sigma_a / sigma_s Check Failing with RGBDensity uniformgrid Mediums

[shadeops]

Howdy,

I was playing with the uniformgrid mediums using rgb densities and have been occasionally running into a failure during the render. I was able to reproduce the issue with a simple scene.

There error is:
pbrt-v4/src/pbrt/cpu/integrators.cpp:1025
FATAL Check failed: 1 - pAbsorb - pScatter >= -1e-6 with 1 - pAbsorb - pScatter = -0.0011489391, -1e-6 = -0.000001

The example I made is a bit contrived as it has significant increases in density in neighboring voxels of the maxDensityGrid. Its also worth nothing I've only run into this with the RGBDensity, I haven't encountered this with FloatDensity.

Here is an example scene -
max_density.zip
It errors out almost immediately, but here is the debug start command -

Rendering failed at pixel (734, 205) sample 0. Debug with "--debugstart 734,205,0"

Rendered on the CPU with https://github.com/mmp/pbrt-v4/tree/08f863d3786

[mmp]

Good one--thanks for reporting it!

Fixed now (and the code is simpler, too...)