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> One thing people (myself included!) often missed though is that Metropolis is extremely prone to flicker.

Doing PCA across frames basically eliminates that. IIRC Pixar did a paper on the technique.

Actually, the Renderman denoiser is based on work from Disney Research.

PCA would be one way to do that. This recent paper suggests that they may be decomposing the samples mostly by path type, though:

He's referring to the older 2008 paper that did PCA, Statistical Acceleration for Animated Global Illumination: http://graphics.pixar.com/library/ShotRendering/paper.pdf