add jjn and atkinson

main.go

@@ -53,6 +53,10 @@ func main() {
 		new = apply(i, simpleErrorDiffusion())
 	case "floydsteinberg":
 		new = apply(i, floydSteinberg())
+	case "jjn":
+		new = apply(i, jarvisJudiceNinke())
+	case "atkinson":
+		new = apply(i, atkinson())
 	default:
 		fmt.Printf("unknown ditherer option: %s\n", ditherer)
 		os.Exit(2)

quantizer.go

@@ -180,6 +180,66 @@ func floydSteinberg() quantizerFunction {
 	}
 }
 
+func jarvisJudiceNinke() 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 func(x int, y int, c color.Color) color.Color {
+		p := coord{x: x, y: y}
+		l := luminence(c) + errMap[p]
+		delete(errMap, p) // don't let the error map grow too big
+		if l > 0.5 {
+			applyError(d, l-1.0, p, errMap)
+			return color.White
+		}
+		applyError(d, l, p, errMap)
+		return color.Black
+	}
+}
+
+func atkinson() quantizerFunction {
+	errMap := make(map[coord]float64)
+	d := diffusion{
+		divisor: 8.0,
+		matrix: map[coord]float64{
+			{x: 1, y: 0}:  1.0,
+			{x: 2, y: 0}:  1.0,
+			{x: -1, y: 1}: 1.0,
+			{x: 0, y: 1}:  1.0,
+			{x: 1, y: 1}:  1.0,
+			{x: 0, y: 2}:  1.0,
+		},
+	}
+	return func(x int, y int, c color.Color) color.Color {
+		p := coord{x: x, y: y}
+		l := luminence(c) + errMap[p]
+		delete(errMap, p) // don't let the error map grow too big
+		if l > 0.5 {
+			applyError(d, l-1.0, p, errMap)
+			return color.White
+		}
+		applyError(d, l, p, errMap)
+		return color.Black
+	}
+}
+
 // That is, "relative luminance": https://en.wikipedia.org/wiki/Relative_luminance.
 // go's color library doesn't give any information on what "color space" the RGBA is derived from,
 // so we convert to Y'CbCr, which returns luminence directly as the Y component.