Historically, all smoothing was performed in SNAP itself, using a relatively simple smoothing algorithm that applied a weighted average of neighboring data points. One undesirable feature of this algorithm is that it rounds off profiles that are triangular in shape, i.e., the central value of the smoothed profile is systematically less than the unsmoothed profile. The same smoothing radius was used for all profiles.
A disadvantage of smoothing the profiles in SNAP is that you don't see the consequences of the smoothing until SNAP is complete--which requires 12 minutes of CPU time for beam shots. You may find that the chosen smoothing radius was too large or too small, leading to over-smoothed or under-smoothed profiles. Smoothing the profiles in SNAPIN provides immediate feedback on the effects of the selected smoothing procedure. Several smoothing algorithms are offered in SNAPIN.
The smoothing algorithm used in SNAP and SNAPIN was taken directly
from the FILTR6 routine in the TRANSP code. The algorithm uses
a simple local weighted average to generate the smoothing action. The
user specifies a smoothing radius, , for the local
weighted average. The weighting function is triangularly shaped, with
a base width , symmetric about and peaked at the point where
the local average is being computed (see Figure 3).
This function has been found to generate almost equivalent smoothing
action to more complicated weighting functions, with a considerable
savings in CPU time. The formula for the weighting function is:
Figure 3: Triangular Weighting Function
where W(x-x0) is the weight applied of the data point at x for
the smoothing being performed at x0. The algorithm evaluates the
smoothed data by computing the local weighted average about each
point, j, using the weighting function:
where f(t) is the linearly interpolated (i.e., continuous, piecewise
linear) function of the original data (x(1),y(1)) through
(x(n),y(n)).
The original FILTR6 routine offered the capability to suppress smoothing of high amplitude features, i.e., features whose amplitude exceeded a preset threshold. Also, the original FILTR6 routine allowed the smoothing radius to vary in the independent coordinate (i.e., major radius or time). Neither of these features has been retained in SNAP. The smoothing radius is a constant in SNAP and SNAPIN (although in SNAPIN the user can repetitively smooth various sections of the data with different smoothing radii, which amounts to the same thing), and all of the data points within the smoothing interval are smoothed, irrespective of their deviation from the smoothed curve.
No provision is offered in SNAP to restrict the smoothing to a subset of the input profile; the entire profile is subject to smoothing. If however the smoothing is performed in SNAPIN, the user identifies the smoothing region with mouse clicks; data points outside the smoothing region are unaffected by the smooth. Such ``smooths'' can be performed sequentially, e.g., one could smooth the data in several segments. Any part of the data can also be smoothed multiple times in SNAPIN if that is desired.
After a user defines the smoothing parameters in SNAPIN, it will draw a temporary curve illustrating the smoothed data. If the user accepts the smooth, then the temporary curve data is written into the processed array for the source being edited; if it is rejected, the contents of the temporary curve are deleted and nothing new is written into the processed array. Note that if n smooths have been performed on a profile and the user elects to delete the n+1st smooth, the processsed array still represents the n-fold smoothed data; only the most recent one is recinded. The only way to undo multiple smooths is to reread the input data.
When you have smoothed a profile interactively in SNAPIN, the manual smoothing parameter is set and the largest radius used for smoothing that profile is passed through SNAP. Once you have smoothed a profile manually, you cannot request that SNAP smooth it (unless, of course, you reread the profile).
The FILTR6 algorithm offers some control of the behavior of smoothing near the endpoints of the smoothing region: it may be set to keep the original endpoint values fixed, or to set the derivative (with respect to the independent variable, radius or time for example) of the smoothed data zero at the endpoints, or a linear combination of these two limits.
The SM option in SNAPIN (which is the setting used in SNAP) forces the smoothing algorithm to keep the values fixed at both the left and right endpoints of the smoothing region. The SML option keeps the left endpoint fixed, but imposes the condition d/dx = 0 on the smoothed curve on the right endpoint. This option is probably most relevant to smoothing the left-half of a profile, for which one might expect d/dx = 0 at the right side by symmetry. Similarly, SMR option keeps the right endpoint fixed, but imposes the condition d/dx = 0 on the smoothed curve on the left endpoint. This option is probably most relevant to smoothing the right-half of a profile, for which one might expect d/dx = 0 at the left side by symmetry. Finally, the SMG option gives the user control over the smoothing near the endpoints: for each endpoint (left and right), the user specifies a real number between 0 and 1. Here 0 corresponds to fixing the endpoint value and 1 corresponds to imposing d/dx = 0 at the endpoint, while intermediate values imply a linear interpolation between these limits.