Copyright (c) 2021 BitBank Software, Inc.
Written by Larry Bank
[email protected]

An 'embedded-friendly' (aka Arduino) PNG image encoding library

Starting in the late 80's I wrote my own imaging codecs for the existing standards (CCITT G3/G4 was the first). I soon added GIF, JPEG and not long after that, the PNG specification was ratified. All of this code was "clean room" - written just from the specification. I used my imaging library in many projects and products over the years and recently decided that some of my codecs could get a new lease on life as open source, embedded-friendly libraries for microcontrollers.

The PNG image specification was written at a time when computers had megabytes of RAM and conserving memory wasn't a big priority. The memory allocated for encoding the compressed data (zlib) and for holding the uncompressed image can be quite a bit more than is available on modern microcontrollers (usually measured in K bytes). Three goals for this project are: easy to compile+use on all embedded systems, use a minimal amount of RAM and be self-contained. One of the dependencies I like to remove when working on embedded software is malloc/free. When compiling on a system with a tiny amount of RAM, heap memory management might not even exist.

Feature summary:
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• Runs on any MCU with at least 45K of free RAM
  
• No external dependencies (including malloc/free)
  
• Encode an image line by line
  
• Allows encoding pixels of type grayscale, indexed, truecolor or truecolor+alpha
• Supports up to 8-bits per color stimulus
  
• Allows defining a transparent color or alpha palette values
  
• Allows control of the zlib compression effort amount (1-9)
• Arduino-style C++ library class with simple API
  
• Can by built as straight C as well
  

How fast is it?
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The examples folder contains a sketch to measure the performance of encoding a 128x128 image generated dynamically.

Documentation:
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Detailed information about the API is in the Wiki
See the examples folder for easy starting points