(The image above has 15-bit color.)
HiColor is a program and a C library for converting images to 15- and 16-bit RGB color, the color depth of old display modes known as “high color”. I wrote it because I wanted to create images with the characteristic high-color look.
HiColor reduces images to two-byte 15- or 16-bit color. In 15-bit mode images have 5 bits for each of red, green, and blue, with the last bit reserved. In 16-bit mode green, the color the human eye is generally most sensitive to, is given 6 bits.
HiColor implements its own simple file format and converts between this format and PNG.
It can also convert standard PNG to standard PNG with only high-color color values.
(This simulates a roundtrip through HiColor without creating a temporary file.)
HiColor files have either the extension .hic or .hi5 for 15-bit and .hi6 for 16-bit respectively.
By default, HiColor applies the Bayer ordered dithering algorithm to reduce the quantization error (the difference between the original and the high-color pixel). Historical software and hardware used it for dithering in high-color modes. HiColor can also use “a dither” instead. Dithering can be selected or disabled with command-line flags.
Quantized images compress better than their originals, so HiColor can be a less-lossy alternative to the 256-color pngquant. Quantizing a PNG file to PNG preserves transparency (but does not quantize the alpha channel). Conversion to and from the HiColor format does not preserve transparency.
The program is written in C with two external dependencies, libpng and zlib, and builds as a static binary. It is known to work on Linux (aarch64, i386, riscv64, x86_64), FreeBSD, NetBSD, OpenBSD, and Windows 98 Second Edition, 2000 Service Pack 4, XP, 7, and 10.
The library is a single C99 header file. It is designed to be easy to understand and modify at a cost to performance. The design makes it unsuitable for real-time graphics.
The command-line program (but not the library) was vulnerable to malicious PNG files because it used a PNG library intended only for trusted input. The vulnerabilities were fixed in version 0.6.0 by switching to libpng.
PNG files produced by HiColor are not highly optimized. Run them through OptiPNG or Oxipng to significantly reduce their size.
With Bayer dithering or no dithering, there is no generation loss after the initial quantization. Applying “a dither” repeatedly to the same image will result in generation loss. In tests the loss converges to zero after 32 or 64 generations (in 15-bit and 16-bit mode respectively).
HiColor 0.1.0–0.2.1 suffered from generation loss with Bayer dithering due to an implementation error. The error was fixed in version 0.3.0.
HiColor has a Git-style CLI.
The actions encode and decode convert images between PNG and HiColor's own image format.
quantize round-trips an image through the converter and outputs a standard 32-bit PNG.
Use quantize to create high-color images readable by other programs.
info prints information about a HiColor file: version (5 for 15-bit or 6 for 16), width, and height.
HiColor 1.0.0
Create 15/16-bit color RGB images.
usage:
hicolor (encode|quantize) [-5|-6] [-a|-b|-n] [--] <src> [<dest>]
hicolor decode <src> [<dest>]
hicolor info <file>
hicolor (version|help|-h|--help)
commands:
encode convert PNG to HiColor
decode convert HiColor to PNG
quantize quantize PNG to PNG
info print HiColor image version and resolution
version print version of HiColor, libpng, and zlib
help print this help message
options:
-5, --15-bit 15-bit color
-6, --16-bit 16-bit color
-a, --a-dither dither image with "a dither"
-b, --bayer dither image with Bayer algorithm (default)
-n, --no-dither do not dither image
sudo apt install -y build-essential graphicsmagick linpng-dev pkgconf tclsh zlib1g-dev
gmake testsudo pkg install -y gmake GraphicsMagick pkgconf png tcl86
ln -s /usr/local/bin/tclsh8.6 /usr/local/bin/tclsh
fi
gmake testInstall Homebrew. Run the following commands in a clone of the HiColor repository.
brew install libpng tcl-tk
make testsudo pkgin -y install gmake GraphicsMagick pkgconf png tcl zlib
gmake testdoas pkg_add -I gmake GraphicsMagick pkgconf png tcl%8.6
ln -s /usr/local/bin/tclsh8.6 /usr/local/bin/tclsh
gmake testInstall MSYS2. Run the following commands in the MSYS2 mingw32 shell in a clone of the HiColor repository. This will build an x86 executable for Windows.
pacman -Syuu make mingw-w64-i686-gcc mingw-w64-i686-libpng mingw-w64-i686-pkgconf mingw-w64-i686-zlib tcl
make testI wrote HiColor because nothing seemed to support high color. Actually, FFmpeg, GIMP, and ImageMagick can reduce images to 15- and 16-bit color. What differentiates HiColor is being a small dedicated tool and embeddable C library and having its own file format.
MIT.
HiColor uses libpng and zlib. Follow the links for their respective licenses.
“plane in flight” (bordeaux-15bit.png) by olaf wisser.
“houses beside trees” (tests/photo.png) by Orlova Maria.
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