Copyright (c) 2020 BitBank Software, Inc.
Written by Larry Bank
larry@bitbanksoftware.com
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.
After testing graphics on some e-ink displays, I thought that my TIFF G4 decoder could be a useful way to bring highly compressed bitonal images to small memory (e.g. Arduino) devices. This is code I wrote long ago and optimized for slow PCs. The CCITT G4 compression standard was designed for efficiently compressing FAX images, but it does really well with line art and other types of 1-bit per pixel images. Typical lossless compression ratios exceed 10 to 1 and it's very quick to encode and decode. This seemed like the perfect match for embedded microcontrollers to draw complex graphics on e-ink and LCD displays. The other useful part of this code is that the scaled down images can look quite good when the anti-aliasing option is used. This option allows the "rough edges" to look better by simulating smoothness with shades of gray. The output is generated by rendering a higher resolution version of the output and combining 2x2 blocks of 1-bpp pixels into a single grayscale pixel. The combination of fast decode+scaling and 'free' anti-aliasing means that you can take very high resolution images (e.g. 300dpi) and scale them down to good looking output on low resolution displays.
Features:
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- Supports any MCU with at least 2K of RAM (ATmega328 is the simplest I've tested)
- Optimized for speed; the main limitation will be how fast you can copy the pixels to the display.
- TIFF G4 image data can come from memory (FLASH/RAM), SDCard or any media you provide.
- CCITT G4 data can be raw (you provide size info), or contained in a TIFF file structure.
- Simple class and callback design allows you to easily add TIFF G4 support to any application.
- Includes simple color icon drawing function to draw images at any scale, color and with antialiasing.
- The C code doing the heavy lifting is completely portable and has no external dependencies.
- Includes fast anti-aliasing options (2 or 4-bits per pixel output).
- Can scale the decoded image by any fractional amount (smaller or larger).
- The only code required is a callback function to use the pixels (emitted one line at a time).
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. If you use xxd, make sure to add a const/PROGMEM modifier in front of the TIFF data array to ensure that it gets written to FLASH and not RAM by your build environment. I've also written my own tool to do this (https://github.com/bitbank2/bin_to_c). My tool already adds the necessary prefixes for Arduino FLASH data.
One of the ways to allow this code to run on any embedded platform was to define a set of callback functions. These isolate the TIFF decoding logic from the display and file I/O. 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 line of image pixels emitted by the library. If you're displaying a TIFF file from memory (RAM or FLASH), 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. Note: If you're using the ESP32 or ESP8266 (or another MCU which uses the Harvard Architecture) and decoding TIFF images stored in RAM or FLASH, you'll need to use the correct open function (openRAM or openFLASH). For MCUs based on the ARM Cortex-M, they are interchangeable.
Please consult the Wiki for detailed info about each method exposed by the TIFFG4 class. I've also provided a C interface to the library and example code which compiles from a makefile for Linux.
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