[PD] Finding a nearly full quadrilateral in a video frame

[Jean-Marie Adrien]

Hello list
I have to find for an installation the four corners of a bed (a rectangle) seen from above against the background (the floor), and this bed might slide a bit, so from time to time update the coordinates of the four corners. A person might be on the bed or close to it. So the general problem is

		find the coordinates x,y in the video frame (GEM) of a nearly perfect quadrilateral  with a rather simple overall visual configuration  

Apparently, haarcascade techniques  which work fantastically for face / hands (A.Villeret) seem inappropriate because the rectangle or quadrilateral is a too simple shape : 
anyone has a hint ? any object around ? blob ? boundaries ??

Thanks a lot !

Jean-Marie