Eventually the Surface trait will be changed to actually mean the
ability to be processed _into_ a MicropolyBatch. So it's ultimately
nonsensical for MicropolyBatch to implement it.
This uses a normalized version of blackbody radiation, so the
colors still vary but the brightness doesn't vary nearly as
wildly as with genuine blackbody radiation.
The sampling method used before is numerically unstable for very
small lights. That sampling method is still used for large/close
lights, since it works very well for that. But for small/distant
lights a simpler and numerically stable method is used.
It *seemed* to fix the problem I was running into, but it actually
made the SphereLight ray intersection code incorrect, and wa just
avoiding intersections that should have happened.
I should test better before committing. :-)
Thanks to a discovery by Petra Gospodnetic during her GSOC
project, I was able to substantially improve light tree sampling
for lambert surfaces. As part of this, the part of the surface
closure API relevant to light tree sampling has been adjusted to
be more flexible.
These improvements do not yet affect GTR surface light tree
sampling.
More specifically: prior to this, SurfaceLights returned the
shadow ray direction vector to use. That was fine, but it
kept the responsibility of generating proper offsets (to account
for floating point error) inside the lights.
Now the SurfaceLights return the world-space point on the light
to sample, along with its surface normal and error magnitude.
This allows the robust shadow ray generation code to be in one
place inside the renderer code.
Reorganized light and surface traits so that light sources are
surfaces as well, which will let them slide easily into
intersection tests with the rest of the scene geometry.
It's not used right now, but in the future I want shaders to be
able to vary over time and have motion blur. This serves as a
nice little reminder by putting it in the API.
The main change is that SurfaceClosures now have the hero
wavelength baked into them. Since surface closures come from
surface intersections, and intersections are always specific to
a ray or path, and rays/paths have a fixed wavelength, it doesn't
make sense for the surface closure to constantly be converting
from a more general color representation to spectral samples
whenever its used.
This is also nice because it keeps surface closures removed from
any particular representation of color. All color space handling
etc. can be kept inside the shaders.
