Dissertation: Lit Review: Screen Space Radiosity

2.4 Screen Space Radiosity

Chapter 6.4 of ShaderX7, Fast Fake Global Illumination, describes several techniques used to simulate the effects of indirect lighting. Included in these techniques is the author’s implementation of Screen Space Ambient Occlusion and its logical expansion into a fake radiosity effect, i.e. Screen Space Radiosity (SSRad). SSRad simulates a local radiance transfer (a fake radiosity), by using the colour information for the scene in the SSAO calculations. This requires the scene’s colours to be rendered to a texture, in much the same way as the depth and normals. The SSAO term will require three channels instead of one, i.e. RGB, but the actual occlusion calculation remains the same. The occlusion factor is combined with the occluder colour values, resulting in a local radiosity effect. [Briney et al, 2010]      

A similar approach was taken by a member of the GameDev forums, who posted a technique that they dubbed SSGI (Screen Space Global Illumination). This technique also uses a colour buffer, and multiplies the RGB values of the occluder with the AO term. [ArKano22, 2009] However, this implementation also stores the single channel AO term and when outputting the final pixel value, uses the AO term and the colour bleeding term added together. This allows the grey shadows of ambient occlusion and the local radiosity effect to be combined.