[Bf-cycles] Alpha pass independent from transparent option

[Bassam Kurdali]

Hi Brecht, then it's perfect,

- output files dictate the choice,

- renderlayers are always premul.

I had been initially scared you meant the renderdlayers themselves would
end up switching their alpha between key and premul based on the output
file choice!!

Suggestions for the renderlayer nodes: a convenience 'key alpha' check
box that does a convert to key alpha if checked, removing the need for
the extra node... benefits:

1- removes need for an extra node in some setups.
2- communicates to the user that the renderlayer is (unless you check
it) with premul alpha.


On Sat, 2012-12-08 at 20:47 +0100, Brecht Van Lommel 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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