clean up command-line parsing and add some error-diffusion experiments

README.md

@@ -0,0 +1,9 @@
+# dither
+
+This repo implements a number of the dithering methods detailed on <https://surma.dev/things/ditherpunk/>.
+
+light.png, light-hires.jpg, dark.png, and dark-hires.jpg are from Surma's post, used under CC BY-NC-SA 4.0.
+
+david.png is taken from the wikipedia article on dithering, where it was released into the public domain.
+
+I took cricket.jpg.

color_quant.go

@@ -0,0 +1,120 @@
+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)
+}

main.go

@@ -1,8 +1,10 @@
 package main
 
 import (
+	"flag"
 	"fmt"
 	"image"
+	"image/color"
 	_ "image/jpeg"
 	"image/png"
 	"math/rand"
@@ -10,25 +12,94 @@ import (
 	"time"
 )
 
-func main() {
+type coord struct {
-	if len(os.Args) == 1 || len(os.Args) > 4 || os.Args[1] == "help" {
+	x, y int
-		fmt.Printf(`usage: %s <path/to/image.ext> <dither_option> <path/to/output.png>
-  Supported dither options are: noop; naive; randomnoise; bayer{0,1}; bayer{0,1}n
-  Supported input image formats are jpg, png
-`, os.Args[0])
-		os.Exit(0)
 }
 
-	filename := os.Args[1]
+// provided x, y, and color at location, return a color
-	ditherer := os.Args[2]
+type quantizerFunction func(int, int, color.Color) color.Color
-	outfile := os.Args[3]
+
+// apply sequentially applies a quantizing function to an image and returns the result
+func apply(i image.Image, f quantizerFunction) image.Image {
+	out := image.NewRGBA(image.Rect(0, 0, i.Bounds().Max.X, i.Bounds().Max.Y))
+
+	b := out.Bounds()
+	for y := b.Min.Y; y < b.Max.Y; y++ {
+		for x := b.Min.X; x < b.Max.X; x++ {
+			out.Set(x, y, f(x, y, i.At(x, y)))
+		}
+	}
+
+	return out
+}
+
+type ditherOpts struct {
+	raw    string
+	level  int
+	invert bool
+}
+
+func (o *ditherOpts) Set(input string) error {
+	for _, p := range input {
+		switch p {
+		case 'n':
+			o.invert = true
+		case '0':
+			o.level = 0
+		case '1':
+			o.level = 1
+		default:
+			return fmt.Errorf("unknown option %s", string(p))
+		}
+	}
+	return nil
+}
+
+func (o *ditherOpts) String() string {
+	return o.raw
+}
+
+func main() {
+	ditherOpts := &ditherOpts{}
+
+	filenameFlag := flag.String("in", "", "Input filename. PNG or JPG formats accepted.")
+	verboseFlag := flag.Bool("v", false, "Include diagnostic log lines.")
+	paletteFlag := flag.String("p", "bw",
+		`Palette to use. Defaults to 'bw': black-and-white.
+Pass 'sixteen' for the classic sixteen-color palette.
+Not all dither options support palettes; unsupported ditherers will ignore this flag.`)
+	dithererFlag := flag.String("d", "naive",
+		`Ditherer to use. Defaults to 'naive', which just flattens to nearest color.
+Other options: 'noise', 'bayer', 'error', 'floydsteinberg', 'jjn', 'atkinson'.`)
+	flag.Var(ditherOpts, "o",
+		`Supply options for the selected ditherer.
+Currently only relevant to the bayer ditherer, which defaults to 'level 0, non-inverted'.
+To change: '1' applies a level-one bayer diffuser; '1n' would invert it, '0' would be the default.`)
+	outputFlag := flag.String("out", "output.png", "Output filename. Outputs in PNG format.")
+
+	flag.Parse()
+
+	// extract all those pointers
+	filename := *filenameFlag
+	ditherer := *dithererFlag
+	palette := *paletteFlag
+	outfile := *outputFlag
+	verbose := *verboseFlag
+
+	if filename == "" || outfile == "" {
+		fmt.Printf("Usage of %s:\n", os.Args[0])
+		flag.PrintDefaults()
+		os.Exit(0)
+	}
 
 	i, codex, err := loadImage(filename)
 	if err != nil {
 		fmt.Printf("error loading %s; %v\n", filename, err)
 		os.Exit(255)
 	}
+	if verbose {
 		fmt.Printf("loaded %s using %s\n", filename, codex)
+	}
 
 	t := time.Now()
 
@@ -37,24 +108,50 @@ func main() {
 	case "noop":
 		new = apply(i, noOp)
 	case "naive":
+		if palette == "bw" {
 			new = apply(i, naiveBW)
-	case "randomnoise":
+		} else if palette == "sixteen" {
+			new = apply(i, naivePalette(sixteencolors))
+		}
+	case "noise":
 		rand.Seed(time.Now().UnixNano())
+		if palette == "bw" {
 			new = apply(i, randomNoise)
-	case "bayer0":
+		} else if palette == "sixteen" {
-		new = apply(i, bayerDithering(0, false))
+			new = apply(i, randomNoisePalette(sixteencolors))
-	case "bayer0n":
+		} else {
-		new = apply(i, bayerDithering(0, true))
+			fmt.Printf("unknown palette option for %s ditherer: %s\n", ditherer, palette)
-	case "bayer1":
+			os.Exit(2)
-		new = apply(i, bayerDithering(1, false))
+		}
-	case "bayer1n":
+	case "bayer":
-		new = apply(i, bayerDithering(1, true))
+		if palette == "bw" {
-	case "simpleerror":
+			new = apply(i, bayerDithering(ditherOpts.level, ditherOpts.invert))
+		} else if palette == "sixteen" {
+			new = apply(i, colorBayer(ditherOpts.level, sixteencolors))
+		} else {
+			fmt.Printf("unknown palette option for %s ditherer: %s\n", ditherer, palette)
+			os.Exit(2)
+		}
+	case "error":
+		if palette == "bw" {
 			new = apply(i, simpleErrorDiffusion())
+		} else if palette == "sixteen" {
+			new = apply(i, simpleColorErrorDiffusion())
+		} else {
+			fmt.Printf("unknown palette option for %s ditherer: %s\n", ditherer, palette)
+			os.Exit(2)
+		}
 	case "floydsteinberg":
 		new = apply(i, floydSteinberg())
 	case "jjn":
+		if palette == "bw" {
 			new = apply(i, jarvisJudiceNinke())
+		} else if palette == "sixteen" {
+			new = apply(i, colorJarvisJudiceNinke())
+		} else {
+			fmt.Printf("unknown palette option for %s ditherer: %s\n", ditherer, palette)
+			os.Exit(2)
+		}
 	case "atkinson":
 		new = apply(i, atkinson())
 	default:
@@ -62,15 +159,20 @@ func main() {
 		os.Exit(2)
 	}
 
+	if verbose {
 		fmt.Printf("dithering took %s\n", time.Since(t))
+	}
 
 	err = saveImage(outfile, new)
 	if err != nil {
 		fmt.Printf("error saving %s; %v\n", outfile, err)
 		os.Exit(255)
 	}
+
+	if verbose {
 		fmt.Printf("saved output to %s\n", outfile)
 	}
+}
 
 func loadImage(filename string) (image.Image, string, error) {
 	r, err := os.Open(filename)

quantizer.go

@@ -1,28 +1,10 @@
 package main
 
 import (
-	"image"
 	"image/color"
 	"math/rand"
 )
 
-// provided x, y, and color at location, return a color
-type quantizerFunction func(int, int, color.Color) color.Color
-
-// apply sequentially applies a quantizing function to an image and returns the result
-func apply(i image.Image, f quantizerFunction) image.Image {
-	out := image.NewRGBA(image.Rect(0, 0, i.Bounds().Max.X, i.Bounds().Max.Y))
-
-	b := out.Bounds()
-	for y := b.Min.Y; y < b.Max.Y; y++ {
-		for x := b.Min.X; x < b.Max.X; x++ {
-			out.Set(x, y, f(x, y, i.At(x, y)))
-		}
-	}
-
-	return out
-}
-
 // noOp just clones colors from one image to another, to validate file handling.
 func noOp(_, _ int, c color.Color) color.Color {
 	return c
@@ -57,10 +39,6 @@ type bayer struct {
 	matrix map[coord]float64
 }
 
-type coord struct {
-	x, y int
-}
-
 func (b *bayer) valueAt(x, y int) float64 {
 	return b.matrix[coord{x: x % b.side, y: y % b.side}]
 }