[argyllcms] Re: Shadow detail problem
- From: "Dave Wagner" <dave.wagner@xxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Thu, 7 Feb 2008 09:54:59 -0600
On Feb 1, 2008 5:29 AM, Graeme Gill wrote:
> 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.
> >
> > 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 finally resolved my problem. I was using the sRGB profile from
http://www.color.org/srgbprofiles.xalter with BPC for my working
profile in Gimp. When I used the sRGB profile without BPC, the shadow
detail returned.
So, here's a final clarification question: Which combination is
"correct," assuming that gimp is not doing BPC...
1) Use sRGB_no_BCP as a source profile to argyll's profile utility AND
sRGB_no_BCP as the RGB working space?
2) Use sRGB_with_BPC for profile generation, sRGB_no_BCP for working space?
3) Use sRGB_no_BCP for profile generation, sRGB_with_BPC for the working space?
Thanks again for all of your help!
-Dave
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