View on GitHub


Read/write access to PNG images in pure Ruby.

Download this project as a .zip file Download this project as a tar.gz file


This library can read and write PNG files. It is written in pure Ruby for maximum portability. Let me rephrase: it does NOT require RMagick or any other memory leaking image library.


Also, have a look at OilyPNG. OilyPNG is a mixin module that implements some of the ChunkyPNG algorithms in C, which provides a massive speed boost to encoding and decoding.


  require 'chunky_png'

  # Creating an image from scratch, save as an interlaced PNG
  png =, 16, ChunkyPNG::Color::TRANSPARENT)
  png[1,1] = ChunkyPNG::Color.rgba(10, 20, 30, 128)
  png[2,1] = ChunkyPNG::Color('black @ 0.5')'filename.png', :interlace => true)

  # Compose images using alpha blending.
  avatar = ChunkyPNG::Image.from_file('avatar.png')
  badge  = ChunkyPNG::Image.from_file('no_ie_badge.png')
  avatar.compose!(badge, 10, 10)'composited.png', :fast_rgba) # Force the fast saving routine.

  # Accessing metadata
  image = ChunkyPNG::Image.from_file('with_metadata.png')
  puts image.metadata['Title']
  image.metadata['Author'] = 'Willem van Bergen''with_metadata.png') # Overwrite file

  # Low level access to PNG chunks
  png_stream = ChunkyPNG::Datastream.from_file('filename.png')
  png_stream.each_chunk { |chunk| p chunk.type }

For more information, see the project wiki or the RDOC documentation on

Security warning

ChunkyPNG is vulnerable to decompression bombs, which means that ChunkyPNG is vulnerable to DOS attacks by running out of memory when loading a specifically crafted PNG file. Because of the pure-Ruby nature of the library it is very hard to fix this problem in the library itself.

In order to safely deal with untrusted images, you should make sure to do the image processing using ChunkyPNG in a separate process, e.g. by using fork or a background processing library.


The library is written by Willem van Bergen for, and released under the MIT license (see LICENSE). Please contact me for questions or remarks. Patches are greatly appreciated!

Please check out the changelog on to see what changed in all versions.

P.S.: The name of this library is intentionally similar to Chunky Bacon and Chunky GIF. Use Google if you want to know _why. :-)