png is a pure Erlang library for creating PNG images. It can currently create 8 and 16 bit RGB, RGB with alpha, indexed, grayscale and grayscale with alpha images.

png requires at least Erlang 17.0, since it uses maps.

Include the library in your rebar configuration:

{deps, [
    {png, ".*", {git, "https://github.com/yuce/png.git", "master"}}]}.

Alternatively, for rebar 3:

{deps, [png]}.

random:seed(erlang:now()),
Width = 30,
Height = 30,
% define an 8bit RGB palette with 4 colors:
Palette = {rgb, 8, [{255, 0, 0}, {128, 255, 128}, {64, 64, 255}, {0, 0, 0}]},
{ok, File} = file:open("sample.png", [write]),

% We create an 8bit indexed PNG
Png = png:create(#{size => {Width, Height},
                   mode => {indexed, 8},
                   file => File,
                   palette => Palette}),

% make the png image row by row
AppendRow = fun(_) ->
                % An image row is composed of palette indices for indexed PNGs
                Row = lists:map(fun(_) -> random:uniform(4) - 1 end,
                                    lists:seq(1, Width)),
                png:append(Png, {row, Row}) end,
lists:foreach(AppendRow, lists:seq(1, Height)),
% need to finalize the image
ok = png:close(Png),
ok = file:close(File).

Creating a PNG image using png is a 3-step process:

1. Pass configuration and initialize the image,
2. Append the pixels, row by row or all at once,
3. Finalize the image.

The configuration currently is a map which consists of the dimensions (size) of the image, color mode and bits per pixel (or index), an optional palette (for indexed PNGs) and finally a callback which is called every time png needs to write some data.

Width = 100,
Height = 150,
ColorMode = indexed,
Bits = 8,
Palette = {ColorMode, Bits, [{255, 0, 0}, {0, 0, 128}]},
Callback = fun(IoData) -> io:format("Received: ~p~n", [IoData]) end,
Config = #{size => {Width, Height},
           mode => {ColorMode, Bits},
           palette => Palette,
           call => Callback},

There is a shortcut for writing to a file instead of a callback, using the file key:

{ok, File} = file:open("my.png", [write]),
Config = #{size => {Width, Height},
           mode => {ColorMode, Bits},
           palette => Palette,
           file => File},

Once we have the configuration, let's start creating the image:

Png = png:create(Config),

png:create writes the PNG header and the palette to the callback or the file and returns a map with the configuration + some state data.

Next step is appending pixels to the image.

The representation of a pixel depends on the color mode and bits. Here's a summary of currently supported image modes:

Where, L is the luminance, A is alpha (opacity), I is the palette index, R, G and B are red, green and blue respectively.

A palette is required for indexed images. The palette is represented as a tuple of color mode, bits and list of red, green, blue tuples:

ColorMode = indexed,
Bits = 8,
Red = {255, 0, 0},
Green = {0, 255, 0},
Blue = {0, 0, 255},
Black = {0, 0, 0},
Palette = {ColorMode, Bits, [Red, Green, Blue, Black]},

png supports adding pixels row by row (PNG specification uses the term scanline), a list of rows or as raw data. Being able to add partial image data is important when you want to keep the required memory low, e.g., creating an image on the fly in response to a web request and sending it in chunks.

Here are supported data layouts:

Note that, each row (scanline) must start with a filter method ID. Currently only filter 0 (no filter) is supported (see PNG specification for more information). For row and rows, png prepends the filter method ID to each row, but for data, you must prepend 0 to every row yourself.

Appending image data is done with png:append (surprise!). An example:

Data = {row, <<1, 2, 3>>},
Png = png:append(Png, Data),

You can call png:append as much as as necessary. After appending all data, you must finalize the image with png:close:

ok = png:close(Png)

There are some examples on https://github.com/yuce/png/tree/master/examples which show the usage and adding PNG chunks manually.