREAM. STREAM examines the map, returning an error if the map appears to be invalid or the number of area groups is beyond the range it can handle (currently 64). Otherwise, it only uses the map as an argument to the Areas utility subroutine, ARDRLN. The user must also pass in a user-definable masked drawing subroutine. A simple version of this subroutine, named (like the argument) STUMSL, is supplied with the Streamlines utility, and may suffice for basic masked drawing operations. There is a separate entry describing the arguments and usage of STUMSL in this section.

Note that in order to keep the interface as simple as possible, Streamlines locally declares the area identifier and area group arrays passed to the masked drawing subroutine. The dimension of these arrays is set to the value of the FORTRAN PARAMETER IPAGSZ, currently set to 64.

The P array argument is intended for future enhancement of the Streamlines utility and is not currently used or examined in any way. It may be given a dummy value that need not be initialized.

3.5.2 Arguments

U (REAL 2 dimensional array, dimensioned as specified in last call to STINIT, input): By default, assumed to contain the first dimensional axis components of the vector field. However, if PLR is non-zero, it is treated as containing the vector magnitudes.

V (REAL 2 dimensional array, dimensioned as specified in last call to STINIT, input): By default, assumed to contain the second dimensional axis components of the vector field. However, if PLR is non-zero, it is treated as containing the vector angles.

P (REAL, input, DUMMY): This argument is currently ignored. May be assigned a dummy value.

IAM (INTEGER array, unknown dimension, input): Area map array previously established by calls to routines in the Areas utility. It is checked to see that the number of area groups is within the limits allowed by Streamlines, then simply passed through to an Areas routine for further processing. Required only if MSK is set to a non-zero value; otherwise it is ignored and may be assigned a dummy value.

STUMSL (EXTERNAL subroutine, input): User-definable masked drawing subroutine. See the entry, "STUMSL", in this section for a discussion of the usage and argument list required for this subroutine. Required only if MSK is set to a non-zero value; otherwise it is ignored and may be assigned a dummy value.

WRK (REAL array, dimensioned as specified in last call to STINIT, input/output): Work array used to store normalized values of the U and V vector components while the streamlines are calculated. It also holds bit flags indicating whether a grid location is eligible for starting a streamline or placing a directional arrowhead.

3.6 STRSET

Resets all parameters to their initial default values.

3.6.1 Usage

To reset all parameters to their default values:

      CALL STRSET
There are no arguments

3.6.2 Arguments

None.

3.7 STSETC (CNM,CVL)

This routine is called to set the value of a parameter of type CHARACTER.

3.7.1 Usage

Use the statement 

      CALL STSETC (CNM,CVL)
to give the parameter whose name is CNM the character value CVL.

3.7.2 Arguments

CNM (CHARACTER, input) is the name of a parameter to be given a character value. Only the first three characters of CNM are examined. The three characters may either be entirely upper or entirely lower case; mixed case is not recognized. It is recommended that the rest of the character string be used to improve the readability of the code. For example, instead of 'ZFT', use 'ZFT - Zero Field Text String'.

CVL (CHARACTER, input) is an expression, the value of which is to be given to the parameter specified by CNM.

3.8 STSETI (CNM,IVL)

This routine is called to set the value of a parameter of type INTEGER.

3.8.1 Usage

Use the statement

      CALL STSETI (CNM,IVL)
to give the parameter whose name is CNM the integer value IVL.

3.8.2 Arguments

CNM (CHARACTER, input) is the name of a parameter to be given an integer value. Only the first three characters of CNM are examined. The three characters must either be entirely upper or entirely lower case; mixed case is not recognized. It is recommended that the rest of the character string be used to improve the readability of the code. For example, instead of 'CKP', use 'CKP - Check Progress Iteration Count'.

IVL (INTEGER, input) is an expression, the value of which is to be given to the parameter specified by CNM.

3.9 STSETR (CNM,RVL)

This routine is called to set the value of a parameter of type REAL.

3.9.1 Usage

Use the statement 

      CALL STSETR (CNM,RVL)
to give the parameter whose name is CNM the real value RVL.

3.9.2 Arguments

CNM (CHARACTER, input) is the name of a parameter to be given a real value. Only the first three characters of CNM are examined. The three characters may either be entirely upper or entirely lower case; mixed case is not recognized. It is recommended that the rest of the character string be used to improve the readability of the code. For example, instead of 'DFM', use 'DFM - Differential Magnitude'.

RVL (REAL, input) is an expression, the value of which is to be given to the parameter specified by CNM.

3.10 STUIXY (XUS,YUS,XDA,YDA,IST)

The user-modifiable routine STUIXY inversely maps a single point on the streamline from user to data coordinate space.

3.10.1 Usage

STUIXY is a user-modifiable routine provided to support custom mappings of the data coordinate space. The user does not invoke it directly. Instead, whenever the parameter MAP specifies a mapping not handled by Streamlines internally (i.e., when MAP is set to a value other than 0, 1, or 2), Streamlines calls STUIXY once for each incremental step in the creation of a streamline. The default version of STUIXY simply performs an identity mapping. In order to implement a custom mapping, you must pick a unique mapping code (a positive integer greater than 2), and then modify each of the three routines, STUMXY, STUIXY, and STUMTA to recognize and respond consistently to the chosen code. In the standard distribution of NCAR Graphics, these three routines reside in a single file, "stumxy.f". STUMXY maps a point from data to user coordinate space, STUIXY inversely maps a point from user to data coordinate space, and STUMTA, which is likely to be the most difficult to implement, finds the tangent angle of the streamline at a point in NDC space. The user does not call STUIXY.

STUIXY has access to a common block called STMAP that contains a number of variables used to record the current transformation state. In order to accommodate a variety of mapping implementations, STMAP provides more information than normally required. Consider the values stored in STMAP as strictly read-only. One essential member of this common block is IMAP, which contains the value currently assigned to the MAP parameter.

In STUIXY, the implementor of a custom mapping needs to add code of the form:

      IF (IMAP .EQ. <chosen_map_code>) THEN
          ...map XUS to XDA
          ...map YUS to YDA
          ...if error, set IST to a negative value
          RETURN
      ENDIF
Note that after the return from STUIXY, Streamlines checks to ensure that XDA and YDA are within the current boundaries of the data coordinate system.

When implementing a custom mapping, you may wish to look at the coding of the pre-defined mappings (0, identity mapping; 1, EZMAP projections; and 2, polar coordinate mapping) in the file "stmpxy.f". For these mappings, the subroutine STIMXY is the equivalent of STUIXY; it has an identical interface and may serve as a model for your implementation. For more information, see the section titled Coordinate Systems in Streamlines.

3.10.2 Arguments

XUS (REAL, input): The X coordinate of a point in the user coordinate system.

YUS (REAL, input): The Y coordinate of a point in the user coordinate system.

XDA (REAL, output): The X coordinate of the point in the data coordinate system.

YDA (REAL, output): The Y coordinate of the point in the data coordinate system.

IST (REAL, output): Status of the mapping operation: 0 indicates success, negative values indicate that the mapping failed; positive values are reserved and should not be used by the implementor of a mapping routine.

3.11 STUMSL (XCS,YCS,NCS,IAI,IAG,NAI)

This routine is the user-definable external subroutine used to draw masked streamlines. The default version of the routine draws any polyline all of whose area identifiers are greater than or equal to zero.

3.11.1 Usage

"STUMSL" is the name given to the default version of the masked streamline drawing routine, and it is also the name given to the argument through which the external subroutine is passed to STREAM. However, you may choose any acceptable FORTRAN identifier as the name of a user-defined version of the routine. The substitute routine must have an argument list equivalent to the default version of STUMSL. Also, whether or not the default version is used, the subroutine that calls STREAM should contain an external declaration of the routine, such as:

      EXTERNAL STUMSL
If the MSK parameter is greater than 0, signifying that masking is to be performed, Streamlines sends one or more sets of X and Y polyline coordinate arrays to the area masking routine, ARDRLN, for each streamline. ARDRLN subdivides the polyline into pieces such that each smaller polyline has a single area identifier with respect to each area identifier group, then makes a call to STUMSL for each polyline piece. While the default version of STUMSL only checks to see that none of the area identifiers are negative, a user-defined version could perform more complicated decision processing based on knowledge of the meaning of specific area identifier groups and/or area identifier values. Note that, before invoking STUMSL, ARDRLN modifies the user coordinate space by making the following calls:

      CALL GETSET(VPL,VPR,VPB,VPT,WDL,WDR,WDB,WDT,LLG)
      CALL SET(VPL,VPR,VPB,VPT,VPL,VPR,VPB,VPT,1)
These calls temporarily turn the user to NDC mapping into an identity, allowing the user to call any of the routines, CURVE, CURVED, or the GKS routine GPL, to render the polygon piece, without worrying about a possible non-identity mapping between user and world coordinates.

The current version of Streamlines supports masked drawing with up to 64 area groups. Streamlines will exit with an error message if an area map with more than 64 groups is passed to it.

3.11.2 Arguments

XCS (REAL array, assumed size NCS, input): Array of X coordinates of the points defining the polyline with the given set of area identifiers.

YCS (REAL array, assumed size NCS, input): Array of Y coordinates of the points defining the polyline with the given set of area identifiers.

NCS (INTEGER, input): Number of points; assumed size of the X and Y coordinate arrays, XCS and YCS.

IAI (INTEGER array, assumed size NAI, input): Array of area identifier values. Each value represents the area identifier with respect to the area group in the area group array with the same array index.

IAG (INTEGER array, assumed size NAI, input): Array of area-identifier groups.

NAI (INTEGER, input): Number of area identifier groups. The current version of Streamlines supports up to 64 area groups.

3.12 STUMTA (XDA,YDA,XUS,YUS,XND,YND,DU,DV,TA,IST)

Given the coordinates of a point on the streamline in data, user, and NDC space, and the interpolated, normalized components of the vector at the point relative to data coordinate space, the user-modifiable routine STUMTA finds the directional angle of the streamline relative to NDC space at the point.

3.12.1 Usage

STUMTA is a user-modifiable routine provided to support custom mappings of the data coordinate space. The user does not invoke it directly. Instead, whenever the parameter MAP specifies a mapping not handled by Streamlines internally (i.e., when MAP is set to a value other than 0, 1, or 2), Streamlines calls STUMTA once for each incremental step in the creation of a streamline. The default version of STUMTA simply returns the angle implied by the incremental vector components passed to it: that is, it returns ATAN2(DV,DU). In order to implement a custom mapping, you must pick a unique mapping code (a positive integer greater than 2), and then modify each of the three routines, STUMXY, STUIXY, and STUMTA to recognize and respond consistently to the chosen code. In the standard distribution of NCAR Graphics, these three routines reside in a single file, "stumxy.f". STUMXY maps a point from data to user coordinate space and STUIXY inversely maps a point from user to data coordinate space. STUMTA, which is likely to be the most difficult to implement, finds the tangent angle of the streamline at a point in NDC space.

STUMTA has access to a common block called STMAP that contains a number of variables used to record the current transformation state. In order to accommodate a variety of mapping implementations, STMAP provides more information than normally required. Consider the values stored in STMAP as strictly read-only. One essential member of this common block is IMAP, which contains the value currently assigned to the MAP parameter.

When implementing a non-linear mapping, an iterative differential technique will most likely be required. Look at the routine, STMPTA, in 'stmpxy.f', which handles the pre-defined mappings, for examples of the method. Both the default transformation (MAP set to 0), in order to account for possible log scaling of the user coordinate axes, and also the EZMAP projection (MAP set to 1) use such a technique. Basically the idea is that the vector components must be proportionally reduced in size enough that an effectively "instantaneous" angle can be calculated, but they must not become so small that the calculation is adversely affected by the floating point precision available for the machine. Additionally, checks must be put in place to prevent the increment from stepping off the edge of the coordinate system space. The pre-defined mappings step in the opposite direction to find the angle whenever an increment in the original direction would fall off the edge.

3.12.2 Arguments

XDA (REAL, input): The X coordinate of a point on the streamline in the data coordinate system.

YDA (REAL, input); The Y coordinate of a point on the streamline in the data coordinate system.

XUS (REAL, input): The X coordinate of the point in the user coordinate system.

YUS (REAL, input): The Y coordinate of the point in the user coordinate system.

XND (REAL, input): The X coordinate of the point in NDC space.

YND (REAL, input): The Y coordinate of the point in NDC space.

DU (REAL, input): The interpolated value of the normalized component of the vector at the point, with direction parallel to the X axis of the data coordinate system.

DV (REAL, input): The interpolated value of the normalized component of the vector at the point, with direction parallel to the Y axis of the data coordinate system.

TA (REAL, output): The directional angle of the streamline at the point relative to NDC space.

IST (REAL, output); Status of the mapping operation: 0 indicates success, negative values indicate that the mapping failed; positive values are reserved and should not be used by the implementor of a mapping routine.

3.13 STUMXY(XDA,YDA,XUS,YUS,IST)

The user-modifiable routine STUMXY maps a single point on the streamline from data to user coordinate space. The argument list for STUMXY parallels that of the internal Streamlines routine, STMPXY, used for the predefined mappings employed when the MAP parameter has a value between 0 and 2.

3.13.1 Usage

STUMXY is a user-modifiable routine provided to support custom mappings of the data coordinate space. The user does not invoke it directly. Instead, whenever the parameter MAP specifies a mapping not handled by Streamlines internally (i.e., when MAP is set to a value other than 0, 1, or 2), Streamlines calls STUMY as needed during the process of creating a streamline. The default version of STUMY simply performs an identity mapping. In order to implement a custom mapping, you must pick a unique mapping code (a positive integer greater than 2), and then modify each of the three routines, STUMXY, STUIXY, and STUMTA to recognize and respond consistently to the chosen code. In the standard distribution of NCAR Graphics, these three routines reside in a single file, "stumxy.f". STUMXY maps a point from data to user coordinate space, STUIXY inversely maps a point from user to data coordinate space, and STUMTA, which is likely to be the most difficult to implement, finds the tangent angle of the streamline at a point in NDC space. The user does not call STUIXY.

STUMXY has access to a common block called STMAP that contains a number of variables used to record the current transformation state. In order to accommodate a variety of mapping implementations, STMAP provides more information than normally required. Consider the values stored in STMAP as strictly read-only. One essential member of this common block is IMAP, which contains the value currently assigned to the MAP parameter.

In STUMXY, the implementor of a custom mapping needs to add code of the form:

      IF (IMAP .EQ. <chosen_map_code>) THEN
          ...map XDA to XUS
          ...map YDA to YUS
          ...if error, set IST to a negative value
          RETURN
      ENDIF
Note that after the return from STUMY, Streamlines checks to ensure that XUS and YUS are within the current boundaries of the user coordinate system.

When implementing a custom mapping, you may wish to look at the coding of the pre-defined mappings (0, identity mapping; 1, EZMAP projections; and 2, polar coordinate mapping) in the file "stmpxy.f". For these mappings, the subroutine STMPXY is the equivalent of STUMXY; it has an identical interface and may serve as a model for your implementation. For more information, see the section titled Coordinate Systems in Streamlines.

3.13.2 Arguments

XDA (REAL, input): The X coordinate of a point on the streamline in data coordinate space.

YDA (REAL, input): The Y coordinate of a point on the streamline in data coordinate space.

XUS (REAL, output): The X coordinate of the point in user coordinate space.

YUS (REAL, output): The Y coordinate of the point in user coordinate space.

IST (REAL, output) Status of the mapping operation: 0 indicates success, negative values indicate that the mapping failed; positive values are reserved and should not be used by the implementor of a mapping routine.

4.0 OBSOLETE SUBROUTINES

4.1 EZSTRM (U,V,WORK,IMAX,JMAX)

EZSTRM is a front-end to STRMLN with a simpler interface. Like STRMLN, it may be used to create a streamline field flow plot in a single call.

4.1.1 Usage

U and V are 2-dimensional vector component arrays. IMAX must be given the value of the actual size of the first dimension of both these arrays (implying that the first dimensions must be the same for both), while JMAX is the assumed size of the second dimension. The WORK array must, at minimum, be of length 2*IMAX*JMAX. Assuming the default value of the compatibility mode parameter, CPM, Streamlines always performs a SET call and will draw a perimeter around the plot using a call to PERIM when accessed through the EZSTRM interface. By modifying the value of CPM, however, you may take more control over the utility than originally possible using this entry point. For instance, you can override the default value of the SET parameter, or use the Version 3.2 coordinate system mapping routines instead of the old FX and FY functions. Nevertheless, when creating new code, users are strongly encouraged to use the STINIT/STREAM interface, since its capabilities are greater and more likely to improve with time.

4.1.2 Arguments

U (REAL 2-dimensional array, dimensioned IMAX xn: n >= JMAX, input) By default, assumed to contain the first dimensional Cartesian components of the vector field. However, if PLR is non-zero, it is treated as containing the vector magnitudes.

V (REAL 2-dimensional array, dimensioned IMAX xn: n >= JMAX, input) By default, assumed to contain the second dimensional Cartesian components of the vector field. However, if PLR is non-zero, it is treated as containing the vector angles.

WORK (REAL array, dimensioned n: n>= 2*IMAX*JMAX working space): User provided work array used to store the normalized vector component values, and also to keep track of the grid boxes eligible for starting a streamline or placement of a directional arrow.

IMAX (INTEGER, input) Actual size of the first dimension of arrays U and V

JMAX (INTEGER, input) Assumed size of the second dimension of arrays U and V.

4.2 FX (X,Y)

Given the X and Y coordinates of a point on the streamline in the grid coordinate system, the function FX returns the X coordinate of the point in user coordinate space.

4.2.1 Usage

The user does not invoke the function, FX, directly. In older versions of NCAR Graphics, the user modified the functions, FX and FY, in order to implement custom mappings from grid coordinates to user coordinates. These functions were used throughout NCAR Graphics, not just in the Streamlines utility. For compatibility with existing code, calls to any of the Streamlines primary entry points predating Version 3.2 (EZSTRM or STRMLN), by default use FX and FY to map from grid to user coordinates. However, by appropriately setting the compatibility mode parameter, CPM, the user can choose whether the FX and FY functions or the new mapping routines are invoked when using any of the primary entry points supported by Streamlines.

Unlike the Version 3.2 mapping routines, whose input coordinates are in the data coordinate system, FX takes input in the grid coordinate system. Therefore, any required conversions into the data coordinate system must be performed within the function prior to the mapping into user coordinates. The common block STMAP, available for inclusion within the FX routine, supplies the information necessary to perform the conversion into data coordinate space, as well as for the mapping from data to user coordinates. The default version of FX simply performs an identity mapping from grid to user coordinate space.

4.2.2 Arguments

X (REAL, input): The X coordinate of a point on the streamline in the grid coordinate system

Y (REAL, input) The Y coordinate of a point on the streamline in the grid coordinate system

4.3 FY (X,Y)

Given the X and Y coordinates of a point on the streamline in the grid coordinate system, the function FX returns the X coordinate of the point in user coordinate space.

4.3.1 Usage

The user does not invoke the function, FY, directly. In older versions of NCAR Graphics, the user modified the functions, FX and FY, in order to implement custom mappings from grid coordinates to user coordinates. These functions were used throughout NCAR Graphics, not just in the Streamlines utility. For compatibility with existing code, calls to any of the Streamlines primary entry points predating Version 3.2 (EZSTRM or STRMLN), by default use FX and FY to map from grid to user coordinates. However, by appropriately setting the compatibility mode parameter, CPM, the user can choose whether the FX and FY functions or the new mapping routines are invoked when using any of the primary entry points supported by Streamlines.

Unlike the Version 3.2 mapping routines, whose input coordinates are in the data coordinate system, FY takes input in the grid coordinate system. Therefore, any required conversions into the data coordinate system must be performed within the function prior to the mapping into user coordinates. The common block STMAP, available for inclusion within the FY routine, supplies the information necessary to perform the conversion into data coordinate space, as well as for the mapping from data to user coordinates. The default version of FY simply performs an identity mapping from grid to user coordinate space.

4.3.2 Arguments

X (REAL, input): The X coordinate of a data point in the grid coordinate system

Y (REAL, input): The Y coordinate of a data point in the grid coordinate system

4.4 STRMLN (U,V,WORK,IMAX,IPTSX,JPTSY,NSET,IER)

STRMLN plots a streamline representation of a flow field, given two 2-dimensional vector component arrays, U and V. Certain characteristics of the plot may be controlled by adjusting the values given to the input arguments. Other less frequently changed characteristics may be affected by modifying the initial values assigned to members of the common blocks STR02 and STR03 at the beginning of the executable code section of STRMLN. In addition, depending on the value given to the compatibility parameter, CPM, certain options may be controlled by setting internal parameters using STSETx routines.

4.4.1 Usage

Beginning with version 3.2 of NCAR Graphics, the STRMLN entry point has been recoded as a front end to the STINIT/STREAM interface to Streamlines. The compatibility mode parameter, CPM, controls the degree to which a call to the Version 3.2 STRMLN emulates the older call. Appropriate settings of CPM can separately answer each the following three questions regarding the level of emulation:

Given the default value of CPM, all these questions are answered in the affirmative, and a call to STRMLN gives a reasonably faithful emulation of the older version's behavior. However, even in this case, it is possible to use the parameter setting routines to control the behavior of features that have no counterpart in the older version of STRMLN, as long as the feature is accessible without calling the new interface. For instance, you could control the streamline linewidth by setting the LWD parameter, but on the other hand you could not draw streamlines masked to an area map because doing so requires direct invocation of STREAM with the proper input arguments.

The following two tables show how the STRMLN input arguments and STR02/STR03 common block members map into internal parameters currently supported by Streamlines:

Input ArgumentInternal Parameter
NSETSET (NSET = 0 is approximately equivalent to SET = 1)

Common Block MemberInternal Parameter
INITASGD
INITBAGD
ITERPCKP
ITERCCKX
IGFLGTRP
ICYCCYK
IMSGSVF
UVMSGUSV
UVMSGVSV
DISPLVNL
AROWLARL (AROWL as fraction of grid box size is converted to ARL as fraction of viewport width)
CSTOPSSP (CSTOP as fraction of grid box size is converted to SSP as fraction of viewport width)
DISPLDFM (DISPL as fraction of grid box size is converted to DFM as fraction of viewport width)
DISPC/DISPLCDS (The critical displacement multiplier CDS is calculated as the ratio of DISPC to DISPL)

See the parameter descriptions in the next section for an explanation of the meaning of these parameters. Descriptions of the behavior of NSET and the STR02 and STR03 common block members is available in the source code for STRMLN.

4.4.2 Arguments

U (REAL 2-dimensional array, dimensioned IMAX x n: n >= JPTSY, input): By default, assumed to contain the first dimensional Cartesian components of the vector field. However, if PLR is non-zero, it is treated as containing the vector magnitudes.

V (REAL 2-dimensional array, dimensioned IMAX x n: n >= JPTSY, input): By default, assumed to contain the second dimensional Cartesian components of the vector field. However, if PLR is non-zero, it is treated as containing the vector angles.

WORK (REAL array, dimensioned n: n>= 2*IPTSX*JPTSY, working space): User provided work array used to store the normalized vector component values, and also to keep track of the grid boxes eligible for starting a streamline or placement of a directional arrow.

IMAX (INTEGER, input): Actual value of the first dimension of arrays U and V.

IPTSX (INTEGER, input): Number of contiguous elements along the first dimensional axis containing data to be processed in each of the arrays, U and V.

JPTSY (INTEGER, input): Number of contiguous elements along the second dimensional axis containing data to be processed in each of the arrays, U and V.

NSET (INTEGER, input) Flag that controls how and when the SET call is invoked. If NSET is 0, STRMLN makes a SET call to establish a standard viewport and window boundaries coincident with the array coordinate boundaries. PERIM is called to draw a border. If NSET is greater than zero, STRMLN does not call SET or PERIM. If NSET is less than zero, STRMLN calls SET to establish window boundaries coincident with the array grid coordinate boundaries but does not modify the viewport or call PERIM.

IER (INTEGER, output) If no error involving the ICYC common block variable (or, depending on the compatibility mode, the CYK internal parameter) is detected IER contains the value 0 on exit from STRMLN. If, when STRMLN is invoked, ICYC (or CYK) is erroneously set to indicate that the data is cyclic, STRMLN still processes the data using non-cyclic interpolation formulas, but returns the value of -1 in IER.

5.0 PARAMETERS

The behavior of Streamlines is controlled by parameters stored in internal common blocks accessible to its routines that require access to them.

5.1 Parameter Names

Each of the parameters of Streamlines is identified by a three-character name that in general has some mnemonic relationship to its function. For example, SSP is the name of a parameter that specifies the amount of space Streamlines tries to maintain between adjacent streamlines and MSK is the name of a parameter that allows the user to specify whether or not to mask the streamlines to an area map.

5.2 Parameter Types

Each parameter is intrinsically of type INTEGER, or REAL, but the type of the parameter is not implied by the form of its name; one must read the description of the parameter to determine its type. Note that Streamlines does not currently support any parameters of type CHARACTER.

5.3 Parameter Defaults

Each parameter has a default value which is intended to yield a reasonable, useful, default behavior. To modify the behavior of Streamlines routines, one changes the values of the appropriate parameters at the appropriate time.

5.4 Parameter Access

The value of a parameter may be set using STSETI or STSETR. Each of these routines has as its first argument a character string beginning with the three-character name of the parameter and as its second argument an expression of the type implied by the last character of the routine name ("I" for "INTEGER" or "R" for "REAL"). Similarly, the current value of a parameter may be retrieved using STGETI or STGETR, each of which has as its first argument the name of the parameter and as its second argument a variable of the type implied by the last character of the routine name.

In general, once a parameter is given a particular value, it retains that value until it is given a new value by the user. However, a number of parameters are read-only to the user; their values may be retrieved by the STGETx routines but an attempt to set them using the STSETx routines will result in an error. These parameters are intended to provide the user with useful information about the state of the program that may be helpful in determining how to perform the next operation.

5.5 Parameter-Name Arguments

Only the first three characters of the parameter-name argument in calls to the parameter access routines are examined. The user is encouraged to add additional text in order to make the code more understandable. For example, a call to set the streamline spacing might read

      CALL STSETR ('SSP - Streamline Spacing',0.02)
Note that since the dimension of the parameter name variable is defined as 'CHARACTER*(*)', there is no limit to the number of characters used to comment the call.

5.6 Automatic Type Conversion

Normally, one would use STSETI to set parameters of type INTEGER and STSETR to set parameters of type REAL. However, since automatic conversion is done as necessary, this need not be the case. One could, for example, use either of the two statements

      CALL STSETI ('USV - U Array Special Value',-9999)
      CALL STSETR ('USV - U Array Special Value',-9999.0)
to set the U array special value (which is intrinsically REAL) to -9999.0. The first has the virtue of being two characters shorter. Similarly, one could use either of the two statements

      CALL STSETI ('TRP - Interpolation Method',1)
      CALL STSETR ('TRP - Interpolation Method',1.0)
to set the interpolation method parameter (which is intrinsically INTEGER) to 1. (In this case, of course, the first of the two statements seems to make better sense.) If a REAL parameter is to be given a non-integral value (like 3.14159), then STSETR must be used, of course. Note that the argument passing rules of FORTRAN must still be respected: an INTEGER argument passed to STSETR will have the usual "undefined" results.

5.7 Automatic Restriction of Parameter Values

Some parameters may take on any possible value of the proper type. Others are restricted to a certain range. In some cases, out-of-range parameters are simply constrained to some value within the range. In other cases, where it seems dangerous to second-guess what the user has in mind, an error is returned and the program halts.

5.8 Parameters Categorized

This section contains a list of all Streamlines parameters categorized by function along with a short descriptive phrase. It also notes which of the categories may safely be set in between the call to STINIT and STREAM. The section following is an alphabetical listing of all parameters including a detailed description.

5.8.1 Miscellaneous Control Parameters

These parameters provide miscellaneous control of the Streamlines utility. When using the STINIT/STREAM interface, these parameters may be set between the calls to STINIT and STREAM.

CPM - Compatibility Mode - Integer

SST - Streamlines Statistics Output Flag

5.8.2 Coordinate System Control

These parameters (along with the mapping subroutine, STMPXY or STUMXY) establish the mapping through a series of coordinate systems starting with the array indices that define the basic grid and ending with the Normalized Device Coordinates. Any non-default values must be set prior to calling STINIT.

SET - SET Call Flag - Integer

XC1 - X Coordinate Value at Array Index 1 (Data) - Real

XCM - X Coordinate Value at Array Index M (Data) - Real

YC1 - Y Coordinate Value at Array Index 1 (Data) - Real

YCN - Y Coordinate Value at Array Index N (Data) - Real

WDL - Window Left Boundary (User) - Real

WDR - Window Right Boundary (User) - Real

WDB - Window Bottom Boundary (User) - Real

WDT - Window Top Boundary (User) - Real

VPL - Viewport Left Boundary (NDC or Fractional) - Real

VPR - Viewport Right Boundary (NDC or Fractional) - Real

VPB - Viewport Bottom Boundary (NDC or Fractional) - Real

VPT - Viewport Top Boundary (NDC or Fractional) - Real

VPS - Viewport Shape - Integer

MAP - Map Transformation Code - Integer

TRT - Transformation Type - Integer

5.8.3 Input Data Control

These parameters control the interpretation of the input data, and allows the user to place restrictions on the range of data represented. Any non-default values must be set prior to calling STINIT.

SVF - Special Value Flag - Integer

USV - U Array Special Value - Real

VSV - V Array Special Value - Real

PLR - Polar Input Mode - Integer

CYK - Cyclical Data Flag - Integer

5.8.4 Rendering Control

These parameters allow the user to control the rendering of the streamline flow field plot. Parameters in this category may be set between the call to STINIT and STREAM.

GBS - Grid Based Spacing - Integer

MSK - Mask To Area Map Flag - Integer

LWD - Streamline Linewidth - Real

SGD - Stream Starting Grid Increment - Integer

AGD - Arrow Placement Grid Increment - Integer

AMD - Arrow Minimum Distance - Real

CKP - Check Progress Iteration Count- Integer

CKX - Check Crossover Iteration Count - Integer

TRP - Interpolation Method - Integer

VNL - Vector Normalization Value - Real

CDS - Critical Displacement Multiplier - Real

SSP - Streamline Spacing Value - Real

DFM - Streamline Differential Magnitude

ARL - Arrow Head Length - Real

5.8.5 Read-Only Informational Parameters

These parameters may be used to retrieve information about the state of the Streamlines utility. The may be retrieved at any time, but may not contain useful information at all stages of the processing.

ERR - Error Information - Integer

5.9 Parameter Description

Parameter descriptions follow, in alphabetical order. Each description begins with a line giving the three-character mnemonic name of the parameter, the phrase for which the mnemonic stands, and the intrinsic type of the parameter. Note that the phrases such as "the value of XXX is x" where 'XXX' is the name of a parameter and x is an INTEGER or REAL value, are freely used to mean "the Streamlines internal common block variable associated with parameter XXX holds the value x".

5.9.1 AGD - Arrow Placement Grid Increment - Integer

This parameter allows you a degree of control over directional arrowhead spacing in the field flow plot. Streamlines allows a maximum of one directional arrowhead for each grid box, where a grid box is the space between adjacent integer grid coordinates in the grid coordinate system along both dimensions. However, Streamlines actually places an arrowhead only if both lower grid indices, modulo the value of AGD, are equal to 0. If AGD is set to 2, for instance, Streamlines places an arrowhead on the first streamline to enter each grid box with even-numbered lower grid indices.

If the transformation pipeline is everywhere linear from grid coordinate space to NDC space, then the spacing of the arrowheads should be more or less uniform over the field flow plot. However, if there is a non-linearity anywhere in the pipeline, the arrowheads will probably be more crowded in some areas than in others. Future enhancements to the Streamlines utility are expected to provide uniform arrowhead spacing across non-linear grid to NDC space mappings. The default value of AGD is 2.

5.9.2 AMD - Arrow Head Minimum Distance - Real

AMD allows you to specify, as a fraction of the viewport width, a minimum distance between adjacent directional arrowheads along a single streamline. If the data grid is transformed in such a way that adjacent grid cells become very close in NDC space, as for instance in many map projections near the poles, you can use this parameter to help reduce the otherwise cluttered appearance of these regions of the plot. Note that currently, whenever AMD has a positive value, the first arrowhead that would otherwise be drawn for each streamline is always eliminated. If AMD is less than or equal to 0.0, then no arrowheads are eliminated. The default value of AMD is 0.0.

5.9.3 ARL - Arrow Head Length - Real

ARL defines the length of each of the two lines used to create the directional arrow head. If the parameter GBS is set to 0, ARL has units "fraction of viewport width"; if GBS is set to 1, ARL has the units "fraction of grid box width". The default value of ARL is 0.012 when GBS has the value 0 and 0.33 when GBS has the value 1. Setting GBS causes ARL to be reset to its appropriate default value.

5.9.4 CDS - Critical Displacement Multiplier - Real

CDS specifies the minimum amount the streamline must grow as a multiple of the basic differential step size each time the stream progress is checked in order for the streamline not to be terminated. The nominal differential step size is specified by DFM in NDC space, and the progress is checked each CKP iterations. Points of convergence or divergence typically cause stream growth to diminish and the streamline eventually to be terminated. The default value of CDS is 2.0, meaning that any time a streamline does not increase in length a minimum of 2.0*DFM in NDC over the previous check, it is discontinued and a new streamline is begun if possible.

5.9.5 CKP - Check Progress Iteration Count- Integer

The parameter CKP specifies the number of iterations through the streamline building loop between each check of the streamline growth. If the distance between current position of the streamline and the position saved at the time of the previous check is less than a minimum amount, defined as the value of CDS times the value of DFM in NDC space, then the current streamline is terminated and a new one begun if possible. The default value of CKP is 35.

5.9.6 CKX - Check Crossover Iteration Count- Integer

CKX specifies the number of iterations through the streamline building loop between checks for streamline crossover, that is, one streamline growing closer than a certain distance (as specified by the parameter SSP) to previously created streamline. A negative value of CKX causes Streamlines to check for crossover only when a new grid box is entered. At each crossover check, the current streamline position is compared with a sampling of previous streamline positions retained in an internal circular list. The length of this list is fixed by the FORTRAN PARAMETER IPLSTL, currently set to 750. Since up to this number of comparisons are performed at each crossover check, the frequency with which these checks are performed can have a noticeable impact on performance. By default, CKX has the value -99, causing Streamlines to check for crossover only on entrance to a new grid box.

5.9.7 CYK - Cyclical Data Flag - Integer

Use this parameter to specify that the data in the vector field arrays is cyclical: that is, it repeats with a period of M-1 (M, the input parameter to STINIT) along the first dimensional axis. If the flag is set to specify that the data is cyclical, Streamlines checks to see if the field data meet certain criteria. If they do, then an internal cyclical flag is set, causing the normalized vector interpolation routines to consider data from the opposite ends of the dataset when interpolating near the first dimensional dataset boundaries. If the criteria are not met, then Streamlines sets an error flag, retrievable by the user through the parameter ERR; processing continues without interruption, except that Streamlines now interpolates near the first dimensional end points without consideration of data at the opposite end. The test the data must pass in order to be deemed cyclical is that for each subscript value along the second dimensional axis, the first element and the last element along the first dimensional axis must be identical. A value of 0 for CYK means that the data is to be considered non-cyclical; any other value means that Streamlines should test for the cyclical condition. The default value of CYK is 0.

5.9.8 CPM - Compatibility Mode - Integer

Controls the degree of compatibility between versions of the Streamlines utility prior to NCAR Graphics 3.2 and the current version. You can independently control three behaviors using the nine settings provided:

Note, however, that when using the Version 3.2 entry points STINIT and STREAM, only the third behavior option has any meaning.

When CPM is set to 0, its default value, the Streamlines utility's behavior varies depending on whether you access it through one of the pre-Version 3.2 entry points (STRMLN and EZSTRM), or through the STINIT/STREAM interface. Otherwise, positive values result in invocation of the older coordinate mapping routines (FX and FY). Negative values cause the Version 3.2 mapping routines to be used instead. When using the pre-Version 3.2 interface only, odd values of CPM cause the data values in the common blocks, STR02 and STR03, to override corresponding values initialized in the Version 3.2 STDATA block data subroutine, or set by the user calling STSETx routines. Values of CPM with absolute value less than or equal to two cause the NSET argument to STRMLN to take precedence over the SET parameter.

Here is a table of the nine settings of CPM and their effect on the operation of the Streamlines utility:

ValueUse FX and FYUse STR02, STR03Use NSET
-4nonono
-3noyesno
-2nonoyes
-1noyesyes
0old - yes; new - no (*)yesyes
1yesyesyes
2yesnoyes
3yesyesno
4yesnono

(*) Old means EZSTRM or STRMLN entry point; new, STINIT/STREAM. Only the first column is applicable to the behavior of the STINIT/STREAM interface

5.9.9 DFM - Streamline Differential Magnitude

DFM specifies the length of the differential magnitude step size used by Streamlines. If the parameter GBS is set to 0 DFM has units "fraction of viewport width"; if GBS is set to 1, DFM has the units "fraction of grid box width". When the Version 3.2 mapping routines are used, DFM directly affects processing time and the resulting plot precision. In general, smaller values of DFM cause Streamlines to take more, smaller steps in the construction of a streamline, resulting, within the limits of the processor\'s floating point resolution, in longer execution times and a more precise plot. Process memory requirements are not affected. If the compatibility mode parameter is set such that the older mapping routines, FX and FY, are invoked instead, DFM no longer has any effect on the plot, since in this case the step size is determined by the setting of the parameter VNL as a fraction of the grid box width. The default value of DFM is 0.02 when GBS has the value 0 and 0.33 when GBS has the value 1. Setting GBS causes DFM to be reset to its appropriate default value.

5.9.10 GBS - Grid Based Spacing

The parameter GBS controls the interpretation of several parameters that play a critical role in the appearance of the streamline plot. These parameters are DFM, SSP, and ARL. When GBS has the value 0, the values of these parameters are treated as having units of "fraction of viewport width". If GBS has the value 1, the values are treated as having the units of "fraction of grid box width". Whenever you set GBS, the three affected parameters are reset to default values appropriate to the units; therefore you must set GBS prior to setting any non-default values for DFM, SSP, or ARL. You may find that using the grid-based spacing method causes Streamlines to adapt more gracefully to variations in the density of the data grid. Currently, the default value of GBS is 0; however, in the next release this may change.

5.9.11 LWD - Streamline Linewidth - Real

LWD controls the linewidth used to draw the streamlines. Note that since the linewidth in NCAR Graphics is always calculated relative to a unit linewidth that is dependent on the output device, you may need to adjust the linewidth value depending on the output conditions to obtain a pleasing plot. LWD affects the linewidth of the directional arrowheads as well as the streamlines themselves. The arrowhead length also increases somewhat when the linewidth is greater than the default. However, the arrowhead length parameter still affects the length. The default is 1.0, specifying a device-dependent minimum linewidth.

5.9.12 MAP - Map Transformation Code - Integer

MAP defines the transformation between the data and user coordinate space. For a complete explanation of the various coordinate systems used by the Streamlines utility, see the section Coordinate Systems in Streamlines. Other relevant information is contained in the discussion of the user-modifiable mapping subroutines, STUIXY, STUMXY, and STUMTA, in the discussion of the transformation type parameter, TRT, and also in the section, The SET Call. Three MAP parameter codes are reserved for pre-defined transformations, as follows:

ValueMapping transformation
0 (default)Identity transformation between data and user coordinates: array indices of U and V are linearly related to user coordinates. Note however that a non-linear transformation is still possible from user to NDC coordinates.
1 EZMAP transformation: first dimension indices of U and V are linearly related to longitude; second dimension indices are linearly related to latitude.
2Polar to rectangular transformation: first dimension indices of U and V are linearly related to the radius; second dimension indices are linearly related to the angle in degrees.

If MAP has any other value, the user-modifiable mapping subroutines are invoked. The default version of these routines simply perform identity mappings, without considering the semantic value of the TRT parameter. The effect is the same as if one were to set both MAP and TRT to zero. Note that, while the Streamlines utility does not actually prohibit the practice, the user is advised not to use negative integers for user-defined mappings, since other utilities in the NCAR Graphics toolkit attach a special meaning to negative mapping codes.

For all the predefined mappings, the linear relationship between the grid array indices and the data coordinate system is established using the four parameters, XC1, XCM, YC1, and YCN. The X parameters define a mapping for the first and last indices of the first dimension of the data arrays, and the Y parameters do the same for the second dimension. If MAP is set to a value of one, you need to be careful to ensure that the SET parameter is given a value of zero, since the EZMAP routines require a specific user coordinate space for each projection type, and internally call the SET routine to define the user to NDC mapping. Otherwise, you may choose whether or not to issue a SET call prior to calling STINIT, modifying the value of SET as required.

5.9.13 MSK - Mask To Area Map Flag - Integer

Use this parameter to control masking of streamlines to an existing area map created by routines in the Areas utility. When MSK is greater than 0, masking is enabled and an area map must be set up before calling STREAM. The area map array and, in addition, the name of a user-definable masked drawing routine, must be passed as input parameters to STREAM. There are two states for the MSK parameter, as follows:

ValueEffect
<= 0 (default)No streamline masking.
>0The subroutine ARDRLN is called internally to decompose the streamlines into segments contained entirely within a single area group. ARDRLN calls the user-definable masked drawing subroutine.

See the discussion of the default version of the user-definable subroutine, STUMSL, for further explanation of masked drawing of streamlines

5.9.14 PLR - Polar Input Mode - Integer

When PLR is greater than zero, the vector component arrays are considered to contain the field data in polar coordinate form: the U array is treated as containing the vector magnitude and the V array as containing the vector angle. Be careful not to confuse the PLR parameter with the MAP parameter polar coordinate mode. The MAP parameter relates to the location of the vector, not its value. Here is a table of values for PLR:

ValueMeaning
0 (default)U and V arrays contain data in cartesian component form.
1U array contains vector magnitudes; V array contains vector angles in degrees.
2U array contain vector magnitudes; V array contains vector angles in radians.

5.9.15 SET - SET Call Flag - Integer

Give SET the value 0 to inhibit the SET call STINIT performs by default. Arguments 5-8 of a SET call made by the user must be consistent with the ranges of the user coordinates expected by Streamlines. This is determined by the mapping from grid to data coordinates as specified by the values of the parameters XC1, XCM, YC1, YCN, and also by the mapping from data to user coordinates established by the MAP parameter. See the descriptions of those parameters and the sections 2.3 and 2.6, Coordinate Systems in Streamlines and The SET Call. The default value of SET is 1.

5.9.16 SGD - Stream Starting Grid Increment - Integer

This parameter gives you a degree of control over the number and density of streamlines in the field flow plot. The Streamlines utility never begins a streamline in any grid box that has previously had a streamline pass through it, where a grid box is defined as the space between adjacent integer grid coordinates in the grid coordinate system along both dimensions. By setting SGD to a value greater than 1, you can reduce the number of grid boxes initially eligible for starting a streamline. A grid box is considered initially eligible for starting a streamline only if both the lesser indices that establish the grid box, modulo the value of SGD, equal 0. If SGD is set to 2, for instance, every grid box with even-numbered lower grid indices would be initially eligible for starting a streamline. As the streamlines grow and pass through grid boxes that were initially eligible, these boxes too are marked ineligible, further reducing the boxes where a stream can be started.

If the transformation pipeline is everywhere linear from grid coordinate space to NDC space, then this scheme for starting streamlines usually produces a more or less uniform spacing of the streamlines over the field flow plot. However, if there are non-linear transforms anywhere in the pipeline, the streamlines will probably be more crowded in some areas than in others. Future enhancements to the Streamlines utility are expected to address this issue, and also perhaps to provide options for intentional non-uniform spacing based on flow intensity. The default value of SGD is 2.

5.9.17 SSP - Streamline Spacing Value - Real

The streamline spacing parameter establishes the minimum distance a streamline in progress is allowed to approach existing streamlines before being terminated. If the parameter GBS is set to 0, SSP has units "fraction of viewport width"; if GBS is set to 1, SSP has the units "fraction of grid box width". In general, within either system of units, larger values of SSP increase the distance between streamlines, and have a tendency to create more, but shorter stream lines. The spacing is only checked at intervals, so streamlines sometimes approach closer than the specified distance. The checking frequency is adjustable using the streamline crossover checking parameter, CKX. The streamline starting grid increment parameter, SGD, also affects the overall streamline density. The default value of SSP is 0.015 when GBS has the value 0 and 0.5 when GBS has the value 1. Setting GBS causes SSP to be reset to its appropriate default value.

5.9.18 SST - Streamline Statistics Output Flag -Integer

If SST is set to one, STREAM writes a summary of its operations to the default logical output unit, including the number of streamlines plotted and the total differential step count. Here is a sample of the output:

 STREAM Statistics
                Streamlines plotted: 119
      Total differential step count: 2903
The differential step count actually counts the number of iterations through the main streamline construction loop, and can be used to help gauge the trade-offs between the increased processing time required for smaller differential step sizes and the resulting differences in plot quality.

5.9.19 SVF - Special Value Flag - Integer

The special value flag controls special value processing for the U and V vector component data arrays. Special values may appear in either the U or V array or in both of them. When any of the four points surrounding the current streamline end contain a special value, the streamline is terminated, and a new one started, if possible. Streamlines allows special value processing to be turned on or off, as follows:

ValueEffect
0 (default)Neither the U nor the V array is examined for special values
non 0Whenever the streamline under construction enters a new grid box, the U and V array values at each corner of the box are examined for special values. The interpolation method parameter, TRP, is overridden, causing Streamlines to use bi-linear interpolation only.

The U and V special values are defined by setting parameters USV and VSV. Streamlines only uses bi-linear interpolation when special value processing is in effect, because the Bessel interpolation method quadruples the requirement for good data points (from 4 to 16) surrounding the current stream end point.

5.9.20 TRP - Interpolation Method - Integer

Use TRP to control which of two interpolation methods Streamlines should use in determining the normalized flow components for each point in the streamline. The choices are as follows:

ValueInterpolation Method
0 (default)Use the 16-point Bessel interpolation method where possible; otherwise, near the data set boundaries use 12, 9 or 4 point interpolation methods, depending on the situation.
non 0Use 4-point bi-linear interpolation at all points.

Note that Streamlines forces use of the 4-point bi-linear interpolation method if the SVF parameter is set to turn on special value processing.

5.9.21 TRT - Transformation Type - Integer

The transformation type parameter, TRT, qualifies the mapping transformation specified by the MAP parameters, as follows:

ValueEffect
-1Direction, magnitude, and location are all transformed. This option is not currently supported by any of the pre-defined coordinate system mappings.
0Only location is transformed
1 (default)Direction and location are transformed

This parameter allows you to distinguish between a system that provides a mapping of location only into an essentially cartesian space, and one in which the space itself mapped. To understand the difference, using polar coordinates as an example, imagine a set of wind speed monitoring units located on a radial grid around some central point such as an airport control tower. Each unit's position is defined in terms of its distance from the tower and its angular direction from due east. However, the data collected by each monitoring unit is represented as conventional eastward and northward wind components. Assuming the towers' location is at a moderate latitude, and the monitoring units are reasonably 'local', this is an example of mapping a radially defined location into a nearly cartesian space (i.e. if the northward components were all set to 0.0, the streamlines defined by the eastward components would all be parallel straight lines. One would set MAP to two (for the polar transformation) and TRT to zero to model this data on a plot generated by the Streamlines utility.

On the other hand, picture a set of wind data, again given as eastward and northward wind components, but this time the center of the polar map is actually one of the earth's poles. In this case, the eastward components do not point in a single direction; instead they outline a series of concentric circles around the pole. This is a space mapping transformation: one would again set MAP to two, but TRT would be set to one to transform both direction and location.

Changing the setting of this parameter affects the end results only when a non-uniform non-linear mapping occurs at some point in the transformation pipeline. For this discussion a uniform linear transformation is defined as one which satisfies the following equations:

Xout = Xoffset + Sconstant * Xin
Yout = Yoffset + Sconstant * Yin
If the Sconstant is not the same for both equations then the mapping is non-uniform.

This option is currently implemented only for the pre-defined MAP parameter codes, 0 and 2, the identity mapping and the polar coordinate mapping. However, it operates on a different stage of the transformation pipeline in each case. The polar mapping is non-linear from data to user coordinates. The identity mapping, even though necessarily linear over the data to user space mapping, can have a non-uniform mapping from user to NDC space, depending on the values given to the input parameters of the SET call. This will be the case whenever the LL input parameter specifies a logarithmic scaling or the viewport and the user coordinate boundaries do not have the same aspect ratio. Thus for a MAP value of 2, TRT affects the mapping between data and user space, whereas for MAP set to 0, TRT influences the mapping between user and NDC space.

5.9.22 USV - U Array Special Value - Real

USV is the U vector component array special value. It is a value outside the range of the normal data used to indicate that there is no valid data for this grid location. When the special value flag parameter, SVF, is non-zero, each time a streamline enters a new cell Streamlines will check for this special value in the U array at each of the four corners of the grid box. Any time the special value is discovered, the current streamline is terminated and a new one started if possible. The default value given to USV is 1.0 * 1012.

5.9.23 VNL - Vector Normalization Value - Real

The parameter, VNL, determines the value Streamlines uses to normalize the vector flow field, before beginning the streamline construction loop. When Streamlines is used with the pre-Version 3.2 mapping routines, FX and FY, the value of VNL determines the step size in the grid coordinate system used to construct the streamlines, as a fraction of the grid box size. When using FX and FY, smaller values of VNL result in smaller steps, more processing time and, within the limits of the processor's floating point accuracy, a higher precision plot. However, if the mapping has non-linearities, the grid size does not remain constant over the transformation and the step size can vary greatly, resulting in discontinuities in certain areas of the plot.

Streamline's new mapping routines define the streamline differential magnitude in NDC space, ensuring a constant step size over the whole plot, notwithstanding any non-linearity in the transformation. When using the new mapping routines, the parameter DFM controls the step size in NDC space, and VNL is not adjustable by the user. (See the discussion of the compatibility mode parameter, CPM, for a discussion of how to switch between the old and new mapping routines.) The default value of VNL is 0.33.

5.9.24 VPB - Viewport Bottom - Real

The parameter VPB has an effect only when SET is non-zero, specifying that Streamlines should do the call to SET. It defines a minimum boundary value for the bottom edge of the viewport in NDC space, and is constrained to a value between 0.0 and 1.0. It must be less than the value of the Viewport Top parameter, VPT. The actual value of the viewport bottom edge used in the plot may be greater than the value of VPB, depending on the setting of the Viewport Shape parameter, VPS. The default value of VPB is 0.05.

5.9.25 VPL - Viewport Left - Real

The parameter VPL has an effect only when SET is non-zero, specifying that Streamlines should do the call to SET. It defines a minimum boundary value for the left edge of the viewport in NDC space, and is constrained to a value between 0.0 and 1.0. It must be less than the value of the Viewport Right parameter, VPR. The actual value of the viewport left edge used in the plot may be greater than the value of VPL, depending on the setting of the Viewport Shape parameter, VPS. The default value of VPL is 0.05.

5.9.26 VPR- Viewport Right - Real

The parameter VPR has an effect only when SET is non-zero, specifying that Streamlines should do the call to SET. It defines a maximum boundary value for the right edge of the viewport in NDC space, and is constrained to a value between 0.0 and 1.0. It must be greater than the value of the Viewport Left parameter, VPL. The actual value of the viewport right edge used in the plot may be less than the value of VPR, depending on the setting of the Viewport Shape parameter, VPS. The default value of VPR is 0.95.

5.9.27 VPS - Viewport Shape - Real

The parameter VPS has an effect only when SET is non-zero, specifying that Streamlines should do the call to SET; it defines the desired viewport shape, as follows:

ValueEffect
<0.0The absolute value of VPS specifies the shape to use for the viewport, as the ratio of the viewport width to its height,
0.0The viewport completely fills the area defined by the boundaries specifiers, VPL, VPR, VPB, VPT
>0.0, <1.0

(0.25, default)

Use R = (XCM-XC1)/(YCN-YC1) as the viewport shape if MIN(R, 1.0/R) is greater than VPS. Otherwise determine the shape as when VPS is equal to 0.0.
>= 1.0Use R = (XCM-XC1)/(YCN-YC1) as the viewport shape if MAX(R, 1.0/R) is less than VPS. Otherwise make the viewport a square.

The viewport, whatever its final shape, is centered in, and made as large as possible in, the area specified by the parameters VPB, VPL, VPR, and VPT.

5.9.28 VPT - Viewport Top - Real

The parameter VPT has an effect only when SET is non-zero, specifying that Streamlines should do the call to SET. It defines a maximum boundary value for the top edge of the viewport in NDC space, and is constrained to a value between 0.0 and 1.0. It must be greater than the value of the Viewport Bottom parameter, VPB. The actual value of the viewport top edge used in the plot may be less than the value of VPT, depending on the setting of the Viewport Shape parameter, VPS. The default value of VPT is 0.95.

5.9.29 VSV - V Array Special Value - Real

VSV is the V vector component array special value. It is a value outside the range of the normal data used to indicate that there is no valid data for this grid location. When the special value flag parameter, SVF, is non-zero, each time a streamline enters a new cell Streamlines will check for this special value in the V array at each of the four corners of the grid box. Any time the special value is discovered, the current streamline is terminated and a new one started if possible. The default value given to VSV is 1.0 * 1012.

5.9.30 WDB - Window Bottom - Real

When STINIT does the call to SET, the parameter WDB is used to determine argument number 7, the user Y coordinate at the bottom of the window. If WDB is not equal to WDT, WDB is used. If WDB is equal to WDT, but YC1 is not equal to YCN, then YC1 is used. Otherwise, the value 1.0 is used. The default value of WDB is 0.0.

5.9.31 WDL - Window Left - Real

When STINIT the call to SET, the parameter WDL is used to determine argument number 5, the user X coordinate at the left edge of the window. If WDL is not equal to WDR, WDL is used. If WDL is equal to WDR, but XC1 is not equal to XCM, then XC1 is used. Otherwise, the value 1.0 is used. The default value of WDL is 0.0.

5.9.32 WDR - Window Right - Real

When STINIT does the call to SET, the parameter WDR is used to determine argument number 6, the user X coordinate at the right edge of the window. If WDR is not equal to WDL, WDR is used. If WDR is equal to WDL, but XCM is not equal to XC1, then XCM is used. Otherwise, the value of the STINIT input parameter, M, converted to a real, is used. The default value of WDR is 0.0.

5.9.33 WDT - Window Top - Real

When STINIT does the call to SET, the parameter WDB is used to determine argument number 8, the user Y coordinate at the top of the window. If WDT is not equal to WDB, WDT is used. If WDT is equal to WDB, but YCN is not equal to YC1 then YCN is used. Otherwise, the value of the STINIT input parameter, N, converted to a real, is used. The default value of WDT is 0.0.

5.9.34 XC1 - X Coordinate at Index 1 - Real

The parameter XC1 specifies the X coordinate value that corresponds to a value of 1 for the first subscript of the U and V, vector field component arrays. Together with XCM, YC1, and YCN it establishes the mapping from grid coordinate space to data coordinate space. If XC1 is equal to XCM, 1.0 will be used. The default value of XC1 is 0.0.

5.9.35 XCM - X Coordinate at Index M - Real

The parameter XCM specifies the X coordinate value that corresponds to the value of the STINIT input parameter, M, for the first subscript of the U and V vector component arrays. Together with XC1, YC1, and YCN it establishes the mapping from grid coordinate space to data coordinate space. If XC1 is equal to XCM, the value of M, converted to a real, will be used. The default value of XCM is 0.0

5.9.36 YC1 - Y Coordinate at Index 1 - Real

The parameter YC1 specifies the Y coordinate value that corresponds to a value of 1 for the first subscript of the U, V, vector component arrays as well as for the P scalar data array, if used. Together with YCN, XC1, and XCM it establishes the mapping from grid coordinate space to data coordinate space. If YC1 is equal to YCN, 1.0 will be used. The default value of YC1 is 0.0

5.9.37 YCN - Y Coordinate at Index N - Real

The parameter YCN specifies the Y coordinate value that corresponds to the value of the STINIT input parameter, N, for the second subscript of the U and V vector component arrays as well as the P scalar data array, if used. Together with YC1, XC1, and XCM it establishes the mapping from grid coordinate space to data coordinate space. If YC1 is equal to YCN, the value of N, converted to a real, will be used. The default value of YCN is 0.0

5.9.38 ZFC - Zero Field Text Block Color - Integer

If ZFC is greater or equal to zero, it specifies the GKS color index to use to color the Zero Field text block. Otherwise the Zero Field text block is colored using the current GKS text color index. The default value of ZFC is -1.

5.9.39 ZFP - Zero Field Text Block Positioning Mode - Integer

The ZFP parameter allows you to justify, using any of the 9 standard justification modes, the Zero Field text block unit with respect to the position established by the parameters, ZFX and ZFY. The position modes are supported as follows:

ModeJustification
-4The lower left corner of the text block is positioned at ZFX, ZFY.
-3The center of the bottom edge is positioned at ZFX, ZFY.
-2The lower right corner is positioned at ZFX, ZFY.
-1The center of the left edge is positioned at ZFX, ZFY.
0 (default)The text block is centered along both axes at ZFX, ZFY.
1The center of the right edge is positioned at ZFX, ZFY.
2The top left corner is positioned at ZFX, ZFY.
3The center of the top edge is positioned at ZFX, ZFY.
4The top right corner is positioned at ZFX, ZFY.

5.9.40 ZFS - Zero Field Text Block Character Size - Real

ZFS specifies the size of the characters used in the Zero Field graphics text block as a fraction of the viewport width. The default value is 0.033.

5.9.41 ZFT - Zero Field Text String - Character* 36

Use ZFT to modify the text of the Zero Field text block. The Zero Field text block may appear whenever the U and V vector component arrays contain data such that all the grid points otherwise eligible for plotting contain zero magnitude vectors. Currently the string length is limited to 36 characters. Set ZFT to a single space (' ') to prevent the text from being displayed. The default value for the text is 'Zero Field'.

5.9.42 ZFX - Zero Field Text Block X Coordinate - Real

ZFX establishes the X coordinate of the Zero Field graphics text block as a fraction of the viewport width. Values less than 0.0 or greater than 1.0 are permissible and respectively represent regions to the left or right of the viewport. The actual position of the block relative to ZFX depends on the value assigned to the Zero Field Positioning Mode parameter, ZFP. The default value is 0.5.

5.9.43 ZFY - Zero Field Text Block Y Coordinate - Real

ZFY establishes the Y coordinate of the Zero Field graphics text block as a fraction of the viewport height. Values less than 0.0 or greater than 1.0 are permissible and respectively represent regions below and above the viewport. The actual position of the block relative to ZFY depends on the value assigned to the Zero Field Positioning Mode parameter, ZFP. The default value is 0.5.

6.0 ERROR MESSAGES

In this section are listed, in alphabetical order, all the error messages that may be written by Streamlines. Each error message begins with the name of the Streamlines routine in which an error has been detected, followed by a dash, followed by the error message itself. These error messages are written by a call to the error-handling support routine SETER, with a final argument indicating that the error is fatal and that execution should be terminated. 

STGETI OR STGETR - PARAMETER NAME NOT KNOWN - x
The given parameter name ('x') is not a legal parameter name known to Streamlines.

STGETI OR STGETR - PARAMETER NAME TOO SHORT - x
The parameter name ("x") is less than three characters long.

STINIT - U AND/OR V ARRAY DIMENSIONS EXCEEDED
The STINIT input argument, M, specifying the number of array elements to use along the first dimension of the U and V arrays, exceeds the actual first dimension of U and/or V, as specified by the input arguments LU and LV. 

STREAM - TOO MANY AREA GROUPS
The area map passed as the argument, IAM, to STREAM contains more area groups than allowed by the Streamlines utility, currently 64. This error can occur only if the mask to area map parameter, MSK, has a value greater than 0, specifying that streamlines are to be drawn masked to an area map.

 STREAM - INVALID AREA MAP
The area map passed as the argument, IAM, to STREAM is judged to be invalid because it contains a negative value for the number of area groups. This error can occur only if the mask to area map parameter, MSK, has a value greater than 0, specifying that streamlines are to be drawn masked to an area map.


STSETI OR STSETR - PARAMETER NAME NOT KNOWN - x
The given parameter name ('x') is not a legal parameter name known to Streamlines.

STSETI OR STSETR - PARAMETER NAME TOO SHORT - x
The parameter name ("x") is less than three characters long.

STSETI OR STSETR - PARAMETER VALUE OUT OF RANGE - x
An attempt has been made to set the parameter named 'x' to a value outside the range allowed by the Streamlines utility.

7.0 EXAMPLES

On a Unix system on which NCAR Graphics has been installed you may retrieve, compile, and execute all examples involving the Streamlines utility by executing the command

    ncargex -streamlines
Alternatively, you can obtain and run a specific example by giving the command

    ncargex example_name
To obtain the code without compiling or executing it, use the command

    ncargex -n example_name
If, after examining the code and perhaps modifying it, you wish to compile and execute the program, use the commands

    ncargf77 -o example_name *.f
    ./example_name
(Note that the form of the above command assumes you have the source for only one example in the current directory.) The metafile created by executing an example can be viewed using the command

    ctrans metafile_name
(It may be necessary to set some environment variables and/or to set other options on the "ctrans" command line.)

See the section Five quick steps for creating and viewing your plot in the NCAR Graphics Fundamentals document for more details.

tstrml: Frame 1 of 1

The test program "tstrml" demonstrates compatibility of the Streamlines utility with the pre-Version 3.2 interface. This code calls the STRMLN entry point to plot the streamline field flow of a functionally derived vector field. Due to changes in the internal algorithms the resulting plot is not identical to the plot produced by the Version 3.1 version of STRMLN, but it is very similar, and the information content is the same.

stex01: Frame 1 of 1

The example "stex01" illustrates the polar coordinate mapping supplied with Streamlines. The plot actually consists of two uniform fields plotted one over the other. The two mutually perpendicular fields are mapped to a polar coordinate space, one parallel to the radial axis and the other perpendicular to it. Besides illustrating polar coordinate axes, this example provides a good test of the accuracy of the differentially generated streamlines, since the streamlines perpendicular to the radial axis should theoretically, and, in fact, do, form a series of concentric circles. You can affect the quality of the circles as a whole depending on the value you give to the differential magnitude parameter, DFM, but the generated streamlines show no tendency to deviate from a basically circular path.

stex02: Frame 1 of 5

Example "stex02" has 5 frames, where each frame shows the effect of a different SET call on a simple uniform field (both components at all locations have a value of 1.0). For comparison, the streamlines drawn in magenta are overlaid on a vector plot in green of the same field. The first four frames show each of the four coordinate axis options available through the LL argument to SET (X and Y linear, X linear and Y log, X log and Y linear, X and Y log). The final frame shows the effect of creating a linear coordinate system, where a unit length on the X axis has a different size than a unit length on the Y axis. The intent of this example is simply to show that Streamlines (and Vectors) can now generate correct plots when used any of the SET call coordinate system options. The example incidentally illustrates use of the compatibility mode parameter, CPM. Although it invokes the old interfaces, STRMLN and VELVCT, it sets CPM to activate Version 3.2 features like the new mapping routines.

stex03: Frame 1 of 10

The example "stex03" produces 10 frames, one for each of the supported EZMAP projections. Except for the Lambert conformal conic projection, all the frames show the maximum possible area of the globe visible with that projection. Again, a uniform field is shown, and the streamlines overlay a plot of the same data generated by calls to routines in the Vectors package. Note that, while for some of the projections the whole surface of the globe is visible, others show only a portion of the globe. You do not need to worry about adjusting the grid boundaries to the visible part of the projection. Streamline data outside the visible boundaries is simply discarded.

See the section, Field flows, in NCAR Graphics Fundamentals, UNIX Version, for descriptions of more examples involving the Streamlines utility.

INDEX

A

C

D

E

G

L

M

P

S

T

U

V

W

X

Y

Z