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Monitor color profiles

Aug 23, 2012, 14:56
Post: #1
Monitor color profiles
Hi all,

I have a rather technical question about color profiles. If my computer (Macbook Air running Mac OS X 10.7) has a system-wide display profile, do I have to also give this profile to GIMP as a monitor profile? Why?

As far as I understand, the display profile attempts to adjust the colors sent to the display, in order to make up for inaccuracies in the way the display shows colors. I thought this was achieved by a look-up table (LUT) inside the graphics card, that simply matches each R, G, or B value to the proper one. However, if this is already done by the graphics card, why would GIMP ever need to know about it? I would've thought GIMP could just assume the display is perfect, and the graphics card's LUT would make sure that the colors displayed are as close as possible to the intended colors.

I must be missing something important---what is it? Choosing the right profile in the 'Monitor profile' option in my GIMP certainly has an effect, and without doing it, colors in GIMP look different from colors in, e.g., Preview.

Thanks for all your help!
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Aug 28, 2012, 03:12
Post: #2
RE: Monitor color profiles
Thanks, ofnuts, that does make sense. However, I wasn't doing any manipulation of the image. If I just open a picture in Gimp and the monitor profile is not assigned, the colors of this picture will look different from Preview (less saturated in my case). Without altering the image at all, if I then assign the correct monitor profile from Preferences, the colors change to match those in Preview.
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Sep 11, 2012, 17:04 (This post was last modified: Sep 11, 2012 17:05 by legendre17.)
Post: #3
RE: Monitor color profiles
Ok, I think I've managed to solve my own question. Essentially the point is that the monitor profile helps GIMP take care of any imperfections in the monitor calibration.

I had gotten things mixed up (as, apparently, most people do): calibration and profiling are two different things. You calibrate your display by changing things like brightness and by using the video card's look-up tables, but this need not bring your display exactly to sRGB (indeed, you don't even necessarily have to calibrate at all). One obvious reason why you wouldn't want your display to be calibrated to sRGB is if you had a wide gamut display. Calibrating it to sRGB would mean you'd lose the benefit of the wide gamut. The up-side of calibrating (even if imperfectly) is that on a calibrated display, non-color-aware applications can still have reasonably accurate colors.

Even for displays that have a gamut comparable to sRGB, the calibration could reduce the available gamut or the color resolution, so displays are usually calibrated to some state that is only an approximation of sRGB. After that, a colorimeter is used to profile the display -- which means making a map between device colors (the numbers that are fed to the video card) and physical colors, represented in some standard way, like the CIEXYZ system. GIMP uses the embedded profile in a picture to convert RGB picture values to CIEXYZ values, then needs the display profile to map CIEXYZ to display values. Without this last map, displayed colors can be inaccurate.
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