# Form factor for rectangular shaped patch? [for radiosity]

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Working on a simple radiosity demo, and im currently approximating all of my patches with disks, using the form factor:
A * cos (Theta_E) * cos (Theta_R)
---------------------------------
PI * R^2 + A

(from nvidia paper) It works pretty well, except my patches are acually quads (not disks) so I get disk shaped artifacts when two surfaces are very close to each other. I am aware of the general point-polygon form factor, but it seems to give very strange results (not to mention is quite a bit more complex than the disk-disk) Does anyone know of a quick closed form form factor between two quads?

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I feel bad - I knew this answer a few days ago but I didn't have time to post and I figured someone else would :(

Anyway, read the paper "Modelling the interaction of Light Between Diffuse Surfaces", that is the original paper on radiosity and they assume quad patches. If you need help understanding the formula I can go into more details - if you can't dig up the paper email me (my first name followed by my company, readyatdawn) and I'll forward you the paper.

Good luck!

-= Dave