[argyllcms] Re: Perceptual intent in Display profiles?

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Fri, 20 Nov 2009 10:51:17 +1100

Michel Joly de Lotbinière wrote:
But the remaining problem is this: shadow detail was removed from
images when viewed in a color managed application with the video LUT
loaded, while it was present when viewed in a non-managed viewer with
the video LUT loaded.

So, was the dispcal ambient light -a option the right correction to
use to solve the problem of disappearing color-managed shadow detail?

Probably not. The problem described in what you quoted is because
the source profile and destination profiles don't have matching
black points, and the process being used doesn't map one to the other.
[Another trap is to use the V2 sRGB profiles from the ICC website -
 they are broken, don't use them. Use the sRGB profile supplied
 with Argyll V1.1.0 RC1 instead.]

To fix this you either need to use BPC (Black Point Compensation)
if it is available in your CMM and will work with the gamut mapping
mode you are using, or if you are using Argyll to create your
CLUT destination profile, make sure you give it the source
profile as its source gamut, and when using the profile use an
intent that does gamut mapping (such as perceptual or saturation).

How is it possible to get those other intents into the generated
profile so I can try out the gamut mapping method? At the moment, the
LUT profile just has the "relative colorimetric" tables in it.

Supply the source profile as the argument to the -s or -S flags.

What options do I give colprof to generate the other tables? I'm
uncertain: what to specify as a reasonable "source color space" -S
option that would enclose most of the usual colors in digital
photography images? I.e. something a little larger than sRGB, but not
too large? Or use the LCD manufacturer's supplied icc profile (without
calibration curve) for the LCD panel, that the iccview.de comparision
measures as 80% of the sRGB volume--should I use that? Or should I use
some reasonable profile for reflective media (e.g. a typical inkjet
printer)--but as I'm not looking at reflective media while editing
pictures, how can that be a correct choice?

You can try various things, but ideally you want to use a profile
for the space your image files are in. Note that if your image
files are in a very large gamut colorspace (such as Lab or ProPhoto etc.),
you will probably have to supply a supplementary gamut too (-g),
if you don't want the output to be over compressed and dull.

Graeme Gill.

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