Federico Mena Quintero wrote:
> And yes, the blend tool should do the right thing with alpha values,
> i.e. premultiply them before compositing them in. I'll submit a patch
> for it in the afternoon.
>
<rant_and_rave>
I don't think this episode uncovered an alpha-related bug, so what needs to be fixed?
Mr. Turner's example (unwittingly, perhaps) specified two different gradients (in the
unmultiplied
color space). And two different results were obtained. What went wrong?
*Should* the gradient tool work in the premultiplied space, while GIMP mostly functions
in the unmultiplied one? Won't this give users different alpha characteristics in
different places?
Won't they be better served with a consistent alpha channel behavior of the
unmultiplied
space in *most* places? Especially since - for better or worse - the rendering
pipeline functions
with unmultiplied alpha only? The gaussean blur HAS to pull premultiplied tricks. No
surprises
there either, for just as Mr. Lamb observed with premultiplied alpha, the unmultiplied
space is
no cure for the blues either. They happen to be two (mutually exclusive) conventions
that can
be applied to the alpha channel, and one has a Real Slick compositing algorithm
associated with
it, blessed by the Computer Graphics Gurus.
In light of this, then, I think we should be very careful in preserving a consistent
behavior when
it come to how the alpha channel behaves, and warn users when inconsistencies
(invariably)
arise.
</rant_and_rave>
Be good, be well.
Garry