[argyllcms] Re: Shadow detail problem

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Fri, 01 Feb 2008 22:29:53 +1100

Dave Wagner wrote:

However, in the resulting profiles the shadow area is pushed toward
black.  The first viewable black level is around 21 (RGB).  This is
not at all like the results on my CRT at home.

How are you determining this ? Are you speaking about the
vcgt tag in the profile (the calibration), or the
profile ? The latter doesn't in itself change color,
it only has an effect when linked to another device
profile using a CMM (typically the input profile).

Here is a screenshot of the drycreekphoto sensitivity test chart
through the profile and straight in Gimp.  Also included are the
xicclu grey axis plots for the LCD profile and the same plot for my
CRT profile at home:


Looks about what you would expect for an LCD. Somewhere about
L* = 7 you reach the point on the colorimetric lookup where
the display is unable to go any blacker.

I've tried using profile -k z and profile -k x and they seem to have
no effect on the shadow levels (maybe because these switches are for

So, what am I doing wrong?  Or, is it correct for it to flatten out the shadows?

You need to investigate what your system is doing with the profile,
and what you expect of it. Assuming for the moment that the vcgt tag
calibration is being loaded into your system somehow (using
dispwin etc), then if (for instance) your CMM (Whatever Gimp
is using in this case, lcms ?) is taking a grey wedge defined in RGB,
interpreting that in some idealized color space (such as sRGB), and
then doing a relative colorimetric translation of those colors
into your LCD display colorspace, then yes, the shadows will
drop out because the LCD display can't display an ideal 0 black
that sRGB colorspace is.
It won't display anything until the sRGB L* value reaches about 7.

If you want some other sort of behaviour (ie. that the black
level of the idealized input space be smoothly gamut mapped),
then this has to be arranged and asked for somehow, if Gimp
doesn't do this by default.

One approach might be to use some sort of CMM link time
grey axis gamut mapping, such as "black point compensation".
(does lcms and Gimp provide this as an option for the
screen rendering ?)

Another might be to setup the display profile to gamut map
from sRGB using the ICC CLUT gamut mapping mechanism.
(Note that shaper/matrix profiles don't support this
type of gamut mapping, they only support colorimetric
rendering natively). If your source colorspace was
sRGB, then you'd have to specify this as the source
colorspace when making the profile (Argyll profile -S option),
and then select perceptual rendering in your CMM (assuming Gimp
has such control over it's display color rendering).

[Yes this is all a bit ugly, but that's how the ICC
 stuff works.]

I hope this helps.

Graeme Gill.

Other related posts: