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Köszi.

István


2015.11.08. 17:09 keltezéssel, Steve írta:
> https://en.wikipedia.org/wiki/DjVu
>
>
>      Compression
>
> DjVu divides a single image into many different images, then compresses
> them separately. To create a DjVu file, the initial image is first
> separated into three images: a background image, a foreground image, and
> a mask image. The background and foreground images are typically
> lower-resolution color images (e.g., 100 dpi); the mask image is a
> high-resolution bilevel image (e.g., 300 dpi) and is typically where the
> text is stored. The background and foreground images are then compressed
> using a wavelet-based compression
> <https://en.wikipedia.org/wiki/Wavelet_compression#Wavelet_compression>
> algorithm named IW44.^[5]
> <https://en.wikipedia.org/wiki/DjVu#cite_note-djvupaper-5> The mask
> image is compressed using a method called JB2 (similar to JBIG2
> <https://en.wikipedia.org/wiki/JBIG2>). The JB2 encoding method
> identifies nearly identical shapes on the page, such as multiple
> occurrences of a particular character in a given font, style, and size.
> It compresses the bitmap of each unique shape separately, and then
> encodes the locations where each shape appears on the page. Thus,
> instead of compressing a letter "e" in a given font multiple times, it
> compresses the letter "e" once (as a compressed bit image) and then
> records every place on the page it occurs.
>
> Optionally, these shapes may be mapped to UTF-8
> <https://en.wikipedia.org/wiki/UTF-8> codes (either by hand or
> potentially by a text recognition system
> <https://en.wikipedia.org/wiki/Text_recognition>), and stored in the
> DjVu file. If this mapping exists, it is possible to select and copy text.
>
> Since JBIG2 was based on JB2, both compression methods have the same
> problems when performing lossy compression. Numbers may be substituted
> with similar looking numbers (such as replacing 6 with 8) if the text
> was scanned at a low DPI prior to lossy compression.
>
>
>