Once those reflections maps calculated, remap them into the incandescence attribute of your material with a 3d projection (perspective type).
To avoid a washed out material/object, set the colorOffset of your projected texture to -0.1.

When you render the image, your reflected color maps are noisy (like it's been rendered). So let's apply a little blur to those image. Previously, I was using the filterOffset of the tetxure file to blur the image, but it sucks!
Another way of blurring the image, is to use the uvFilterSize (thanks to Duncan for that tip in one of his posts on Highend).
The uvFilterSize calculates how blurry the texture is regarding the distance from the object to the camera.
So, select your texture files, then create a ramp without its 2d placement. Kill the outUVFilterSize connection of the file, and connect the outAlpha of the ramp into the uvFilterSizeX and uvFilterSizeY. Then delete 2 colors of the ramp, set the remaining to 0 and render an IPR session.
Apply a different uvFilterSize per material to have a smooth reflected color map.

For one shader, the hyperShade would look like this.



Now, the higher value the ramp's color is, the blurrier the texture will be.

The time to get a coffee while rendering (long because of the ray-traced shadows), and it should looks great (At least I hope so)



Limitations regarding this method:

- The smaller an object is the more difficult it'll be to see a reflected color on it (because applying a blur to the texture file will mix the colors from a normal to another (see the cube has some green feed_back to its normal 'facing' the camera).



And the final Cornell box image.



And this is the previous image I had made to illustrate the (first) version of this tutorial. You can see the difference...



Once again, I warn people that it's only a guideline for faking radiosity. This method will work in 'most of the cases', but you may need to think about the thing depending of the shot you have to fake, the objects, etc...

Anyway, I hope this tutorial will help you and will give you ideas to enhance your lighting / rendering

Faking radiosity - January 2001 - Emmanuel Campin at pixho.com