What is affine invariant used in computer vision?

The ratio of areas of $ABC$ and $ACD$ is the ratio in which the line $AC$ divides the segment $BD$ (and it is the ratio of the heights of $B$ and $D$ over $AC$ respectively). This later ratio is affine invariant as affine transformations preserve length ratios on any line. Do make sure that your points don't collapse onto a single line though.

• Next oc.optimization control - Is there an "adjacency matrix" for weighted directed graphs that captures the weights?
• Previous big picture - Applications of Math: Theory vs. Practice