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  1. #10
    Programmer schnaader's Avatar
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    The deflate streams in this file aren't recompressed same as the original by the zlib library and the differences are too big, so the transform fails for paq8px(d). To solve this, paq8px(d) would have to integrate preflate, which hasn't been done yet. Preflate stores zlib reconstruction data to solve this problem in these cases.

    Preflate is integrated in Precomp since v0.4.7, so as a workaround, you can use "precomp -cn" and paq8pxd afterwards. Looking at the results from precomp alone shows that it won't make a big difference on this file, though the difference for paq8pxd is a bit larger:

    Code:
    Original:          			16.709.659 bytes
    Precomp 0.4.7 -cn: 			16.949.572 bytes (18 PDF streams, only about 200 KB difference, ~7 KB zlib reconstruction data)
    Precomp 0.4.7 -t+:  			 2.578.516 bytes (only lzma2, don't transform the deflate streams)
    Precomp 0.4.7:      			 2.542.832 bytes (difference ~36 KB)
    
    paq8pxd_v90_mt -s4:			 1.854.287 bytes
    Precomp 0.4.7 -cn | paq8pxd_v90_mt -s4:  1.794.806 bytes (difference ~59 KB)
    There are 18 PDF streams in the file, but they are relatively small. The remaining parts of the file is uncompressed image data (multiple images, 400 KB - 4 MB in size). Detecting those would benefit this file more than using preflate.

    EDIT: When processing the PDF file using qpdf, we can force the uncompressed image data to be processed using deflate which leads to the attached smaller PDF file (3.8 MB):

    Code:
    qpdf --stream-data=compress 7421.pdf 7421-compressed.pdf
    Note that this is a lossy transform, so "--stream-data=uncompress" won't lead back to the original PDF, but it shows how inefficient the image data was stored in the original PDF.
    Attached Files Attached Files
    Last edited by schnaader; 11th November 2020 at 21:54. Reason: added paq8pxd_v90 results
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  2. Thanks:

    brispuss (12th November 2020)

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