some redesign

pkg/dithering/8x8grid.go

@@ -0,0 +1,43 @@
+package dithering
+
+import (
+	"image"
+	"image/color"
+)
+
+type grid struct {
+	color.Gray
+}
+
+func (_ grid) ColorModel() color.Model {
+	return color.RGBAModel
+}
+
+func (_ grid) Bounds() image.Rectangle {
+	return image.Rect(0, 0, 8, 8)
+}
+
+func (g grid) At(x, y int) color.Color {
+	step := int(64 / (g.Gray.Y / 4))
+	if (y*8+x)%step == 0 {
+		return color.Black
+	}
+	return color.White
+}
+
+type uniformGrid struct {
+	image.Image
+	image.Rectangle
+}
+
+func (g uniformGrid) ColorModel() color.Model {
+	return g.Image.ColorModel()
+}
+
+func (g uniformGrid) Bounds() image.Rectangle {
+	return g.Rectangle
+}
+
+func (g uniformGrid) At(x, y int) color.Color {
+	return color.Color(nil)
+}