"Mike G." wrote in message
[re colormanagement in reference to monitor display]
….
To me, I’ve gotten to the point of feeling that the answer lies in the question "Just what goes into the video LUT?"
This is exactly half of the issue: calibration. The other half is characterization.
Calibration is something we all do when we tweak the contrast or brightnes of a monitor. It means adjusting the monitor to a known state by a combination of manual adjustment of the monitor controls, and it often includes software adjustment of the *video* LUT. The end result of calibration is a known appearance of the black and white points, color temp, and gamma. It may be done visually, or using a device.
Characterization, on the other hand, defines the numbers behind the appearance of pure red, green, and blue, aka the primaries (which are not adjustable), as well as gamma. Characterization is the same as profiling, and it defines the translation of numeric color values to displayed color.
Monitor profiles typically use a matrix based mathmatics to convert numeric RGB values to output RGB values. A matrix is a compact representation that works well for CRTs, and for Photoshop’s working color spaces, such as Adobe RGB and sRGB.
Other devices, including certain LCD monitors, printers, and scanners, use a more complex multi-dimensional look up table – called a LUT for short. The icc LUT is different from the th the video LUT.
Let me lay out a scenario, and let’s exclude Windows NT from the discussion, since it doesn’t let software write to the LUT.
Not important, but: NT does have a system interface that allows the LUT to be modified.
First, most current profiling software that’s part of a hardware/software solution (ie GMB, Monaco, Spyder) combines the calibration and profile generation. We start with the monitor Brightness and Contrast adjustments, which essentially set the white point and black point. These are monitor hardware adjustments. Next we set a preset monitor color temperature, or adjust gain on 2 or 3 color guns, depending on the monitor. This sets monitor Color Temperature, and is another monitor hardware adjustment.
Absolutely. The monitor color temperature may also be set via the video LUT in Adobe Gamma (calibraton) or measured (characterization). In general, anything that visibly alters the display is part of calibration, and anything that does not (such as determination of color temp) is calibration.
Next, a number of greyscale and color patches are displayed and automatically read, and the profiling software compares them against known values. From this, an icc profile is generated, generally in the form of a LUT.
The Spyder software has a choice at this point- it can alter the display of colors by changing the video LUT (calibration), or measure the displayed colors, and reflect that color output value in the monitor profile (characterization).
Adobe gamma does a similar operation, using the gamma sliders, and this is part of calibration, not characterization.
I’m assuming (but not sure) that target gamma is factored in at this point. Next, the profiling software loads "information" into the video card LUT, and sets a startup loader which reloads that information each time the system is started.
More specifically, the brightness for each RGB value is determined, and this characterizes the gamma of the display.
So we get to the crux of the question, which is: Is it the icc profile information which the profiling software loads into the video LUT at startup?
The icc profile defines what the video system will display for each RGB value. The profile is not loaded into the video LUT, this is stored somewhere else.
If so, then isn’t in fact the profile available to all apps?
No. What is available to all apps is the video LUT, which will display RGB numbers at a specified color temp, white point, gamma, etc. The video profile is only interesting to color aware applications, meaning those that translate colors from one color space to another. Photoshop does this when it translates colors from your working color space (e.g. Adobe RGB) to your monitor color space.
If not, what *is* loaded into the video LUT? From what I’ve read, my *impression is that the icc LUT *is* what gets loaded to the video LUT, but I’ve not been able to find this clearly stated.
As mentioned, an icc LUT is a different animal from a video LUT. The video LUT is loaded at startup time, by a program such as the spyder software or Adobe Gamma Loader. This is another source of problems, when two different startup apps modify the video LUT, resulting in a doubly corrected display.
I’ve read many times about Photoshop making the monitor profile corrections ‘on the fly’, but don’t know what this means.
Yes, Photoshop converts from your working space to your monitor space on the fly. This results in a particular image looking the same on any display that has been properly calibrated (with Adobe Gamma or a calibration device), provided that image is tagged with its working space profile.
Clearly, Photoshop knows about the monitor profile, assuming it’s been correctly installed. Does it know if it’s been loaded into the video LUT, if in fact that happens? Certainly it must. I can see that a correction (small?) would be needed if the hardware calibration was done to a color temperature other than 6600 deg K, which I believe is the white point of both Adobe RGB and sRGB, and that an even larger correction would be needed if the monitor were calibrated to, say, 5000 or 9300 deg K. Of course if the icc LUT *isn’t* what goes into the video LUT, then the answer becomes clear.
You have all the right puzzle pieces. Photoshop ignores anything havning to do with the video LUT, and uses only information in the profile. The monitor profile, together with the working space profile, contains enough information to convert
an RGB value from your image to a display value.
Finally, from reading the Adobe tech paper referenced by C J Southern, it was not clear to me if the Adobe Gamma loader put profile information into the video LUT. I got the impression it did not. In that case, of course the whole profile correction would be made ‘on the fly’.
Again, it the split between characterization and calibration is key to understanding how this works. The Adobe Gamma Loader deals only with calibration data, primarily the video LUT, which it loads into the video card at startup.
One other point of information, to support older versions of Windows, Adobe Gamma would also place the name of the monitor profile in the the registry, so that Photoshop could find it later and use it for display conversion. This mechanism is still used, even in newer versions of Windows, and for that reason it is good to open, then immediately close, the monitor profile with Adobe Gamma.
Well folks, that’s the scenario, with a lot of options, questions, unknowns, etc. I hope people will jump in on this and add their thoughts. Thanks to all for their ideas to this point, and for their future contributions.
Color calibration is a mess. If you want proof, check out the great heap of obfuscation, er I mean, the font of all color profile wisdom, known as www.color.org .
—
Mike Russell
www.curvemeister.com