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A thread about a tiny bit of tech in Soul (which I hope y'all are enjoying today!)
The "astral plane" world operates by different rules, obviously. For one thing, everything is translucent, which meant many, many things were volumetric. That was fun. Especially the Hall of Everything.
However, the lighting in the astral plane also bent some physical rules. Watch the colors around the characters in particular.
The color fringing reacts to the lighting in the scene, which helps shapes the characters. Normally this would be the job of the shadows (and at Pixar, we're all about the shadows, normally), but astral world plays totally different ball.
Mitch Kopelman and I figured out from the art direction that some of what was needed for this look was to remap the volumetric transmittance from light attenuation into albedo hue shifts and saturation boosts instead.
Basically, instead of self shadowing themselves and getting darker away from the light, the characters instead *filter* light and increasingly aberrate the color response.
I bolted this into the renderer (a physically based path tracer which shades before lighting, so.. things got a little weird), and Junyi Ling and his team used this trick (and lots of other tech, see SIGGRAPH 2020 talks) to make the Soul characters look amazingly otherworldly.
Interesting bit is that this trick would be trivial to express in some shading languages, gets much harder to deal with in a physically based ray tracer, but now we're thinking a lot about how to do non-physical things in PBR in an effort to push new looks.
It's like a new wheel of reincarnation playing out, from 100% NPR to 100% PBR and now slowly back again. This non-physically based lighting is a small trick that's easy to bolt into a PBR renderer, and you'll see more in the future, but it's all very interesting to say the least.
Anyways, that's it, enjoy the movie!
