[shuhao]

Apply an edge finding (both vertical and horizontal) algorithm. Try to scale your image down first (< 100x100) and take the grey scale for optimization.

Try the simplistic kernel of

1 -1

for horizontal edge and

1
-1

for vertical edge,

I assume that your "rectangle" will look like

  / \
 /   \
/     \

as it seems to be on the ground, so go through your filtered image and try to recognize those pattern with something like a machine learning algorithm

Either way, it's pretty tough...