diff --git a/quantizer.go b/quantizer.go
index 16d8b85..9bb41fd 100644
--- a/quantizer.go
+++ b/quantizer.go
@@ -40,7 +40,7 @@ func naiveBW(_, _ int, c color.Color) color.Color {
 // randomNoise injects random noise into the quantization step
 func randomNoise(_, _ int, c color.Color) color.Color {
 	l := luminence(c)
-	if (l + rand.Float64() - 0.5) > 0.5 {
+	if (l + (rand.Float64() - 0.5)) > 0.5 {
 		return color.White
 	}
 	return color.Black
@@ -126,15 +126,14 @@ func simpleErrorDiffusion() quantizerFunction {
 	errMap := make(map[coord]float64)
 	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
 		if l > 0.5 {
-			errMap[coord{x: x + 1, y: y}] = (l - 1.0) / 2.0
-			errMap[coord{x: x, y: y + 1}] = (l - 1.0) / 2.0
-			delete(errMap, coord{x: x, y: y})
+			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)
 			return color.White
 		}
-		errMap[coord{x: x + 1, y: y}] = l / 2.0
-		errMap[coord{x: x, y: y + 1}] = l / 2.0
-		delete(errMap, coord{x: x, y: y})
+		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)
 		return color.Black
 	}
 }