[Bf-cycles] Future of the non-progressive renderer and Cycles strand rendering

Matthew Heimlich matt.heimlich at gmail.com
Fri Jan 4 13:34:04 CET 2013

Brecht (and others),

Just wondering what the status is on non-progressive rendering in
Cycles, and if there are any plans to optimize in the future. In other
path tracers, most notably Arnold, there are huge gains to be had by
decentralizing diffuse, AA, glossy, etc. samples. I know it's an
option right now, but it's currently slower in every case I've thrown
at it.

Also I wanted to know if there are any plans to implement a dedicated
hair shader alongside the upcoming commit for Cycles strand rendering.
The Kajiya-Kay method would be good. The Marschner method would be
even better (http://www.cs.cornell.edu/~srm/publications/SG03-hair.pdf).

I posted a couple of features from Arnold in the Cycles hair thread
over on Blender Artist that would benefit Cycles rendering greatly if
implemented. I'll repost them here for the mailing list as well.

First is a minimum pixel width value. Instead of relying on AA passes
to clear up hairs, which can take hundreds of passes for thin
furs/hairs, you set a minimum pixel value. As long as hairs are larger
than this value, nothing special happens. But as soon as the camera
zooms out or something else changes automatic alpha transparency kicks
in along the length of the hair to automatically blend the tips. In
practice, this more than cuts render times in half. Transparency
values are linked to auto-transparency, which would be the equivalent
of max transparency in Cycles.

Second, the diffuse cache. This is a toggleable option in the
specialized hair shader that bakes indirect illumination values into
the hair control points, which are then the only points sampled during
rendering rather than sampling the entire length of the hair geometry.
They are then interpolated between. In scenes with more than one
diffuse bounce, this again results in render times being cut by nearly

I'd like to hear developer thoughts on these topics, as hair is a huge
boon to Cycles and I think we can all agree that it should be as fast
and feature full as we can possibly make it.

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