Houdini 12.5 geometry light point clouds

[danw]

I'm trying to make use of the new point cloud feature for geometry lights in Houdini 12.5, but there's a few too many unexplained quirks and details for me to get my head around. Can anyone explain how it works behind the scenes?

Firstly, it seems that when I specify a geometry node (in this case a fog volume), it will generate a point cloud fine. Then I tell it not to auto-generate, and read the pre-baked pointcloud from disk. This works fine too, but if I then remove the reference to the original geometry, and use the pointcloud exclusively, it will emit light with a darker patch near the object itself. I would assume if point clouds were being used, it would skip normal geometry lighting entirely, but it appears to combine both together.

Secondly, I've been loading the point cloud back in, and looking at the contents. The attributes that appear to contribute are "A" "weight" and "volume". I'm guessing volume is a representation of the physical size of each point in the cloud. I'm not clear on exactly what effect A and weight have. I can't seem to work out in either case how the numbers correlate with the values in the volume they were generated from. There doesn't appear to be any attribute in the generated point cloud which actually holds information about the volume densities/values it was generated from - not even position, as they're arranged in regular axis-aligned grids, rather than having more points for higher density.

This lead to an interesting test - I was testing out how different volume values altered the values generated in the point cloud - I would presume more light would be emitted from higher values, but it actually appears to emit uniform light from all non-zero points in the volume.

I tested it with a volume which had a constant density of 1.0 in a cylinder around the y-axis. I then made the same cylinder with a linear falloff from 1.0 at the axis, to 0.0 at the edge of the cylinder, and it emitted identical light onto the plane I had placed just below it. If I make the value a constant 0.001 instead, it emits exactly the same again.

Is there a glitch in here?

I'm trying to get to the point where I can generate my own custom point cloud to light with, but I just can't work out how the point attributes relate to the lighting. "A" and "weight" both seem to have similar, but different effects.

Oh, not sure if it matters, but this was all done using "Physically Correct" light attenuation.

Edited March 24, 2013 by danw