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Colour Correction with a colour passport basics
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- Background Correct the image (is this stage necessary before colour calibrating, if so I would have to take a grey image as well as the image with the colour chart?)
- Convert RGB to XYZ, going by http://brucelindbloom.com/index.html?Eqn_RGB_to_XYZ.html i would first inverse compand. This would be done by first putting each r,g, and b pixel to the power gamma (v = V^gamma) . One thing im confused about at this stage is what RGB coulourspace would i be using (eg. Adobe, CIE, sRGB??). The second part of this would be using the transform matrix to run those values into R,G and B, this matrix is also dependnt on the RGB colourspace and also the white space. Is it essential to use the right white space or is there a standard one (e.g. D65?)
- For the XYZ to Lab conversion I am a little confused where the white reference values values Xr, Yr and Zr come from. Are there any tables like this http://brucelindbloom.com/index.html?WorkingSpaceInfo.html that would give those values?
- Yes, you must do background correction to flatten the image before you calibrate the color (develop the RGB-to-XYZ transform).
- No. Again, you cannot use any "book formulas" whether from Bruce Lindbloom's site, easyrgb.com, or any other site. If you do so you will have lost any ability for your color calibration to compensate (take ito account) changing light levels, and will be throwing anway any possibility of comparing results to instruments such as colorimeters or spectrophotometers. You cannot do that. Again, you need to use least squares to come up with the transform, not use one from a web site or book. I'd use D65/10. It's pretty much an industry standard (except in publishing where D50 is popular).
- For converting from XYZ to LAB, you can use the reference chromaticities here: https://www.easyrgb.com/en/math.php
- Background Correct (I would then white balance the image after this)
- Colour Correct RGB-RGB
- Calibrate RGB-XYZ
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