Visualizing individual light rays

[h-OUS-e]

Not really an issue but just a question, what is the best way to visualize individual light rays coming out of the emitter, being refracted, and hitting a camera?

thanks!

[njroussel]

You can write a custom integrator which only samples lights paths according to your pre-defined behaviour. That would allow you to get a rendering where you only see the contribution in the final image of those light paths.

If you want to get individual light paths (handful), then you can simulate them "by-hand" by scripting the integrator for those few rays which would allow you to effectively save the bounces to some data structure which you can later use in your visualization.

[h-OUS-e]

Thank you! And following up with a couple of questions.

1. Is there any reference on how to write your own integrator plugin? Or an example of a custom integrator written in python instead of C+ to be able to write one from there?

2. Is the particle tracer (ptracer) the only integrator where light rays are emitted from the source rather than the camera?

3. How would I script the individual light rays in the integrator? don't these rays come out of the emitter?

4. What would be the easiest way to calculate the number of rays coming out of the emitter, and the number of those rays that have made it to some surface.

[njroussel]

1. We have a few. Here's a simple one: https://github.com/mitsuba-renderer/mitsuba3/blob/master/src/python/python/ad/integrators/direct_reparam.py. If you're not interested in differentiation you can inherit of mi.SamplingIntegrator. You only need to implement the sample() method.

2. Yes.

3. In this tutorial we essentially implement an Integrator and some of the boilerplate around it without having to deal with the whole mi.SamplingIntegrator interface. This is what I meant, by scripting something specifically for the behaviour you want.

4. Keep a counter per shape. You can get the shape information inside of si.shape where si is a mi.SurfaceInteraction3f which you'll typically get after a ray intersection.

[h-OUS-e]

That is super helpful thank you!