pngtile
Constant-time/memory tile-based handling of large PNG images.
ABOUT:
pngtile is a library (and associated command-line utility) offering efficient random access to partial regions of
very large PNG images (gigapixel range).
For this purpose, the library linearly decodes the PNG image to an uncompressed memory-mapped file, which can then
be later used to encode a portion of this raw pixel data back into a PNG image.
NOTES:
The command-line utility is mainly intended for maintining the image caches and testing, primary usage is expected
to be performed using the library interface directly. A simple Cython wrapper for a Python extension module is
provided under python/.
There is a separate project that provides a web-based tile viewer using Javascript (implemented in Python as a
WSGI application).
The .cache files are not portable across different architectures, nor are they compatible across different cache
format versions.
The library supports sparse cache files. A pixel-format byte pattern can be provided with --background using
hexadecimal notation (--background 0xFFFFFF - for 24bpp RGB white), and consecutive regions of that color will
be omitted in the cache file, which may provide significant gains in space efficiency.
COMPILING:
The library depends on libpng and pthreads. The code was developed and tested using:
* libpng 1.2.15~beta5-3ubuntu0.1
* NPTL 2.7 (glibc 2.7-10ubuntu5)
To compile, simply execute
make
The libpngtile.so will be placed under lib/, and the 'pngtile' binary under bin/.
USAGE:
Store the .png data files in a directory. You must have write access to the directory when updating the caches,
which are written as a .cache file alongside the .png file.
Provide any number of *.png paths as arguments to the ./bin/util command. Each will be opened, and automatically
updated if the cache doesn't exist yet, or is stale:
pngtile -v data/*.png
Use -v/--verbose for more detailed output.
To render a tile from some image, provide appropriate -W/-H and -x/-y options to pngtile:
pngtile data/*.png -W 1024 -H 1024 -x 8000 -y 4000
The output PNG tiles will be written to temporary files, the names of which are shown in the [INFO] output.
To force-update an image's cache, use the -U/--force-update option:
pngtile --force-update data/*.png
Alternatively, to not update an image's cache, use the -N/--no-update option.
TODO/BUGS:
At this stage, the library is primarily designed to handle a specific set of PNG images, and hence does not support
all aspects of the PNG format, nor any other image formats.
The pt_images opened by main() are not cleaned up before process exit, due to the asynchronous nature of the tile
render operation's accesses to the underlying pt_cache object.