ColourClashNet

A basic collection of classes to handle image color reduction.

Purposes

These tools can be used to create displayable images for old computer video modes, using various color reduction algorithms, color quantization, and dithering.

Reduction recipe

To reduce colors on source images for old systems' video mode we need to know:

The output computer color space, aka the number of bits the system uses to represent a color. Source image usually has 3 color channels (red, green, and blue) with 8-bit per channel, for a total of 16 million selectable colors. Old systems usually have 3 channels with 3 or 4-bit for color, for a total of 512 or 4096 selectable colors, or even a fixed color palette.
The maximum number of colors displayable on video modes. Most computers can display only 16 colors on screen.
The hardware limitations. Most computers use tricks to minimize ram consumption, like using tiles or doubling pixels techniques.

How to

Simply create a ColorTransform class, select color quantization, the maximum number of colors, and a dithering algorithm. Feed it with an input image and get the transformed image.

Processing

graph TD;
    Source_Image-->Source_Color_Palette;
    Source_Color_Palette-->Color_Quantization;
    Color_Quantization-->Color_Reduction_Algorithm;
    Color_Reduction_Algorithm-->Dithering;
    Dithering-->Output_Image;

Limitations

Processing algorithms don't support alpha channels.

To do

A lot of stuff, like sprite masking and assembly code generation.

Support

Quantization

Quantization reduces the number of bits for colors channel. The lower value, the less shadow of colors.

Quantization Level	Red Shades	Green Shades	Blue Shades	Total selectable colors	Notes
8-8-8	256	256	256	16,777,216	24 bit images, SVGA, Amiga AGA
6-6-6	64	64	64	262,144	Amiga HAM8, VGA
5-6-5	32	64	32	65536	16 Bit Hi Color
5-5-5	32	32	32	32768	15 Bit Hi Color, SNES
4-4-4	16	16	16	4096	Amiga OCS/ECS/HAM6, Atari STE, NeoGeo
3-3-3	8	8	8	512	Atari ST, Sega Genesis, PC Engine
2-2-2	4	4	4	64	EGA
1-1-1	2	2	2	8	Basic Color Schemes

To generate an image for Sega Genesis, the proper 3-3-3 color quantization should be selected before processing image.

Original Image (22000 colors)	5-6-5 Quantization	4-4-4 Quantization	3-3-3 Quandtization	1-1-1 Quantization

Dithering

Dithering spreads the color reduction approximation errors over the entire image. After the color reduction process, dither tries to simulate the missing color using the remaining color information. Ditherting strength can be adjusted to obtain better results.

16 Color Image Without Dithering	2x2 Ordered Dithering	4x4 Ordered Dithering	Floyd Steimberg Dithering	Jarvis, Judice and Ninke Dithering

Color Reduction

There are lots of classic color reduction algorithms. The majority simply reduce the number of colors, while others try also to adapt the source image according to the system screen mode limitations. Here some examples.


Original Image (22000 colors)

SVGA 256 Colors	VGA 256 Colors	EGA 16 Colors

AMIGA HAM8 Mode	AMIGA HAM6 Mode	AMIGA OCS 16 Colors with Copper

ATARI STE 16 Colors	ATARI ST 16 Colors

Commodore 64 HiRes Mode	Commodore 64 LoRes Mode

Zx Spectrum	Zx Spectrum Alternative

Amstrad CPC Mode0	Amstrad CPC Mode1	Amstrad CPC Mode2

zioben/ColourClashNet