i_stroke_set data type in preparation for stroke computation.
- Added convolution of difference images to extract strokes better (kernel has positive values for pixels within the stroke radius, negative values for those between 1x and 2x of the radius away from the center, and 0 elsewhere). Kernel is normalized such that values add up to 1.
- Result is somewhat promising (insides of solid areas (e.g. people) are removed, but borders remain).
- Attempts to use local non-maxima suppression don't behave quite right since we can't just scan a N x N neighborhood to remove non-maximum pixels. Ideally we want to know in which directions to scan (normal to the stroke), but we don't have normals here. Time to convolve with X and Y derivatives?