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Reducing color depth without modifiying most prominent color?

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    Solved Reducing color depth without modifiying most prominent color?

    Hi everybody,

    is there a way to reduce the color depth of an color image without modifying the most prominent color(s)?

    I often use scans of documents or technical drawings and have to mail them to other people. The images usually consist of more than 90% plain white (#FFFFFF), plus a some gray or black and maybe some other colors. To reduce the file size, I would like to decrease the color depth to 8 Bit and save as a highly compressed PNG.

    However, the white background color is altered when reducing the color depth. It goes more grayish or blueish, or whatsoever.

    Any ideas how to avoid this? Or is this a change request for an upcoming version?

    Best, Rod

    #2
    If the background is pure white, then reducing the colour depth won't change it. Presumably the scanned paper colour is some distance from 255,255,255. I just tried with a screen shot of your post, for which the background colour is 250,250,250 and that is 251,251,251 after reducing the colour depth. I then increased the contrast to get the attached image, which is whiter than the original @ 255,255,255
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      #3
      Originally posted by Bhikkhu Pesala View Post
      If the background is pure white, then reducing the colour depth won't change it.
      Negative. I tried again on Version 4.38, with a technical drawing.

      The background color changed from 255, 255, 255 to 254, 254, 254. This is definitely not a major change, but sometimes I get changes that are clearly visible.

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        #4
        Ok, got it. The option "Use Best Color Quality" (introduced in V. 4.36?) does the trick.

        If it is not used, the white background color seems to move towards the mean color of the image.

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