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On rare occasions, decreasing color depth to 1bpp gives surprising results

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    On rare occasions, decreasing color depth to 1bpp gives surprising results

    Hello there :]

    This is my first post here, I'll try not to do anything wrong ^^'.
    As stated in one of the stickies of the Bug Report sections, since the issue is about how the program behaves compared to my expectation but does not crash or give any error message, I'll post it here instead of there — even though I'm pretty much convinced it is indeed some kind of bug.

    Version of IrfanView : 4.28
    OS : Windows XP Home SP3
    Number of processor cores used : 2

    So, a few days ago at the company I work for, a user had to batch convert about 2 thousands JPEG files to TIFF/CCITT4. It went fine except for a single file, where the "colors" appeared somewhat as if we had previously inverted them : clear areas had become black and dark ones had become white.

    Since I can't give away copies of this document, I made a test with my collection of anime/games wallpapers and related pictures, and got the same issue for 7 pictures (out of 1936 total).

    So, I made a few more tests on these 7 pictures and found out what follows :
    1. the pictures with which the issue happens are correctly opened and displaying alright within IrfanView itself
    2. the problem doesn't seem to come from the TIFF saving options we had used but from the Decrease Color Depth function itself
    3. checking or not the "Use Floyd-Steinberg dithering" doesn't seem to affect the result much
    4. checking or not the "Make grayscale image" doesn't make much difference either (as one could expect from a 1bpp conversion )
    5. using the "Negative" function before decreasing color depth does not give the exact the same output
    6. the problem does not happen if I use the "Convert to Grayscale" feature from the Image menu before decreasing color depth to 1bpp
    7. seems like the problem mostly (only ?) happens with quite large pictures (several thousand pixels width and height, a few dozen megabytes in memory), but of course, not all of them — that would be too simple to troubleshoot, heh ? ;D



    Now, here is an archive (about 35 MB) that contains the files which I got the issue with : http://www.multiupload.com/KTQY21A34D

    And below are the results of 1bpp conversion for one of these files.
    1 - first one, which I consider unsatisfactory (click on the preview for full sized picture):


    2 - the second, which is what I would have expected, and could get after using Grayscale conversion before decreasing color depth to 1bpp :



    So, considering how rare the issue seems to be, it's not very important, but still, I think it would be nice if it could be fixed

    Have a nice day :}
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