PNGdec

Copyright (c) 2021 BitBank Software, Inc.
Written by Larry Bank
bitbank@pobox.com

What is it?

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

Why did you write it?

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.

What's special about it?

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 decoding 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 48K of free RAM
  
• No external dependencies (including malloc/free)
  
• Decode an image line by line with a callback function
  
• Decode an image to a user supplied buffer (no callback needed)
  
• Supports all standard options except interlacing (too much RAM needed)
  
• Function provided to turn any pixel format into RGB565 for LCD displays
  
• Optionally disable zlib's internal CRC check - improves speed by 10-30%
• 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 decoding a 240x200 image of varying bit depths. Here's the results when run on a few common MCUs:

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