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https://github.com/dbohdan/hicolor

🎨 Convert images to 15/16-bit RGB color with dithering
https://github.com/dbohdan/hicolor

color-quantization color-reduction dithering high-color image-conversion image-format image-library image-processing quantization retro-graphics

Last synced: about 1 month ago
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🎨 Convert images to 15/16-bit RGB color with dithering

Host: GitHub
URL: https://github.com/dbohdan/hicolor
Owner: dbohdan
License: mit
Created: 2021-07-04T16:07:53.000Z (about 3 years ago)
Default Branch: master
Last Pushed: 2024-05-31T08:18:23.000Z (3 months ago)
Last Synced: 2024-05-31T22:39:18.816Z (3 months ago)
Topics: color-quantization, color-reduction, dithering, high-color, image-conversion, image-format, image-library, image-processing, quantization, retro-graphics
Language: C
Homepage:
Size: 643 KB
Stars: 193
Watchers: 5
Forks: 5
Open Issues: 1
Metadata Files:
- Readme: README.md
- License: LICENSE
- Authors: AUTHORS

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README

# HiColor

![A building with a dithered gradient of the sky behind it.
A jet airplane is taking off in the sky.](bordeaux-15bit.png)

*(The image above has 15-bit color.)*

HiColor is a program and library for converting images to 15- and 16-bit RGB color,
the color depth of old display modes known as [“high color”](https://en.wikipedia.org/wiki/High_color).
I wrote it because I wanted to create images with the characteristic high-color look.

## Contents

- [Description](#description)
- [Known bugs and limitations](#known-bugs-and-limitations)
- [Usage](#usage)
- [Building](#building)
- [Alternatives](#alternatives)
- [License](#license)

## Description

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](format.md) 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](https://en.wikipedia.org/wiki/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”](https://pippin.gimp.org/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](https://pngquant.org/).
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 minimal dependencies and builds as a static binary by default.
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.

## Known bugs and limitations

### Security

The command-line version of HiColor (but not the library) uses [cute_png](https://github.com/RandyGaul/cute_headers) to read PNG files.
cute_png is intended for trusted input.
This means that a maliciously-crafted PNG file could hack the HiColor CLI.
To be safe, only feed HiColor PNG files you created yourself.
Recompress PNG files from the Internet with a trusted program.

### PNG file size

PNG files produced by HiColor are not optimized.
Run them through [OptiPNG](http://optipng.sourceforge.net/) or [Oxipng](https://github.com/shssoichiro/oxipng) to significantly reduce their size.

### Generation loss

With Bayer dithering or no dithering, there is no [generation loss](https://en.wikipedia.org/wiki/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.

## Usage

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 it to create high-color images readable by other programs.
`info` displays information about a HiColor file: version (`5` for 15-bit or `6` for 16), width, and height.

```none
HiColor 0.5.0
Create 15/16-bit color RGB images.

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 program version
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
```

## Building

### Debian/Ubuntu

```sh
sudo apt install -y build-essential graphicsmagick tclsh
gmake test
```

### Cross-compiling for Windows

The following commands build a 32-bit executable for Windows.

```sh
sudo apt install -y build-essential gcc-mingw-w64-i686
gmake hicolor.exe
# Wine, Tcl, and GraphicsMagick are needed only for testing.
sudo apt install -y graphicsmagick tclsh wine
gmake test-wine
```

## Alternatives

I wrote HiColor because nothing seemed to support high color.
I was wrong about that.
Actually,
[FFmpeg](https://www.madox.net/blog/2011/06/06/converting-tofrom-rgb565-in-ubuntu-using-ffmpeg/),
[GIMP](https://docs.gimp.org/2.10/en/gimp-filter-dither.html),
and
[ImageMagick](https://www.imagemagick.org/Usage/quantize/#16bit_colormap)
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.

## License

MIT.

### Photos from Unsplash

[“plane in flight”](https://unsplash.com/photos/AwtncJT1qKs) (`bordeaux-15bit.png`) by olaf wisser.

[“houses beside trees”](https://unsplash.com/photos/PWBXQJ7PUkI) (`tests/photo.png`) by Orlova Maria.

#### License

> Unsplash grants you an irrevocable, nonexclusive, worldwide copyright license to download, copy, modify, distribute, perform, and use photos from Unsplash for free, including for commercial purposes, without permission from or attributing the photographer or Unsplash. This license does not include the right to compile photos from Unsplash to replicate a similar or competing service.