From b8b7ff71671e0412004f2398851f5a861b0f7d8d Mon Sep 17 00:00:00 2001
From: David Ashby <delta.mu.alpha@gmail.com>
Date: Mon, 11 Jan 2021 19:42:42 -0500
Subject: [PATCH] make the error-propagation handling generic

---
 quantizer.go | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/quantizer.go b/quantizer.go
index 9bb41fd..0c0340e 100644
--- a/quantizer.go
+++ b/quantizer.go
@@ -122,18 +122,28 @@ 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 {
+		target := coord{x: currentPixel.x + c.x, y: currentPixel.y + c.y}
+		errMap[target] = errMap[target] + (quantError * (i / 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,
+	}
 	return func(x int, y int, c color.Color) color.Color {
-		l := luminence(c) + errMap[coord{x: x, y: y}]
-		delete(errMap, coord{x: x, y: y}) // don't let the error map grow too big
+		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 {
-			errMap[coord{x: x + 1, y: y}] = errMap[coord{x: x + 1, y: y}] + ((l - 1.0) / 2.0)
-			errMap[coord{x: x, y: y + 1}] = errMap[coord{x: x, y: y + 1}] + ((l - 1.0) / 2.0)
+			applyError(diffusionMatrix, 2.0, l-1.0, p, errMap)
 			return color.White
 		}
-		errMap[coord{x: x + 1, y: y}] = errMap[coord{x: x + 1, y: y}] + (l / 2.0)
-		errMap[coord{x: x, y: y + 1}] = errMap[coord{x: x, y: y + 1}] + (l / 2.0)
+		applyError(diffusionMatrix, 2.0, l, p, errMap)
 		return color.Black
 	}
 }