When you assign a display profile with vcgt curves to an image...it changes the preview of the image to reflect all the bumps, hills and valleys seen in the RGB curves demonstrated above that corrects for the nonlinearity/nonuniform appearance of the display at the time of calibration/profiling of the other display.
I don't think that's true. The vcgt curves can be embedded in a profile, but they are not part of the icc spec and they are generally disregarded in icc workflows. The only time I see them used is when I set a new profile for a monitor on the system level (Mac OSX). In that case the system reads the vcgt data from the profile and uploads it to the video card. After that they're forgotten about and I think photoshop rightly ignores them.
I've thought a bit about this over the last couple days and think I was wrong in my previous post (with some caveats) and think that colorreason's idea is not as far off the mark as we may think.Consider this:
You have a sRGB image that is tagged and you are working in a profiled/calibrated environment. You send it to me, also in a calibrated environment. We can be pretty confident that we are seeing the same color appearance. Right? That's the point of color management—everyone is calibrated to a known standard and the image is tagged. We're good—this is the way it's supposed to work.2.
You convert that sRGB image to your monitor profile and send it to me. This is the same scenario as no. 1. The monitor profile might be different, but it is still a color profile built around a know standard. I can still convert from it to PCS, sRGB, or anything else. We'll still agree on a color framework.3.
You convert the sRGB image to your monitor profile and send it to me untagged, but with a copy of the monitor profile. I then apply the monitor profile and we are identical to No. 2.4.
You have untagged RGB data from a non-color-managed application on your profiled system. You can apply your monitor profile in a color managed app like photoshop without changing the color. You send this to me with your monitor profile. I also can apply your profile and we are back at No.3, which is the same as No. 2. Assuming your monitor profile accurately describes your monitor, I can apply the profile and we'll be speaking about the same color. This means that I can get a decent idea about what color looks like on your monitor in your non-color-aware apps with the caveats given below.5.
What you can't
do: unplug your monitor and plug it into m video card using your profile. Unless my system behaves identically, we'll need to recalibrate and built a new profile. Caveat:
There will be some differences. We might have gamut issues—you might have a wide gamut monit that I clearly can't simulate on a normal monitor. I might be calibrated to a different white point and luminosity so the absolute colorimetric date may be different. Monitors next to each other may look different and may give different readings to a measurement device. But assuming that we account for what out eyes do with chromatic adaptation and such, we can be confident that the color appearance will be in the ballpark. Just because you are at 6500K and I'm at 5500K doesn't mean we can't agree on color.
The video card, vcgt problem is not as much of an issues as we might think because the profiles for the monitor are created after
the vcgt has been uploaded to the card. We can think of the video card/monitor as one thing that we are profiling so long as we don't change them.
I've attached an unusual profile for an unusual monitor. You should be able to apply it to images and see what I'm seeing on this non-standard monitor.