Actually, I see it, now that the script is done and the tests can be performed
without bruising my knuckles. It isn't as bad as the Color mode, but it's
there. Decomposing and recomposing an unaltered layer copy of an image changes
the colours a little. Having two layers, top blurred, and bringing into it the
L component of the lower layer, accentuates noise somewhat, not as badly as the
Color mode. Iterating this procedure a number of times on the blurred layer,
however, soon breaks down the image, with severe colour artifacts.
Good news: the YCbCr_ITU_R709 decomposition works much better. Iteration tests
brought no breakdown. The test on an unaltered layer copy of original also
doesn't alter colours.
A commenter in the original bug report about the Color mode did say the R709
worked well in his tests.
----- Original Message ----
> From: Rupert Weber <g...@leguanease.org>
> To: gimp-developer@lists.XCF.Berkeley.EDU
> Sent: Mon, August 9, 2010 4:45:00 PM
> Subject: Re: [Gimp-developer] Getting new layer modes fit for inclusion
> On 08/09/2010 04:16 PM, Charlie De wrote:
> >> From Rupert:
> >> (as far as I can tell, the current Decompose Lab functions are
> >> broken, anyway).
> > What makes you say that? And how severe is the problem?
> > Right this minute I'm writing a script with LAB Decompose...
> Well, 'different' might have been nicer than 'broken' -
> The conversion doesn't account for gamma:
> As a result, the L* channel is very light (check the histogram). I have
> no clue how that influences a* and b*.
> Also, it uses the conversion matrix for PAL/SECAM RGB. (Maybe PAL/SECAM
> has gamma 1.0? Then the missing gamma correction would be correct.)
> Anyway, unless your image is in PAL/SECAM RGB, the conversion in
> Decompose is incorrect.
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