dither/color_quant.go

package main

import (
	"image/color"
	"math/rand"
)

var bwpallette = color.Palette{
	color.Black,
	color.White,
}

var sixteencolors = color.Palette{
	color.RGBA{0, 0, 0, 255},       // black
	color.RGBA{0, 0, 127, 255},     // navy
	color.RGBA{0, 0, 255, 255},     // blue
	color.RGBA{0, 127, 0, 255},     // green
	color.RGBA{0, 255, 0, 255},     // lime
	color.RGBA{127, 0, 0, 255},     // maroon
	color.RGBA{255, 0, 0, 255},     // red
	color.RGBA{0, 127, 127, 255},   // teal
	color.RGBA{127, 0, 127, 255},   // purple
	color.RGBA{127, 127, 0, 255},   // olive
	color.RGBA{0, 255, 255, 255},   // aqua
	color.RGBA{255, 0, 255, 255},   // fuchsia
	color.RGBA{255, 255, 0, 255},   // yellow
	color.RGBA{127, 127, 127, 255}, // gray
	color.RGBA{192, 192, 192, 255}, // silver
	color.RGBA{255, 255, 255, 255}, // white
}

func permuteColor(c color.Color, i uint8) color.Color {
	r, g, b, a := c.RGBA()
	return color.RGBA{
		uint8(r>>8) + i,
		uint8(g>>8) + i,
		uint8(b>>8) + i,
		uint8(a >> 8),
	}
}

// naivePalette smashes each pixel to its closest color in the pallette.
func naivePalette(p color.Palette) quantizerFunction {
	return func(_, _ int, c color.Color) color.Color {
		return p[p.Index(c)]
	}
}

// randomNoisePalette injects random noise into the quantization step
func randomNoisePalette(p color.Palette) quantizerFunction {
	r := len(p)
	return func(_, _ int, c color.Color) color.Color {
		// I think the proper theory here is probably "only try and randomize within one palette swatch in either direction".
		// it might be possible to instead permute the color selected _from the palette_ (i.e. modify the result of p.Index(c))...
		// ...but I think for that to work you'd need a proper "ordering" for the colors.
		noise := rand.Intn(256) / r
		rc := permuteColor(c, uint8(noise))
		return p[p.Index(rc)]
	}
}

// color permutation algo: https://en.wikipedia.org/wiki/Ordered_dithering#Algorithm
func colorBayer(level int, p color.Palette) quantizerFunction {
	b := newBayer(level)
	r := len(p)
	return func(x, y int, c color.Color) color.Color {
		v := float64(r) * (b.valueAt(x, y) - 0.5)
		rc := permuteColor(c, uint8(v))
		return p[p.Index(rc)]
	}
}

func colorError(errMap map[coord]float64, d diffusion, palette color.Palette) quantizerFunction {
	r := float64(len(palette))
	return func(x int, y int, c color.Color) color.Color {
		p := coord{x: x, y: y}
		rc := permuteColor(c, uint8(r*errMap[p]))
		delete(errMap, p) // don't let the error map grow too big
		nc := palette[palette.Index(rc)]
		l := luminence(c) - luminence(nc)
		applyError(d, l, p, errMap)
		return nc
	}
}

func simpleColorErrorDiffusion() quantizerFunction {
	errMap := make(map[coord]float64)
	d := diffusion{
		divisor: 2.0,
		matrix: map[coord]float64{
			{x: 1, y: 0}: 1.0,
			{x: 0, y: 1}: 1.0,
		},
	}
	return colorError(errMap, d, sixteencolors)
}

func colorJarvisJudiceNinke() quantizerFunction {
	errMap := make(map[coord]float64)
	d := diffusion{
		divisor: 48.0,
		matrix: map[coord]float64{
			{x: 1, y: 0}: 7.0,
			{x: 2, y: 0}: 5.0,

			{x: -2, y: 1}: 3.0,
			{x: -1, y: 1}: 5.0,
			{x: 0, y: 1}:  7.0,
			{x: 1, y: 1}:  5.0,
			{x: 2, y: 1}:  3.0,

			{x: -2, y: 2}: 1.0,
			{x: -1, y: 2}: 3.0,
			{x: 0, y: 2}:  5.0,
			{x: 1, y: 2}:  3.0,
			{x: 2, y: 2}:  1.0,
		},
	}
	return colorError(errMap, d, sixteencolors)
}