dithering done :)

pkg/dithering/8x8grid.go

@@ -18,16 +18,19 @@ func (_ grid) Bounds() image.Rectangle {
 }
 
 func (g grid) At(x, y int) color.Color {
-	step := int(64 / (g.Gray.Y / 4))
-	if (y*8+x)%step == 0 {
+	n := g.Gray.Y / 4
+	if n == 0 {
 		return color.Black
 	}
-	return color.White
+	step := int(64 / n)
+	if (y*8+x)%step == 0 {
+		return color.White
+	}
+	return color.Black
 }
 
 type uniformGrid struct {
 	image.Image
-	image.Rectangle
 }
 
 func (g uniformGrid) ColorModel() color.Model {
@@ -35,9 +38,23 @@ func (g uniformGrid) ColorModel() color.Model {
 }
 
 func (g uniformGrid) Bounds() image.Rectangle {
-	return g.Rectangle
+	// just need to return something
+	return g.Image.Bounds()
 }
 
 func (g uniformGrid) At(x, y int) color.Color {
-	return color.Color(nil)
+	dx, dy := g.Image.Bounds().Dx(), g.Image.Bounds().Dy()
+	return g.Image.At(x%dx, y%dy)
+}
+
+type Dithering struct {
+	image.Image
+}
+
+func (d Dithering) ColorModel() color.Model { return color.GrayModel }
+func (d Dithering) Bounds() image.Rectangle { return d.Image.Bounds() }
+func (d Dithering) At(x, y int) color.Color {
+	c := color.GrayModel.Convert(d.Image.At(x, y))
+	return uniformGrid{grid{c.(color.Gray)}}.
+		At(x, y)
 }