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Old 07-27-2008, 03:37 PM   #1
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Epub file sizes

Has anyone noticed the size difference in Epub files and the same books in other formats?? It seems that the Epub's are smaller.


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Old 07-28-2008, 12:17 AM   #2
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Quote:
Originally Posted by jerryleejr View Post
Has anyone noticed the size difference in Epub files and the same books in other formats?? It seems that the Epub's are smaller.
I did notice that Epub seems to be a touch smaller than LIT; I haven't compared it to Mobipocket, but it may well be. Most pre-Epub formats have all kinds of optimizations to decrease CPU and RAM usage in an e-book rendering application, like using lightweight compression and separately compressing fairly small file chunks (to facilitate seeking without needing to de-compress the entire file). Epub instead uses the fairly good DEFLATE algorithm across entire files, which is as good as you can get without requiring that a renderer decompress everything in the book in order to render anything.
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Old 07-28-2008, 12:43 AM   #3
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Its completely a block size thing. In LRF for example every chapter is separately compressed. If you follow adobes guidelines and put every chapter into a separate file then I suspect you wont see any significant size differences between LRF and epub.

MOBI on the other hand compresses the entire text sream, but it supports 3 levels of compression

0 - no compression
1 - palmdoc (a lightweight compression scheme)
2 - huffman coding (this should actually outperform DEFLATE for pure text)
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Old 07-28-2008, 10:43 AM   #4
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As kovid indicates, some compression is more effective the more that gets compressed. While other compression schemes can actually enlarge some files, e.g. RLE compression.
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Old 07-28-2008, 02:13 PM   #5
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High compression on Mobi files takes forever even on a fast machine. Personally I never use it. Mobi advise its use only for extremely large books, like dictionaries.
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Old 07-28-2008, 02:18 PM   #6
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Now that I know that Epub files can be "unzip", I would imagine that the Epub files use a variant of the LZW compression. With it, typically large files compress more than smaller files due to the nature of the compression methodology. Therefore if the individual chapters are compressed individually, there will be (normally) less compression than if the entire book is compressed as a unit.
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Old 07-28-2008, 04:09 PM   #7
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EPUB uses standard Flate compression of the ZIP container.
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