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: http://dave.wagner.googlepages.com/Profile_issues.jpg
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 CMYK?). 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.