Add canvas, dither, sprites, clock and pixel elements

pixel/dither.go

@@ -0,0 +1,39 @@
+package pixel
+
+import "fmt"
+
+var bayer = [4][4]int{{0, 8, 2, 10}, {12, 4, 14, 6}, {3, 11, 1, 9}, {15, 7, 13, 5}}
+
+// Dither fills b below the level threshold, a elsewhere.
+func (g Canvas) Dither(x, y, w, h int, a, b Slot, level int) {
+	checkLevel(level)
+	if level <= 8 {
+		g.Rect(x, y, w, h, a)
+		g.cells(x, y, w, h, b, func(t int) bool { return t < level })
+		return
+	}
+	g.Rect(x, y, w, h, b)
+	g.cells(x, y, w, h, a, func(t int) bool { return t >= level })
+}
+
+// Veil dithers s over what is already there.
+func (g Canvas) Veil(x, y, w, h int, s Slot, level int) {
+	checkLevel(level)
+	g.cells(x, y, w, h, s, func(t int) bool { return t < level })
+}
+
+// Checker is a half Veil: every other dot.
+func (g Canvas) Checker(x, y, w, h int, s Slot) { g.Veil(x, y, w, h, s, 8) }
+
+func (g Canvas) cells(x, y, w, h int, s Slot, on func(t int) bool) {
+	for j := range h {
+		row := bayer[(g.gy+y+j)&3]
+		g.bits(x, y+j, w, 1, s, func(i int) bool { return on(row[(g.gx+x+i)&3]) })
+	}
+}
+
+func checkLevel(level int) {
+	if level < 0 || level > 16 {
+		panic(fmt.Sprintf("pixel: dither level %d outside 0–16", level))
+	}
+}