[argyllcms] Re: Calibrating problems in darks areas

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Thu, 20 Dec 2007 13:12:41 +1100

Frédéric Mantegazza wrote:

As I said before, my monitor has a green color cast. As soon as I load the LUT, it is OK. But then, when I use the profile with the color engine (I tried all softwares supporting color management under Linux!), all value under 15 turn black. I attached 2 screenshots : one with sRGB as monitor profile, and one with my generated monitor profile.

What do you mean "all value under 15 turn black" ? In which
color space ?

The device profile converts from PCS to device.
What profile are you using to convert from device to PCS ?
How are they linked (what intent etc.) ? What is your
input profiles black response like ?

All these things have a great bearing on what you end up with,
and in figuring out such puzzles you need to analyse it step
by step, and compare what you expect to measure, which what you
actually measure. (icclu and spotread are your friends in this sort
of situation).

For instance, shaper/matrix profiles can't do any
gamut mapping, they're basically colorimetric. If you
link a colorimetric input profile (ie. shaper/matrix sRGB)
with a colormetric output profile (ie. your shaper/matrix monitor),
then typically the monitor black point is well above
the theoretical 0 of the sRGB, so nothing happens
initially as you increase the sRGB values from black.
You would need to have linked these profiles with some
form of luminance range mapping (ie. something
that maps the black points) to expect the output
to change immediately with a change of input.
[This isn't necessarily the explanation for what
you are seeing, but is typical of the type of
thing you should be looking for.]

$ profile -v -A "901HT" -q m -a s -d mt -T p A901HT

If you're generating a shape/matrix profile, then -dmt -Tp
isn't going to do anything, since there are no B2A CLUT tables,
hence no mechanism to do gamut mapping.

Graeme Gill.

Other related posts: