pdftopdf QPDF: Ghostscript transparency performance optimization#147
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ValdikSS wants to merge 2 commits into
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pdftopdf QPDF: Ghostscript transparency performance optimization#147ValdikSS wants to merge 2 commits into
ValdikSS wants to merge 2 commits into
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…age. To adapt the PDF for printing, pdftopdf wraps each page of the PDF file into XObject, moving all the page properties inside it, and defining new MediaBox / Matrix for the Page. However moving Transparency Group into wrapping XObject prevents Ghostscript from detecting "fake transparency"—situation when the transparency is defined but not really used or needed. Transparency processing significantly slows down Ghostscript. Keep Transparency Group in Page declaration, do not move it into XObject. Fixes: OpenPrinting/cups-filters#569 Link: https://bugs.ghostscript.com/show_bug.cgi?id=707523 Link: OpenPrinting/cups-filters#569
If we're printing multiple PDF pages on a single physical page, and they have different Transparency Group definitions (with different color spaces or other parameters), fall back to the original code where we do not promote Transparency Group into Page declaration.
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Forward-port of the same changes for cups-filters 1.x: OpenPrinting/cups-filters#669
Original issue: OpenPrinting/cups-filters#569
When pdftopdf moves all the Page option definitions into wrapping XObject, moving Transparency Group into XObject declaration prevents Ghostscript from detecting fake transparency (when the transparency is defined but not used), which slows the document rasterization.
Keep Transparency Group in Page declaration unless two or more pages with different transparency options are printed:
number-up: TG is in Pagenumber-up=X, same TGs in each page: TG is in Pagenumber-up=X, different TGs: fall back to TG in XObjectsSee OpenPrinting/cups-filters#569 (comment) and https://bugs.ghostscript.com/show_bug.cgi?id=707523#c6 for more information.
Test case
price.pdf file has
/S /Transparencygroup defined for each page, pages 1-3 have fake transparency with only text, and only page 4 has a picture.I've tested the patch accross my small collection of PDF files, and special transparency testing files (here and also this one), and all of them produce identical byte-to-byte rasterization result with and without the patch.
However I'm not versed in PDF at all, that's why think of this MR as a draft proposal, it need some additional validation that this approach is correct in all cases.