Why does ArgyllCMS scanin report different patch RGB values than darktable or GIMP?
Asked 1/6/2021
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I photographed an IT8 target, disabled most default processing in darktable, set the input/output/internal profiles to linear Rec.709 RGB, and exported a TIFF. In darktable and GIMP, the eyedropper reads about R59 G51 B51 on patch A01, but ArgyllCMS scanin -o reports much lower values (for example A01 ≈ 11.6, 8.6, 8.7). If I export as sRGB, the values are closer but still not the same. Why does scanin give different RGB numbers, and how can I make its readings match what color-managed applications show?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
5y ago
2 Answers
2
Turns out scanin ignores any profile information. Considering profiles would be missing the point for profiling.
But I just wanted a quick way to read the patch values with values similar to what I saw in Gimp or Darktable.
A solution is to bake in the profile. In my case:
tificc IMG_1956it8.tif IMG_1956it8_baked.tif
running scanin on the result gave me RGBs that match what gimp/darktable report.
Originally by user98. Source · Licensed CC BY-SA 4.0
user98
5y ago
0
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scanin is reading the TIFF’s raw pixel numbers and effectively ignoring the embedded ICC profile. That’s expected: when profiling, applying the file’s profile would defeat the purpose of measuring the image data directly.
Darktable and GIMP are color-managed, so their eyedropper values reflect the image after profile interpretation/conversion. If your TIFF is linear Rec.709, those displayed values won’t match what scanin reports from the unconverted data.
If you want scanin to report values similar to what you see in GIMP or darktable, first bake the profile conversion into the TIFF, then run scanin on that converted file. In ArgyllCMS, tificc can do this:
tificc IMG_1956it8.tif IMG_1956it8_baked.tif
Running scanin on the baked TIFF should produce RGB values that closely match the color-managed eyedropper readings.
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