Because of how we track and match images in order to obtain data, and because of the fundamentals of strain theory, we must treat the area of interest as a continuous surface.  Discontinuous surfaces can result in unreliable data and erroneous strains.

Tracking and Matching Data

We apply a random speckle pattern to track our Area of Interest (this is the region of the specimen that you wish to obtain data).  We do not track the individual speckles, rather we track groups of pixels that we call Subsets.  The Subset size is user defined, but in this case let's consider a subset size of 21.  If we have a subset size of 21 pixels, this means that we are tracking squares of 21x21 pixels throughout the surface of the specimen.  In order to track and match these subset points, the subsets themselves must remain continuous.  If the subsets break apart or have discontinuous behavior, then we cannot track them.  Typically subsets that break apart (for example, when the surface cracks) will be dropped from the data, however what data is included in the contour plots is partially determined by factors like subset size and thresholding (which you can change in the Run menu).  What is even more problematic is that sometimes it looks like the subset was dropped near a crack, but some of that false data inside the crack is included in the nearest subset.  We report data in the center of the subset, so if the subset is 21x21, the edge of the contour plot will be a subset that includes 10 pixels outside of where the plot is drawn.  So some of the data that looks like it is outside of the crack, could potentially be including the crack, which would result in unreliable data.  The data within that crack is unreliable because strain theory assumes a continuous surface.

Strain Theory

Strain is a way to quantify how a continuous body deforms.  It is, very simply, the percent change in elongation and it is a measure of how ductile or rigid a material is (however, there are different types of strain tensors available in the software).  If you have a discontinuous surface, like a hole or a crack, if we correlate over that void or crack, then we are essentially putting strain gauges over the crack.  So as a material separates, it can show a huge "strain," but the material has broken so that is not strain, rather than just simply displacements of crack openings. When cracks or discontinuities in the surface occur, our software is designed to typically drop those points.  Again, we can control how much data we choose include with subset size and thresholding.  But we must be careful when looking at strains where there are discontinuous surfaces, because those could be erroneous due to the discontinuous material behavior.

Special Cases: Composites and Textiles

Some materials with a micro-structure, such as composites and textiles, can behave as a continuous material on a macrolevel but then have fibers slipping past each other and discontinuities on a smaller scale.  This is something to note because we could potentially see a shear strain due to materials slipping past each other, when it's, in fact, just slippage and not strain.  On a more macro level, however, the material is deforming as a continuous surface.  It's important to have an idea of how the material is deforming on both a micro and macro scale when looking at fibrous materials.

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.