On Wed, Jul 19, 2000 at 12:53:31AM +1000, David Hodson wrote: > > Can you justify that (all images should be pre-multiplied)? > > Or is this just your unsupported opinion? > > Well, that was attempted editorial humour to some extent, but it's also > the opinion of (for example) Jim Blinn, and Thomas Porter and Tom Duff. > I'd hardly call it unsupported. Ah, but as others have said, these are people working in a totally different area, and at least Tom Duff is most famous for a speed-up hack easily as ugly as pre-multiplied alpha (Duff's device). > > Gimp has no support for pre-multiplied alpha, > > Well, there's my answer. No support. ... and no need for it. With the exception of (IMHO useless) out-of-gamut RGB values, each is equally expressive, plug-ins and tools are free to convert to pre-mult if appropriate but the core uses ordinary RGBA. > A hack? I thought it was a mathematically elegant representation of > an image layer, which is why I see a reason to support it. I'm trying > to find out if anyone else agrees, or if I'm missing something that's > already there, or there's something specific about Gimp and the way > it's used that makes it unnecessary or not useful. Let's rather say "Not a priority" rather than "not useful", but I do not expect pre-mult alpha to be exposed to the user (as opposed to used in plug-ins or for speed-up hacks) in Gimp any time soon. > And even if you consider it a hack, don't people use pre-mult alpha? > Am I the first one to notice this and complain? Notice? I don't know. Complain, yes. You'll see why in a minute I think. > I have no love at all for the TIFF format. (I was present at the birth > of a similarly over-extended format. I should have complained more loudly.) > But that's irrelevant - or at least orthogonal - to the use of pre-mult > alpha. I'm not aware of any other common interchange format which supports the pre-multiplied alpha representation in storage. If we didn't have to load or save it, pre-mult would not be a problem for Gimp. > I've placed a page at: > > www.ozemail.com.au/~hodsond/alpha.html > > Images are just inline PNGs, just grab 'em as they appear; but it's not > really necessary, if you say that Gimp doesn't handle pre-mult alpha, > then that explains the results. <sigh> No the program which produced your example PNG image is broken. The PNG specification requires straight RGBA, pre-multiplied alpha is prohibited and this is spelled out several times. Gimp can't hope to interpret an invalid image correctly. Please identify the program (name, version, vendor etc.) and I will pass the details on to png-implement. Hopefully we can arrange for them to issue an urgent update to their users and if necessary get publicity so that everyone knows not to use this broken program. > Do users have problems with pre- and non-mult alpha? Since they are equivalent I'd guess users remain comfortably unaware. Nick.
