who can tell me what the different about the PRT and the Irradiance Volume? i feel they are very similar. thanks!

In a nutshell...

Yes PRT and Irradiance Volumes are very similar, both are interested in modelling the effect of light and geometry on a given number of points within a scene, and describing those per-sample points using a compact method.

Some differences however :

1. PRT is mainly interested in points that lie on an objects surface, whereas irradiance volumes are interested in points in space (this is not always the case, for example modelling caustics).

2. Irradiance Volumes will include static light sources in the per-sample calculation, PRT is interested solely in HOW the light is transferred at a sample point.

3. Often PRT only considers the upper hemisphere above a sample point (this is not the case when SSS is considered), while irradiance volumes consider the full sphere of visibility.


You can basically look at these concepts as two parts of the same equation :

PRT = Surface Response to light
Irradiance Volume = Samples of Incident Lighting

So you sample an irradiance volume to give you an incident lighting function, which can be combined with a PRT signal to produce runtime radiance values.