On Sun, Sep 17, 2000 at 10:53:40AM -0400, Garry R. Osgood wrote:
> but one can set the example by hand.

Probably some UI improvements needed in the gradient editor then,
other packages I've seen use a separate edit bar for alpha. I would
not advocate such a change to Gimp, but it is at least inspiration.

> This issue was given voice last July by David Hodson
> <[EMAIL PROTECTED]>. He observed that many image generators in the film
> and 3D animation industry, particularly 3D modeling tools, all
> premultiply alpha when making images. (See Alvy Ray Smith's "Image
> Composition Fundamentals",

Yes, it's faster. That's not always enough reason to use it, but
commitment to Duff's beliefs exceeds their value. Check that your
rendering package was written by engineers NOT zealots :)

e.g. Several packages have been happily spitting pre-multiplied
alpha into PNGs. The resulting files are garbage. Gimp won't load
them propely now or ever.

> Wilber is naive about premultiplied alpha. Feed him a premultiplied
> image and he just thinks it has some very nearly transparent pixels
> very near the black point.

Yes, do not feed Gimp pre-multiplied images. Heavens above, if you feed
Gimp CMYK or YCbCr you wouldn't expect anything useful out, so why
do people figure it will mysteriously "guess" that they've input
this mis-guided pre-multiplied format?

If you're writing software that interfaces with Gimp and uses
pre-multiplied alpha internally you need to convert the image data
whenever you move it into or out of Gimp.

The only really important case I'm aware of is TIFF, and that's
already handled, Garry are there are others you know of? TGA will
be handled better eventually, but it's very poorly defined (the
clarified version 2.0 actually contradicts itself at least once
on the subject of alpha)

> Future Gimps should also accommodate pre-multiplied images, and this
> is being
> included for the Gimp 2.0 discussed at GimpCon; it is not a feature
> that will easily retrofit to 1.x Gimps, unless some few people become
> tremendously brave (but not during the freeze, pleeze).

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...


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