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Thread: Cobalp - how to use it?

  1. #1
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    Does anyone could tell me how to use Cobalp by Tilo Strutz?
    I see that it works great for lossless images on Squeeze chart 2007.

    With some help I got a working compile from sources I found here:
    http://www-nt.e-technik.uni-rostock.de/~ts/Softwar e/CoBALP.tar.gz

    But it only accepts grayscale files in some particular format. Plus compression is poor (as for grayscale). Lastly on Squeeze Chart entry there is Cobalp MAX not just Cobalp.
    What am I doing wrong?

  2. #2
    Programmer Bulat Ziganshin's Avatar
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    i encountered just the same problem when tried it

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    Good that I am not alone then .
    Maybe Stephan had a different version in Squeeze Chart 2007?
    But it is weird.

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    Stephan Busch's Avatar
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    Hi guys,

    I used the version below.

    http://squeezechart.freehost.ag/WaveletCoBALP.wav

    download, rename to .zip and start Waveconvert.exe


    Regards

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    Thanks Stephan!
    Works like charm and it has a GUI

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    After decompressing the output BMP is different from the inputted one in two bytes Every compression and decompression cycle after that with the changed file produces bitwise identical files as inputted. What's wrong?

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    Stephan Busch's Avatar
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    Was the option 'color conversion' on?

    We should contact the author. He is living in Hamburg now
    and haven't worked on CoBALP since 2001. Maybe he can take a look on it again.

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    It doesn't depend on that option (archive size, of course, was affected by that option), in both cases bytes 43 and 44 differ:

    Byte 43 is different: Hex 12 vs 00
    Byte 44 is different: Hex 0B vs 00

    When compressing and decompressing this changed file, no bytes are changed during (de)coding. I believe it is just some not too much important information in header, but even though it just isn't bit-identical (original BMP saved in MS Paint)

  9. #9
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    Accordig to this http://atlc.sourceforge.net/bmp.html WaveConvert+ changes file width and height info. Btw, seems that it is false anyway...

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    Programmer Bulat Ziganshin's Avatar
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    lossless media compressors typically doesn't store each and evey bit of headers, they just save parameters they know. for example, TTA strips extended header information

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    Stephan Busch's Avatar
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    I have asked Dr. Tilo Strutz and he said that CoBALP changes 2 bytes in BMP header, but they are seldom used for data and don't touch image in any way (they are not needed for bitwise image reconstruction upon extraction). There only will be the grayscale version open source. He said that experienced programmers do a lossless color space transformation (p.ex. RCT as described in his book) and process the 3 new components Y, Ur and Vr again with the same method but each component separately..
    Further he said he will perhaps work again on CoBALP as the windows implementation is lousy and not stable.

  12. #12
    Programmer Bulat Ziganshin's Avatar
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    i tried cobalp on two images - 24-bit photo (converted from jpeg) and grey-scale variant of the same photo. on grey image it was 15% better than mm+grzip while on 24-bit image difference was only 5%

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    Quote Originally Posted by Stephan Busch
    but they are seldom used for data and dont touch image in any way
    OK, thanks for info! Recompressing PNG also gives the same image with different byteprint, anyway, so it should be alright

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