From 2bada11da1307198a3be2ecb086aab6e78226ba6 Mon Sep 17 00:00:00 2001
From: David Ashby <delta.mu.alpha@gmail.com>
Date: Mon, 11 Jan 2021 19:47:29 -0500
Subject: [PATCH] add floyd-steinberg

---
 main.go      |  2 ++
 quantizer.go | 48 ++++++++++++++++++++++++++++++++++++++++--------
 2 files changed, 42 insertions(+), 8 deletions(-)

diff --git a/main.go b/main.go
index 034c9f2..1f648dd 100644
--- a/main.go
+++ b/main.go
@@ -51,6 +51,8 @@ func main() {
 		new = apply(i, bayerDithering(1, true))
 	case "simpleerror":
 		new = apply(i, simpleErrorDiffusion())
+	case "floydsteinberg":
+		new = apply(i, floydSteinberg())
 	default:
 		fmt.Printf("unknown ditherer option: %s\n", ditherer)
 		os.Exit(2)
diff --git a/quantizer.go b/quantizer.go
index 0c0340e..b7ceaa6 100644
--- a/quantizer.go
+++ b/quantizer.go
@@ -122,28 +122,60 @@ func bayerDithering(level int, invert bool) quantizerFunction {
 	}
 }
 
-func applyError(diffusionMatrix map[coord]float64, divisor float64, quantError float64, currentPixel coord, errMap map[coord]float64) {
-	for c, i := range diffusionMatrix {
+type diffusion struct {
+	matrix  map[coord]float64
+	divisor float64
+}
+
+func applyError(diffusion diffusion, quantError float64, currentPixel coord, errMap map[coord]float64) {
+	for c, i := range diffusion.matrix {
 		target := coord{x: currentPixel.x + c.x, y: currentPixel.y + c.y}
-		errMap[target] = errMap[target] + (quantError * (i / divisor))
+		errMap[target] = errMap[target] + (quantError * (i / diffusion.divisor))
 	}
 }
 
 func simpleErrorDiffusion() quantizerFunction {
 	errMap := make(map[coord]float64)
-	diffusionMatrix := map[coord]float64{
-		{x: 1, y: 0}: 1.0,
-		{x: 0, y: 1}: 1.0,
+	d := diffusion{
+		divisor: 2.0,
+		matrix: map[coord]float64{
+			{x: 1, y: 0}: 1.0,
+			{x: 0, y: 1}: 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(diffusionMatrix, 2.0, l-1.0, p, errMap)
+			applyError(d, l-1.0, p, errMap)
 			return color.White
 		}
-		applyError(diffusionMatrix, 2.0, l, p, errMap)
+		applyError(d, l, p, errMap)
+		return color.Black
+	}
+}
+
+func floydSteinberg() quantizerFunction {
+	errMap := make(map[coord]float64)
+	d := diffusion{
+		divisor: 16.0,
+		matrix: map[coord]float64{
+			{x: 1, y: 0}:  7.0,
+			{x: -1, y: 1}: 3.0,
+			{x: 0, y: 1}:  5.0,
+			{x: 1, y: 1}:  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
 	}
 }