Dynamic Update of Radiosity Simulations
[Work Package 4]

Increasing Image Quality for Animations using Final Gathering

In images generated from radiosity solutions, shadow borders often cannot not be reproduced good enough when high-quality images are seeked for still images od animation sequences. Often, the patch structure still can be recognized, even if radiosity values are interpolated across patches using gouraud shading.

Using a very fine patch grid with patches in sub-pixel size would help, however, besides prohibitive costs a problem arises since viewing parameters and therefore the final size of patches on the screen are not known a priori.

Especially sharp shadow edges cause visible artefacts. A solution to this problem can be a final gathering step, in which illumination for each pixel is calculated during rendering. This of course is expensive in terms of calculation time, but can provide very good results, because it combines the diffuse interreflections captured by the radiosity solution with exact direct illumination. The radiosity solution can be calculated on a relatively coarse mesh, since direct illumination is gathered lateron on a per-pixel basis anyway, saving radiosity calculation time.

The basic idea of this process is to

These steps are depicted in the following figures.

Pure radiosity solution

Coarse radiosity grid

Radiosity solution with direct illumination removed

Direct illumination added by final gathering

Depending on the quality required and on the characteristics of the scene the final gathering might restrict to only gathering from emitters, or it might gather the illumination on each pixel from the complete scene - which requires considerably more time. The following is an animation sequence where only direct illumination due to light sources as been re-gathered every frame.

Animation sequence with direct illumination from light sources updated by a final gathering step for each frame.

Animation sequence with direct illumination from light sources updated by a final gathering step for each frame.

We have implemented these steps in our system, and now can combine it with the update mechanism for radiosity solutions.

This way, the final gathering method can be applied very well when generating animation sequences: The (coarse) radiosity solution is updated quickly by some radiosity update method, like, e.g., the line-space mechanism, which accounts for the indirect effects to be corrected, and the direct illumination is calculated by a final gathering step, ensuring correct illumination on a per-pixel basis for each frame.

Final gathering of course is not restricted to lambertian emitters, but can also take into consideration emitters with a non-diffuse emittance characteristics.

Final gathering in a scene with non-diffuse emitter