Excuse my English !
It’s a good question. I work on global illumination and I tried some things about this.
I just want to calculate point-surface form-factor using OpenGL with this technique :
Set a Specific colour for the surface of interest
Set an other colour for the other surfaces
Draw an Image of the scene, the point of view is defined by the position of the receiver.
Get the frame buffer
Calculate the FF using elementary FF associated to each pixel, each time the pixel of the image as the specified colour.
You can also calculate the illuminance :
Set a unique colour for each radiosity patch
Draw several views with the camera located in a receiver sample point (to get a hemicube)
The elementary point-surface form-factor can be pre-calculated for each pixel
L(pixel) is given by the colour of the visible patch (=pointer to a radiosity mesh)
The illuminance can be computed with a simple summation : E=Sum (FF(pixel)xL(pixel))
The problem is :
You have to get the Frame buffer and make some ( a lot of) calculations with the processor.
PC’s cards are very slow because of the PCI or AGP port so you can just get 10 images (512x512) per second.
A SGI NT 320 or 540 is better because you get 75 images per second.