Nick Lamb wrote:

> Pre-multiplying is a performance hack only, please don't let people
> think of it as something that will cure "black fringes" -- it won't.
> Perhaps that wasn't your intention, but in any case...

Gimp should do premultiplied alpha. Not because it cures acne or
extends human life expectancy by an astounding 3000 precent, but
because people in the professional paint box community use images
formatted to that convention on a regular basis, and Gimp could
be helpful to that crowd if  the dear little creature would
only eat pre-multiplied alpha and composite with it.

That, of course, gives rise to a rub. I don't believe this can
happen transparently to the user, even if the Gimp had premultiplied
capability. I believe you've commented on this in past premultiplied
alpha go-rounds that it is touch-and-go whether a file's meta data
hints about premultiplied alpha, even if the metadata chunk has
a slot for an alpha premultiplied flag. And premultiplied pixels are
otherwise indistinguishable from unmultiplied ones. So it is doubtful
to me that any paint program any time soon can automagically determine
which is which, so a human (or a self-aware equivalent) would have to
do the determination for it. That means (a) education (including the
distinction among types of alpha in standard Gimp documentation) (b)
a clear statement that Gimp is an unmultiplied compositor (though
certain tools and plug-ins necessarily have to internally play the
premultiplied game - blur comes to mind) and (c) If you care about
what you do, figure out how your image sources are doing their alpha.
Perhaps a later Gimp will know how to consume premultiplied alpha
(you'll have to throw some switches) the 1.x Gimps of the next
two years or so won't know how to do it.

Be good, be well


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