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Subject: Re: ACR Calibration



Les,

If you will send me a text table of the RGB values you want to match to, I will send you a customized version of both of my scripts. Please donít just say Bruce Fraserís or Bruce Lindbloomís, I need the numbers you have selected. .

As to the precision, I would like to point out that any of these scripts are going to read the image patch values with a histogram. The histogram is limited to 256 levels (essentially 8-bit precision). This means that any given RGB measurement can be off by plus or minus one. Even if it is right on target in 16-bit mode. This is a limitation of the scripting interface.

It is a good idea to slightly defocus the target image shot. The best materials and lenses will still produce small specular reflections within the image. This and the selection sample size will smooth this out a bit. Otherwise you will see substantial pixel to pixel variations.

The color checker is purposely designed to occupy less than the dynamic range of a typical camera. So it should not reach either end of the histogram when properly exposed. That is why it is good to use a neutral mid tone for the background. And why contrast should be set with the equally distant mid tone patches (3.5 and 6.5) after the center point has been set based on the mid tone (0.5) patch.

In addition, a color checker should not contain any colors that are out of gamut for any color space being tested. This is basically true with this CC and the measured target values at illuminant D65. In sRGB with any other illuminant, there are some target colors that wind up out of gamut. With illuminant A (tungsten) all but ppRGB have some out of gamut colors.

That said, if you calibrate under ppRGB but your workflow is primarily aRGB, sRGB, or cmRGB you may find that your calibration yields out of gamut (clipped) CC colors anyway. I am putting a simple test for this in my next script update.

After calibration if the ends of the histogram have changed substantially you probably have some gamut concerns. This is why you want a mid tone background. Color values outside the range of the black and white patches should be suspect. The yellow tones seem to be particularly vexing in this respect.

If youíre out this way, try some Rahr & Sons from Fort Worth.

Cheers, Rags :-)

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