Finally some dynamic indirect lighting in the renderer!
(this uses a single ambient light source (an environment map) only)
It's fairly lo-res, but it's good for things like "walking through a dark cave" or "a giant spaceship hanging overhead".
On the cpu side, I look for a diffuse occlusion term by 'ray tracing' (they're pre-rasterized bit masks I fetch from a LUT) a simplified sphere-representation of the scene into a tetrahedral grid of cube maps / bit fields.
During rendering, I then construct the occlusion term, for a specific vertex, based on its position and orientation in the grid and use it to interpolate between an indirect term grabbed in screen space and whatever light comes directly from the ambient light source itself.