ExPng is a pure Elixir implementation of the PNG image format. It can read and write PNG files.

Installation

Add :ex_png as a dependency in your project's mix.exs:

def deps do
  [
    {:ex_png, "~> 1.0.0"}
  ]
end

And run:

~$ mix deps.get

Features

• Decodes any image the PNG standard allows. This includes all standard bit depths, color modes, filtering and iterlacing options
• Encodes using any options the PNG standard allows. See Saving Images below for the defaults ExPng uses, and how to override them
• Read/write access to the image's pixels
• Implements Xiaolin Wu's algorithm for drawing antialiased lines
• Works with all Elixir versions >= 1.7

Creating images

Images can be created by decoding an existing PNG file

{:ok, image} = ExPng.Image.from_file("adorable_kittens.png")

by passing in a list of rows of ExPng.Pixel structs

image = ExPng.Image.new([
    [ExPng.Pixel.rgb(100, 200, 200), ExPng.Pixel.rgb(0, 100, 100)],
    [ExPng.Pixel.rgb(50, 100, 200), ExPng.Pixel.rgb(0, 75, 43)],
])

or by creating a blank image with a given width and height

image = ExPng.Image.new(200, 100)

Modifying images

Images can be edited by painting/clearing individual pixels, drawing lines, or erasing the entire canvas. These editing actions do not occur in layers, and change the underlying pixels, so be careful as changes cannot be undone.

image = ExPng.Image.new(32, 32)

# Draws a pixel with the given color at the given coordinate
image = ExPng.Image.draw(image, {5, 8}, ExPng.Pixel.rgb(0, 100, 200))

# Draws a line between two points.
image = ExPng.Image.line(image, {0, 0}, {15, 8}, ExPng.Pixel.black())

# Sets a single pixel to opaque white
image = ExPng.Image.clear(image, {0, 0})

# Colors the entire canvas opaque white
image = ExPng.Image.erase(image)

Saving images

Images can be saved via

ExPng.Image.to_file(image, filename)

When encoding images, ExPng will attempt to find the optimal bit depth and color mode for the image in the following priority order:

1. pure black and white images
2. grayscale images, with or without transparency
3. color images where the number of unique colors is indexable
4. truecolor images with too many colors to index, with or without transparency

Filter type, compression level, and whether to encode the image with Adam7 interlacing can be set when saving an image:

ExPng.Image.to_file(
    image,
    filename,
    filter: ExPng.Image.Filtering.sub,
    compression: 9,
    interlace: true
)

Valid values for compression are integers 0 (no compression) through 9 (max compression).

Valid filter values are .none, .up, .sub, .average, and .paeth, which can also be represented, respectively, by integers 0-5.

By default, ExPng uses the up filter, the default zlib compression level 6, and no interlacing.

Documentation

Complete documentation can be found on hexdocs.pm: ExPng documentation

About

This library is written by Michael Berkowitz and released under the UNLICENSE.

The suite of test images in test/png_suite comes from Willem van Schaik's PngSuite, with some additions made by Willem van Bergen as part of his work on ChunkyPNG, and some added by the author of this library.