![]() Depending on the way the DjVu file was encoded, the meaningful information may be in the foreground, background or the mask layer. If the remaining meaningful layer turns out to be bitonal (black-and-white), then the optimal JBIG2 compression can be used. It is not strictly necessary for the document to still look all right. This approach is suitable for scanned DjVu books that have a multi-coloured background that came from scanner, which in PDF will take up a lot of size. What we can do is extract only the layer that bears text (and maybe some other meaningful foreground information with it) and make the PDF out of it. When converting, all these DjVu layers of a page are combined into one image, which for some reason almost always turns out to be such that JPEG compression doesn't perform very well on it, and as the result, the output file turns out to be several times larger than the original. The DjVu format uses a smart system of layers which allows to separate the text from the background and have a different approach to compression of them, while the PDF format simply uses the JPEG compression. The last step is to assemble the PDF file out of these images.Ĭonverting only the layer that bears text Ĭonverting coloured DjVu to PDF directly often results in unnecessarily large size of the output.In case the conversion to PDF will be done with XnView nconvert, you can apply some editing along the way (see nconvert's self-explaing help: nconvert -help). See Help:Reducing the file size of large PDF or DjVu scanned books. ![]()
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