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Constant-time/memory tile-based handling of large PNG images.

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    very large PNG images (gigapixel range).

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    For this purpose, the library linearly decodes the PNG image to an uncompressed memory-mapped file, which can then

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    be later used to encode a portion of this raw pixel data back into a PNG image.

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    The command-line utility is mainly intended for maintining the image caches and testing, primary usage is expected

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    to be performed using the library interface directly. A simple Cython wrapper for a Python extension module is

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    provided under python/.

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    There is a separate project that provides a web-based tile viewer using Javascript (implemented in Python as a

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    WSGI application).

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    The .cache files are not portable across different architectures, nor are they compatible across different cache

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    format versions.

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    The library supports sparse cache files. A pixel-format byte pattern can be provided with --background using

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    hexadecimal notation (--background 0xFFFFFF - for 24bpp RGB white), and consecutive regions of that color will

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    be omitted in the cache file, which may provide significant gains in space efficiency.

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    The library depends on libpng and pthreads. The code was developed and tested using:

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    To compile dist versions, simply execute

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    The libpngtile.so and pypngtile.so libraries will be placed under lib/, and the 'pngtile' binary under bin/.

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    XXX: If compiling the pypngtile.so library with make fails, then `setup.py build_ext -i` should build it.

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    Store the .png data files in a directory. You must have write access to the directory when updating the caches,

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    which are written as a .cache file alongside the .png file.

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    Provide any number of *.png paths as arguments to the ./bin/util command. Each will be opened, and automatically

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    updated if the cache doesn't exist yet, or is stale:

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        pngtile -v data/*.png

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    Use -v/--verbose for more detailed output.

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    To render a tile from some image, provide appropriate -W/-H and -x/-y options to pngtile:

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        pngtile data/*.png -W 1024 -H 1024 -x 8000 -y 4000

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    The output PNG tiles will be written to temporary files, the names of which are shown in the [INFO] output.

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    To force-update an image's cache, use the -U/--force-update option:

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        pngtile --force-update data/*.png

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    Alternatively, to not update an image's cache, use the -N/--no-update option.

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    At this stage, the library is primarily designed to handle a specific set of PNG images, and hence does not support

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    all aspects of the PNG format, nor any other image formats.

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    The pt_images opened by main() are not cleaned up before process exit, due to the asynchronous nature of the tile

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    render operation's accesses to the underlying pt_cache object.

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