Fixed up the dotted pattern

artist/checkered.go

@@ -0,0 +1,33 @@
+package artist
+
+import "image/color"
+
+// Checkered is a pattern that produces a grid of two alternating colors.
+type Checkered struct {
+	First  Pattern
+	Second Pattern
+	CellWidth, CellHeight int
+}
+
+// AtWhen satisfies the Pattern interface.
+func (pattern Checkered) AtWhen (x, y, width, height int) (c color.RGBA) {
+	twidth  := pattern.CellWidth  * 2
+	theight := pattern.CellHeight * 2
+	x %= twidth
+	y %= theight
+	if x < 0 { x += twidth  }
+	if y < 0 { x += theight }
+
+	n := 0
+	if x >= pattern.CellWidth  { n ++ }
+	if y >= pattern.CellHeight { n ++ }
+	
+	x %= pattern.CellWidth
+	y %= pattern.CellHeight
+
+	if n % 2 == 0 {
+		return pattern.First.AtWhen(x, y, pattern.CellWidth, pattern.CellHeight)
+	} else {
+		return pattern.Second.AtWhen(x, y, pattern.CellWidth, pattern.CellHeight)
+	}
+}

artist/dotted.go

@@ -0,0 +1,30 @@
+package artist
+
+import "math"
+import "image/color"
+
+// Dotted is a pattern that produces a grid of circles.
+type Dotted struct {
+	Background Pattern
+	Foreground Pattern
+	Size int
+	Spacing int
+}
+
+// AtWhen satisfies the Pattern interface.
+func (pattern Dotted) AtWhen (x, y, width, height int) (c color.RGBA) {
+	xm := x % pattern.Spacing
+	ym := y % pattern.Spacing
+	if xm < 0 { xm += pattern.Spacing }
+	if ym < 0 { xm += pattern.Spacing }
+	radius  := float64(pattern.Size) / 2
+	spacing := float64(pattern.Spacing) / 2 - 0.5
+	xf := float64(xm) - spacing
+	yf := float64(ym) - spacing
+
+	if math.Sqrt(xf * xf + yf * yf) > radius {
+		return pattern.Background.AtWhen(x, y, width, height)
+	} else {
+		return pattern.Foreground.AtWhen(x, y, width, height)
+	}
+}