Comment by Animats

If you run a real radiosity renderer, you get proper darkening at corners. Here's a Blender rendering with a good radiosity renderer. Note that some inside corners have darkening, and some don't, depending on how they're lighted.[1][2]

Full radiosity renderers are slow. The image is rendered over and over again, each pass using the previous result, with all surfaces now being light sources. Each pass thus shows the result of light being reflected one more time. The corner-darkening effect comes from indirect lighting from the opposite face, an effect which gets smaller as you approach the corner. With some renderers, you can watch the passes, as the image changes from starkly lit to very realistic. Render farms for movie production spend most of their time doing just this.

Game hardware cannot yet do true radiosity in real time, so there are many ways to fake it. Some better than others.

[1] http://s10.photobucket.com/user/RCRuiz/media/3dTest/Illumina... [2] http://wiki.blender.org/index.php/Doc:2.4/Manual/Lighting/Ra...