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Read/write access to PNG images in pure Ruby.

ChunkyPNG Build Status

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.


  • Decodes any image that the PNG standard allows. This includes all standard color modes, all bit depths, all transparency, and interlacing and filtering options.
  • Encodes images supports all color modes (true color, grayscale, and indexed) and transparency for all these color modes. The best color mode will be chosen automatically, based on the amount of used colors.
  • R/W access to the image's pixels.
  • R/W access to all image metadata that is stored in chunks.
  • Memory efficient (uses a Fixnum, i.e. 4 or 8 bytes of memory per pixel, depending on the hardware)
  • Reasonably fast for Ruby standards, by only using integer math and a highly optimized saving routine.
  • Interoperability with RMagick if you really have to.

Also, have a look at OilyPNG which 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 }

Also check out the screencast on the ChunkyPNG homepage by John Davison, which illustrates basic usage of the library on the ChunkyPNG website.

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

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 CHANGELOG.rdoc 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. :-)

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