[Bf-cycles] Alpha pass independent from transparent option

Daniel Salazar - 3Developer.com zanqdo at gmail.com
Sat Dec 8 23:06:27 CET 2012

Alpha is not "intuitive"! I thought the same thing once but I was wrong. -1
on forcing an alpha pass that's incorrect. Link to my old bug report


I'd recommend reading a good book on compositing like


We need to adapt our compositing tricks to the more modern ways of cycles
and not the other way around :) So far I've managed to do everything I've
needed (minus group lighting but that will come)

don't get offended by this, just trying to help

Daniel Salazar

On Sat, Dec 8, 2012 at 1:47 PM, Brecht Van Lommel <
brechtvanlommel at pandora.be> wrote:

> Hi,
> > So I would make the following suggestion to avoid the ambiguity:
> >
> > 1- do as you said and switch key/premul based on output format (with a a
> > choice if the format allows both) in the output file settings.
> Yes, probably there would be 3 choices: automatic, premul, key. I'm
> not sure about the design yet, it's probably not feasible to make the
> compositor keep track automatically of what the user did. I'm thinking
> there would be some settings that says if the compositor outputs
> premul or key, and based on that it could do automatic conversion to
> the file format then.
> So there would actually be 2 settings, but I would hope that in nearly
> all cases the compositor output type can be set to premul and file
> format to automatic. This should work for the cases where you use
> typical intermediate formats like OpenEXR and TIFF, and do final
> output either without alpha or with alpha in a web format like PNG.
> > 2- allow switching key/premul in renderlayers regardless of output
> > format , and make them a sane and consistent default (regardless of
> > output format chosen, maybe something suitable for openexr).
> Not sure what you mean by this, where this switching would happen. But
> basically anything that comes out of a render engine is naturally
> premul. OpenEXR is always premul, and TIFF has metadata to indicate
> which alpha type was used. For intermediate formats premul seems the
> best option.
> Layers can be converted to key after rendering but there's no
> information to be gained there, and it would give some information
> loss for surfaces that are both emissive and transparent (though
> that's no so common, maybe for fire volumetrics or so). I think it
> would be good to make compositing nodes work by default with premul as
> a convention, so blur type nodes would have to do no conversion, and
> color correction nodes would optionally convert to key and back.
> Brecht.
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