Vic Snap "Show Focus/contrast" feature

This feature was previously labeled "Show co-variance" and then "Show sigma estimate."
 
 The "Show Focus/Contrast" feature may be selected by right clicking the live image in Vic-Snap.  The user is also given the option for three different subset sizes in the same menu.  This is used as a focus tool.  Specifically, this tool shows an estimate of the sigma (the one standard-deviation confidence interval) that is displayed in Vic-2D/3D results.  This sigma estimate simply estimates how easy it'll be to search and track subsets for the given focus/lighting. In order to provide live feedback, the sigma estimate requires less computing time than the sigma results that are provided in the Vic-2D/3D analysis.  Since it's computed in a different and simpler way, it will likely not match the sigma in the results (which is why we determined that labeling the feature "sigma estimate" was confusing and renamed it "focus/contrast"). 

The purpose of this tool is to provide the user some live feedback for focus (and also provides some feedback on pattern quality and lighting).  Specifically, this feature is a measure of the gradient between white and black. We assume that a sharp gradient from white to black means that we have good contrast and good focus. This is why it can be used as a focus tool. This is also why if the lights are moved or the brightness of the image is adjusted, it will results in a change in the "show focus/contrast" map. So if you are using it as a focus tool, you'll want the lighting (and exposure time) to remain fixed as you focus.