Cycles_path_guiding tests

[sherholz (Sebastian Herholz)]

@makizar
The last post pretty much brought it to the point.

Bidir Pathtracing for cycles?

No, GPU is definitely not the issue. The main problem, as Ace already said, is that Cycles allows you to do things that just don’t work with BiDir - such as e.g. ray visibility or changing your shader depending on the incoming direction. In general, for BiDir to work, everything has to be fully symmetric, tracing the same path from the other direction has to produce the same path - but if you have e.g. the path A<->B<->C, where A is diffuse, C is glossy and B is only visible to diffuse rays, tr…

IF Cycles ever plans to implement any bi-directional method, it will probably
come with the catch that many nice features (filter glossy, shadow rays, shaders depending on previous light path expressions (Glossy/Diffuse), …) artists use have to go away to make
it work.

@pixelgrip I know what the people who worked on that image had to do to make the combination of light tracing and path tracing work in their production renderer and this was not easy.