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Possible bug: Decreasing color depth removes too many colors

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    Possible bug: Decreasing color depth removes too many colors

    When using "Decrease color depth", it seems like Irfanview removes too many colors.

    The image above is from an online riddle game and contains 11 unique colors - there's some hidden text above the lower right red bar that's almost white.

    When reducing this image to 256 colors, the resulting image only contains 8 colors, and the hidden text gets blended into the rest of the white. Is this a bug or inteded behaviour? Are somebody else able to reproduce the behaviour?
    Attached Files
    Last edited by Bhikkhu Pesala; 18.08.2010, 01:59 PM. Reason: Attached image

    #2
    I can confirm the loss of 3 colors. PSP also kept 8 colors.
    But I can't confirm it being a bug. This is quite logical behaviour when decreasing the color depth.
    Some colors will not get in the 256 color palette, because they're too near another color, so will be skipped.
    This is certainly the case with this rectangle with text, where the lowest value is 254-254-254.

    In some way I agree, that this shouldn't happen if only 11 colors are present.
    Btw : the password is "EYESTRAIN".

    Moved to Support
    Last edited by Sam_Zen; 04.11.2007, 01:07 AM.
    0.6180339887
    Rest In Peace, Sam!

    Comment


      #3
      Hi, i confirm the same "PASSWORD IS EYESTRAIN"

      However, i had the same problem and found a solution using IrfanView.
      My 24BPP PNG image has only 251 unique colors and wanted to reduce the color depth to 8BPP saving space and then having the ability to edit the palette.
      No way using reduce color depth with IV: just loosing colors (from 251 to 171) and slightly changing original colors (but changing!).

      I found that saving the original 24 BPP PNG using PNGOUT plugin gives an 8BPP PNG with the correct palette and colors. I just used the default options.

      Maybe, if you experience a high saving time.. try to set Max Passes =1.

      cenzo

      Comment


        #4
        I think it's a bug

        Yes, PSP does the same thing, but to my mind it's still a bug. A user will want to reduce the number of bits his image uses, not reduce further the number of unique colours.

        Try it with an image that just contains two colours -- it always makes the image black and white, even when you use the Custom (i.e. 8-bit) option. But it doesn't make the image monochrome when you ask for three colours.

        I find that inconsistent and annoying.

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