Figure A1. Feature selection.

The change in the image intensity across different regions. The red square shows a receptive field at a flat region (left), at an edge (middle), and at a corner (right). For the flat region, a small change in the receptive field location doesn’t change the image intensity (shown in green arrows). At the edge, moving along the edge direction doesn’t change the overall image intensity, but except for that direction, the image intensity shifts especially along the direction perpendicular to the edge. At the corner, the overall image intensity changes in every direction.