Copyright (c) 2020 BitBank Software, Inc.
Written by Larry Bank
I optimize other people's code for a living. This library is a good example of the kind of work I do for my commercial clients; it contains many unique and clever optimizations that allows it to perform better than anything else available. I'm happy to contribute optimized libraries to the open source community in addition to working on commercial projects. Whatever platform you're using, I can make significant improvements to your native code. Please contact me so that I can show you how.
I started working with image and video files around 1989 and soon turned my interest into a Document Imaging product based on my own imaging library. Over the years I added support for more and more formats until I had supported all of the standard ones, including DICOM. I sold my Document Imaging business in 1997, but still found uses for my imaging code in other projects such as retro gaming. I recently went looking to see if there was a good GIF player for Arduino and only found Adafruit's library. Unfortunately it only runs on their ATSAMD51 boards. I thought it would be possible to use my old code to create a universal GIF player that could run on any MCU/SoC with at least 24K of RAM, so I started modifying my old GIF code for this purpose. The focus of this project is speed and to use as little RAM as possible, so there are some limitations.
To save memory, the code limits the maximum image width to 320 pixels. This is just a constant defined in AnimatedGIF.h, so you can set it larger if you like. Animated GIF images have a lot of options to save space when encoding the changes from frame to frame. Many of the tools which generate animated GIFs don't make use of every option and that's helpful because the frame disposal options are not supported in the library code. They can be implemented in your drawing callback if you want. The reason they're not provided with this code is because one of the image disposal options requires you to keep an entire copy of the previous frame and this would prevent it from working on low memory MCUs. If would also require dynamic memory allocation which is another area I avoided with this code.
Designed for Speed
My work is always focused on code optimization, so this project is no different. I've profiled this code and optimized it for 32-bit CPUs. I discovered while testing it that seeking on micro SD cards (on Arduino) is very very slow. I had to use a little extra RAM to buffer incoming data to avoid seeking. The code does need to seek, but it's done very rarely. I also wrote code to buffer and de-chunk some of the incoming LZW data to avoid checking for chunk boundaries in the inner decode loop. There are a number of clever optimizations that should allow this to run faster than any existing GIF solutions on Arduino boards. The speed gained from my decoder will be lost if it takes too long to display the pixels. For this reason, the GIFDRAW callback passes an RGB565 palette in the byte order of your choosing and the example sketches uses functions to write entire lines of pixels to the SPI TFT displays in a single shot. If you implement your own GIFDRAW callback and have to pass 1 pixel at a time to whatever display device you're using, this will cause a major slowdown in the display of the frames.
A note about performance
The chart above shows the total time to decode an 8-frame sequence included in the test_images folder. The decoding speed of your particular image depends on the complexity (how much compressed data) and if/how transparent pixels are used. Small runs of transparent pixels will interfere with the performance of displaying the image on an SPI LCD. This particular image doesn't use transparency, so the time is purely for decoding the data.
- Supports any MCU with at least 24K of RAM (Cortex-M0+ is the simplest I've tested).
- Optimized for speed; the main limitation will be how fast you can copy the pixels to the display. You can use SPI+DMA to help.
- GIF image data can come from memory (FLASH/RAM), SDCard or any media you provide.
- GIF files can be any length, (e.g. hundreds of megabytes)
- Simple C++ class and callback design allows you to easily add GIF support to any application.
- The C code doing the heavy lifting is completely portable and has no external dependencies.
- Does not use dynamic memory (malloc/free/new/delete), so it's easy to build it for a minimal bare metal system.
Acquiring GIF files to play:
You'll notice that the images provided in the test_images folder have been turned into C code. Each byte is now in the form 0xAB so that it can be compiled into your program and stored in FLASH memory alongside your other code. You can use a command line tool called xxd to convert a binary file into this type of text. Make sure to add a const modifier in front of the GIF data array to ensure that it gets written to FLASH and not RAM by your build environment.
The Callback functions:
One of the reasons that this is apparently the first universal GIF library for Arduino is because the lack of available RAM and myriad display options would make it difficult to support all MCUs and displays properly. I decided that to solve this issue, I would isolate the GIF decoding from the display and file I/O with callback functions. This allows the core code to run on any system, but you need to help it a little. At a minimum, your code must provide a function to draw (or store) each scan line of image. If you're playing a GIF file from memory, this is the only function you need to provide. In the examples folder there are multiple sketches to show how this is done on various display libraries. For reading from SD cards, 4 other functions must be provided: open, close, read, seek. There is an example for implementing these in the examples folder as well.
If you're using the ESP32 or ESP8266 and playing GIF images stored in RAM, you'll need to provide the 4 file callback functions or modify the existing ones because RAM and FLASH are in different adddress spaces (Harvard architecture). The code assumes the source of the GIF data is in FLASH and uses memcpy_P() instead of memcpy() to access it.
Please consult the Wiki for detailed info about each method exposed by the AnimatedGIF class.
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