[argyllcms] Re: Argyll 1.5.1: artifacts in dark shadows
- From: "alberto.ferrante@xxxxxxxxxx" <alberto.ferrante@xxxxxxxxxx>
- To: "argyllcms@xxxxxxxxxxxxx" <argyllcms@xxxxxxxxxxxxx>
- Date: Mon, 25 Mar 2013 12:31:31 +0100 (CET)
Dear Graeme,
Graeme Gill wrote:
> Argyll currently creates a
colorimetric B2A table with a
> colorimetric intent. This means that
any colors outside the gamut
> are clipped with minimum delta E -
that's what colorimetric means.
> (Actually, beyond a certain distance
they are clipped with a minimum
> CIECAM02 Delta Jab, although this
transition is what caused the reversal
> problem mentioned above). The
gamut surface of your device is quite
> flat in Jab space near black,
and so the clipped Jab 0,0,0 value doesn't
> quite correspond to RGB
0,0,0.
>
> When you simply link (say) an idealised RGB source to your
profile,
> the source black falls outside the destination gamut, so it
gets clipped
> by the cLUT approximation of the colorimetric (that is
minimum delta E)
> clipping. This may not be quite what you want in the
shadows, since
> delta hue & saturation is given equal weight to delta
luminance. ie.,
> you'd really like a perceptual clipping, but you're
using the colorimetric
> table. [ie. I would say that you were getting
good results previously
> using the colorimetric B2A table, more by
accident than design.]
Thanks for the explanation.
> So what I
suggest is: Don't use colorimetric if you want a perceptual
> mapping
between your source and destination. Use a gamut mapping that
> deals
properly with mapping white and black. You can do this either
> by
using "colprof -s source.icc", colprof -S source.icc",
> or by creating
a device link using "collink -G". You can control what
> intent the
perceptual table or device link is created with using the
> appropriate
intent flags.
This is not really clear to me: what is the source.icc
profile supposed to be?
If I understood well, I should use a monitor
profile. So, I should create a first ICC profile and then use it as
source.icc... Is that correct?
Best,
Alberto
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