[Bf-cycles] Alpha pass independent from transparent option

[Daniel Salazar - 3Developer.com]

Bassam: that sounds nice, what about image input nodes? convert all to
premul based on image type?

Daniel Salazar
patazstudio.com


On Sat, Dec 8, 2012 at 6:52 PM, Bassam Kurdali <bassam at urchn.org> wrote:

> 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.
> > _______________________________________________
> > Bf-cycles mailing list
> > Bf-cycles at blender.org
> > http://lists.blender.org/mailman/listinfo/bf-cycles
>
>
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>
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