What is PurePNG?

PurePNG is pure-Python package for reading and writing PNG.

PurePNG can read and write all PNG formats. PNG supports a generous variety of image formats: RGB or greyscale, with or without an alpha channel; and a choice of bit depths from 1, 2 or 4 (as long as you want greyscale or a pallete), 8, and 16 (but 16 bits is not allowed for palettes). A pixel can vary in size from 1 to 64 bits: 1/2/4/8/16/24/32/48/64. In addition a PNG file can be interlaced or not. An interlaced file allows an incrementally refined display of images being downloaded over slow links (yet it’s not implemented in PurePNG for now).

PurePNG is written in pure Python(that’s why it’s called Pure). So if you write in Python you can understand code of PurePNG or inspect raw data while debugging.

Comparison to other PNG tools

The most obvious “competitor” to PurePNG is PIL. Depending on what job you want to do you might also want to use Netpbm (PurePNG can convert to and from the Netpbm PNM format), or use ctypes to interface directly to a compiled version of libpng. If you know of others, let me know.

PIL’s focus is not PNG. PIL’s focus is image processing, and this is where PurePNG sucks. If you want to actually process an image—resize, rotate, composite, crop–then you should use PIL. You may use PIL Plugin if you want to use both PurePNG and PIL. In PurePNG you get the image as basically an array of numbers. So some image processing is possible fairly easily, for example cropping to integer coordinates, or gamma conversion, but this very basic.

PurePNG can read and write Netpbm PAM files. PAM is useful as an intermediary format for performing processing; it allows the pixel data to be transferred in a simple format that is easily processed. Netpbm’s support for PAM to PNG conversion is more limited than PurePNG’s. Netpbm will only convert a source PAM that has 4 channels (for example it does not create greyscale–alpha PNG files from GRAYSCALE_ALPHA PAM files). Netpbm’s usual tool for create PNG files, pnmtopng, requires an alpha channel to be specified in a separate file.

PurePNG has good support for PNG’s sBIT chunk. This allows end to end processing of files with any bit depth from 1 to 16 (for example a 10-bit scanner may use the sBIT chunk to declare that the samples in a 16-bit PNG file are rescaled 10-bit samples; in this case, PurePNG delivers 10-bit samples). Netpbm handle’s the sBIT chunk in a similar way, but other toolsets may not (e.g. PIL with native plugin).

libpng is made by the PNG gods, so if want to get at all that goodness, then you may want to interface directly to libpng via ctypes. That could be a good idea for some things. Installation would be trickier.


Because PurePNG is written in Python it’s trivial to install into a Python installation. Just use python setup.py install.

There is also “light” mode: you can just copy the ../code/png/png.py file. You can even curl it straight into wherever you need it: curl -LO https://raw.githubusercontent.com/Scondo/purepng/master/code/png/png.py. This “light” module mode contains all features required for PNG reading and writing, while “full” package mode contains extra features like Cython speedup, other format support, PIL plugin etc.

PIL Plugin

In “full” package PurePNG provide plugin for usage with PIL instead of PIL’s native PNG support. This plugin is in very early stage yet can be useful. Just try it with from png import PngImagePlugin


  • PurePNG rely on python’s zlib instead of PIL. So this plugin can be useful when PIL built without zlib support.
  • PurePNG handle sBIT chunk and rescale values if it’s not correctly rescaled on write.
  • PurePNG does not use separate palette or transparency when reading, providing full RGB and alpha channel instead.
  • PurePNG should write gamma


  • PurePNG does not save custom chunks
  • PurePNG does not use zlib dictionary and method (compression level used)

PurePNG compare to PyPNG

PurePNG is fork of PyPNG - nice and simple module to work with png.

If you work with PyPNG in most cases you can use PurePNG as drop-in replace, but few things are changed:

Buffer, not array

PyPNG document that rows in boxed flat row could be any sequence, but in practice even unit-test check that it should be array.array. This changed from array.array to any buffer-compatible sequence.

You can use buffer() or memoryview() functions to fetch row bytes depending on your version of python if you have used tostring() before. And of course you may just use rows as sequence.

Python 2.2 no longer supported

Most features were already broken in Python 2.2 and it couldn’t be fixed. So support of Python 2.2 is completely removed.

Python 2.2 is pretty old, you know?

PNM|PBM|PAM deprecated in module

For now Netpbm image format kept in png module, but it will be moved to a separate module within package. So if you want to work with Netpbm images using PurePNG do not rely on “light” module mode, use “full” package. (see Installation)