- get_displacement
- foreground extraction <http://opensource.graphics/how-to-code-a-nice-user-guided-foreground-extraction-algorithm-addendum/> <http://opensource.graphics/how-to-code-a-nice-user-guided-foreground-extraction-algorithm/>
- normalize
- watershed
- distance

later:
- draw_mandelbrot
- draw_gaussian
- draw_quiver
