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File size expands when decreasing colour depth!

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    File size expands when decreasing colour depth!

    I had an 8-level greyscale JPG image (scanned from a b+w page). I asked Irfanview (3.97) to reduce it to 2 colours to get back to the b+w image, It said that it had done so, but the resulting file was over twice the size, and when read in again it seemed to have been stored with 24 bits/pixel! While there are now only 4 colours (why not two?), I'd also hoped to reduce the file size. Is this expected behaviour or a bug?

    #2
    I might be wrong, but I don't think it is a bug, the problem is that you're saving the file in JPG format. Try this test.
    • Copy a few words of text to the clipboard
    • Paste them into Irfan View to create a B&W Image
    • Click the Info button to confirm that you have just two colours
    • Save as JPG
    • Reopen it and check the Info again. I get 71 colours.

    Save two colour images as GIF or PNG. You can then have transparency.

    There is an option on the save as JPG dialogue to save a greyscale, which I guess is the lowest colour depth that one can have in a JPG image (256). Don't forget to uncheck the option, or all of your colour images will be saved as greyscale.

    /Mod: Moved from the Bug Reports forum
    Last edited by Bhikkhu Pesala; 18.09.2007, 08:32 PM.
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      #3
      There is actually very little difference in file size between saving a 24-bit image with the color planes containing no data, and omiting color completely. But monochrome (2 colors) is not synonymous with grayscale (at least some 16 colors). If you really have monochrome images then it's best to save them as TIFF with CCITT Group 4 compressionm which is better than Deflated PNG.

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        #4
        Thanks both. I'[ll try out the ideas.

        Peter

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