Physically Based Shading in 3Delight 11

December 4th, 2013 . 2 comments

Although the 3Delight/blender addon is mostly abandoned due to lack of time to keep it maintained, I want to at least bring it up to date with the latest 3Delight release, which has had a lot of good updates in the pathtracing/physically based rendering department.

3Delight’s approach to the problem has been to extend some of the commonly used shadeops, seemingly with the intention of making it simple to convert over old shaders, or create simple shaders from scratch. It definitely has advantages in terms of the amount of work required to get something set up, but imo it’s also a bit messy and confusing how it all fits together, especially if you’re familiar with a more common and organised physically based shading infrastructure as in pbrt.

I’d actually already made my own physically based shading setup about a year ago, fully implemented in RSL and based on pbrt (with path tracing, several types of lights and materials, multiple importance sampling, importance sampling of environment maps, …), the main drawback being that it was relatively slow. I’m hoping that the new features in v11 can help here.

So far, with the help of Paolo Berto, I think I’ve got a few things figured out. The main shadeops to be concerned with are:

bsdf()

This is similar to f() in pbrt, it evaluates the value of a bsdf along a given direction.

trace()

Using the similar “distribution” parameters as in bsdf(), this is similar to sample_f() in pbrt, sampling the bsdf to generate outgoing vectors. The big difference here though is that trace() does much more than just sampling the bsdf, it also traces rays, performs multiple importance sampling, and returns pre-integrated results in the form of an RGB colour.

trace() is concerned with incoming light from scene geometry, including physical lights that can be intersected, that can generate a valid pdf and therefore can be used with MIS. This means that lights with delta distributions (eg. point lights, distant lights) are ignored and have to be handled separately. But for area lights and an implicit environment light, trace() can work well.

To take advantage of this, you need to define ‘proper’ area lights in the RIB, using AreaLightSource and giving them some geometry. Then when using trace()’s new “samplearealights: 1″ parameter, trace() will presumably then a) sample the light sources explicitly, generating ray directions and pdfs and also b) consider those area light shapes for intersection when sampling the bsdf and generating rays from that.

At the moment I assume that it does something similar for the environment, if you assign an environment map in the trace() call – that it will (importance?) sample the environment hemisphere and perform MIS with that too.

So what about other lights than area lights? Well, other lighting effects that are generated in shader code don’t give enough information to the renderer to be effectively sampled explicitly in the trace() framework, and neither can lights representing a delta distribution. You can still sample them yourself though, just with an oldschool light loop – i.e. for each light, retrieve Cl and L from the light, multiply by the bsdf() value, compute visibility, and accumulate with your existing colour from trace().

Next: Putting it all together …

§ 2 Responses to Physically Based Shading in 3Delight 11"

  • yah says:

    3Delight and Blender is a nice combination…but abandoned. Oh well.

  • Sawe says:

    I’ve found that the plug-in does what it is supposed to and provides access to the great 3delight renderer. I think there should be some individual artists and compositors out there who only need to render out some 3D-elememts at very high-quality. When you need displacement at pixel level and high quality anti-aliasing it really works. It also provides access to the shading language (sl) that I’ve been able to use after a lot of experimenting
    I wish this plug-in would be developed more because it has potential. As a feature request I wish imager shaders were supported. They can be used for compositing 3D into plates and for adding fast blur to renderpasses. Some control over outputting several aovs would also be great. If there is no time, thanks anyway. Maybe someone else can pick this up otherwise.

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