2.1 Object Oriented Graphics

The idea behind object oriented graphics (OOG) is to supply the user with classes of geometric objects and graph objects which are completely independent of the underlying graphics engine, making it unnecessary for the user to have to learn details of low level interfaces to graphics. Most users do not wish to be bothered with the low-level and often arcane methods of dealing with a graphics engine, let alone having to know the properties of more than one graphics engine, since typically they differ so radically from one another. We believe that the typical user would like to do something like the following: take the results of some calculations and use them to specify geometric objects; hand the geometric objects to graph objects; ask the graph objects to plot themselves.

Unfortunately the goal of a set of high-level graphics objects which are independent of the underlying graphics engines is difficult (nearly impossible) to reach. This is particularly true of the two graphics engines, Gist and Narcisse, which currently underlie the OOG. Gist has far more and better capabilitiues for 2-D graphics than does Narcisse. This means that to supply relatively equivalent 2-D graphics with Narcisse, it would be necessary to write a Python or C wrapper for Narcisse which does the necessary computations. Likewise, although there is considerable overlap, each engine supplies some 3-D capabilities that the other does not, so wrappers supplying extensions to each must be written. At the time of the writing of this manual, only a small part of this work has been done, but we hope to proceed with this work in the future.

Another intrinsic difficulty is that Narcisse is much slower than Gist, so, in particular, real-time animations involving complex figures are simply not feasible in Narcisse. Part of this slowness is due to the fact that the user program and Narcisse (a separate process) communicate data back and forth via sockets, and part is simply that Narcisse internal computations, for whatever reasons, are very slow.

A third problem is that plotting solely to a file is impossible in Narcisse; it is designed to be used interactively. Narcisse plots can be sent to either a binary or ascii file in addition to being sent to a window, but these files are in a format peculiar to Narcisse. A particular Narcisse plot can be sent to a PostScript file only by clicking a button in the Narcisse GUI currently displaying that plot. On the other hand, Gist plots can be sent to an arbitrary choice of windows, PostScript files, and CGM files without interctive intervention.

The tables below indicate to the user which capabilities are available in PyGist and PyNarcisse currently, and what types of devices can be plotted to. We use the term ``not yet'' for features which will someday be implemented, and ``never'' for those which are essentially impossible.

TABLE 1. Geometry Capabilities of PyGist and PyNarcisse

	PyGist	PyNarcisse
curves, including multiple	yes	yes
multiple disjoint lines	yes	not yet
quadrilateral mesh--line plot	yes	not yet
quadrilateral mesh--contour plot	yes	not yet
quadrilateral mesh--filled contour plot	not yet	yes
region plots	yes	not yet
filled polygons	yes	not yet
cell arrays	yes	not yet
2-D animation, real time	yes	never
color bar	yes	yes
axes in 3d plots	gnomon1 only	yes
surfaces--wire mesh, monochrome	yes	yes
surfaces--wire mesh, colored by data	never	yes
surfaces--flat (color filled cells)	yes	yes
surfaces--contours, filled contours	not yet	yes
surfaces--shaded by light source	yes	not yet
3-D mesh--complete cells	never	yes
3-D mesh--isosurface and plane slices	yes	yes
3-D mesh--isosurface and plane slices, split palette	yes	not yet
3-D realtime animation--moving light source	yes	maybe someday
3-D realtime animation--rotation	yes	yes (slow)

1 The gnomon is a small representation of the coordinate axes at the lower left of the picture. The name of an axis is reverse video if it points into the plane of the graph.

TABLE 2. Device Capabilities of PyGist and PyNarcisse

	PyGist	PyNarcisse
Xwindow	yes	yes
multiple Xwindows	yes	yes
Xwindow on remote machine	yes	yes
file(s) only, no Xwindow	yes	never
CGM file(s)	yes	never
PostScript file(s)	yes	only from GUI
multiple files	yes	never
file in self-specific format	no	yes