- From: Christian Brunschen <cb@df.lth.se>
- Date: Thu, 3 Jun 1999 12:36:31 +0200 (CEST)
- To: Raph Levien <raph@acm.org>
- cc: www-svg@w3.org
On Wed, 2 Jun 1999, Raph Levien wrote: > Peter Graffagnino <pgraff@apple.com> wrote: > > These are good comments. The general rule is that for things that > > manipulate color independent of coverage, premultiplication is not > > convenient. But for things that manipulate the image as a whole, > > premultiplication is more natural. For example, I disagree with the > > analysis of the GaussianBlur case. To correctly evaluate a > [...] > > Indeed, all such convolution operations in the Gimp are implemented by > going to premultiplied space, doing the operation, and going back to > separate-alpha space (which is the interchange format). In earlier > versions, this was not done, and we had lots of nice artifacts when > gaussian-blurring partially transparent white layers. > > I would disagree that, in general, premultiplied alpha is a > significant loss of precision. Yes, when converting back to separate > alpha in the small-alpha case, roundoff errors get magnified. But when > you take that pixel and composite it over another pixel, you only get > an LSB or so of roundoff errors. For Web work, that should be > completely satisfactory. But the magnified roundoff error might be pushed through into a ColorMatrix filter rather than being immediately composited onto something else; and in that case, the magnified roundoff error can result in, well, truly weird results beyond the rightful expectations of the user. > Now, if we're talking about high-quality print applications, then 8 > bits is simply not adequate. Going to 16 bits, premultiplied or no, > is the only way to go. My take on the draft is that it's not being > targeted for high-quality printing. Too bad; SVG has a lot of stuff in > it that would be great for that application. Indeed. Perhaps including a parameter that would allow the user to select from a few bit-depths (8 bpp, 16 bpp, 32bpp?) to be used in the filter effects stage? > Maybe SVG2... > > Raph // Christian Brunschen
Received on Thursday, 3 June 1999 06:36:37 UTC