First, let me say thanks for releasing this as open source, it's
looking very impressive. I'm working on the shading system in Blender
and was looking into how this design compares to what I'm planning and
how I could make my implementation compatible such that OSL could be
plugged in at a later time.
I have a question that I couldn't find an answer to in the
documentation or source code. The BSDFClosure has eval_reflect,
eval_transmit and sample functions. Some rendering engines have an
additional function to compute the average reflected or transmitted
color independent of the incoming light direction, by the integrating
the bsdf over the incoming light direction without any lights. In PBRT
for example this function is called rho.
There are a number of uses for this:
* Compute surface albedo, i.e. "unshaded color", to output as a pass
* Compute surface opacity (by doing this for transmission), useful as
a pass for compositing, rasterization with alpha or transparent
* Importance sampling by picking one of two blended bsdf's
proportional to this.
It would be possible to compute this value by doing the integration
each time, calling the sample function a number of times, but this is
not as efficient as it could be. Or these values could be manually
provided by the shader writer. Maybe there is another way to do this
that I'm missing, or perhaps it was left out intentionally, or not
implemented yet, ..?