Add Light Tracing

Gys, there is a setting in LuxCore that is named Add Light tracing, it TOTALLY makes beautiful caustics and works with Sobol.

Is there a way to integrate that into Cycles or Eevee with RTX ?

Hmm, seems there is very little documentation on that option and I couldn’t figure out otherwise quickly what precisely it does.

Apparently it is a step toward a bidrectional pathtracer (but not being a full one). So I’d rule out Eevee which follows a very different concept. Cycles, yes, depending on what you mean by integration. It is not something you can simply put in an addon or so. It would be a component of the tracing engine, meaning the developers would have to write code for that.

Occasionally discussions appear whether Cycles should get bidir tracing, MLT, VCM or such, but there seems to be no serious effort in that direction currently.

This feature was relatively easy to implement in LuxCore, because it already had a bidirectional engine and all materials had the code for rendering by tracing from lights instead of the camera.
However, Cycles does not have this infrastructure in place.

It performs light tracing on the CPU while doing regular pathtracing on the GPU(s). The results are added.
The light tracing is done on the CPU because a) the code already existed (for OpenCL it does not) and b) it is good enough, the CPU can get a light traced caustic clean just as quickly as the GPU renders the rest.

Here you can see a test animation rendered with 40 sec rendertime per frame, specs in the description:


Thanks for the clarification. Does that mean it is a full bidir tracer or imited in some way? Bidir but without MLT or so?

It is Bidir without MIS (multiple importance sampling), which makes it slightly more noisy at the same sample count, but also a lot faster.
You can read more about it here:

1 Like

Thanks, that is a great thread. I always wanted to read up on this kind of internals of Lux (which aren’t in the official documentation).

Whoever hat that Bidir CPU Caustics/PT GPU combo idea is a freaking genius!

I for one welcome our Luxcore overlords!