Created an elliptical border pattern

artist/circlebordered.go

@@ -0,0 +1,33 @@
+package artist
+
+import "math"
+import "image/color"
+
+// EllipticallyBordered is a pattern with a border and a fill that is elliptical
+// in shape.
+type EllipticallyBordered struct {
+	Fill Pattern
+	Stroke
+}
+
+// AtWhen satisfies the Pattern interface.
+func (pattern EllipticallyBordered) AtWhen (x, y, width, height int) (c color.RGBA) {
+	xf := (float64(x) + 0.5) / float64(width ) * 2 - 1
+	yf := (float64(y) + 0.5) / float64(height) * 2 - 1
+	distance := math.Sqrt(xf * xf + yf * yf)
+
+	var radius float64
+	if width < height {
+		// vertical
+		radius = 1 - float64(pattern.Weight * 2) / float64(width)
+	} else {
+		// horizontal
+		radius = 1 - float64(pattern.Weight * 2) / float64(height)
+	}
+
+	if distance < radius {
+		return pattern.Fill.AtWhen(x, y, width, height)
+	} else {
+		return pattern.Stroke.AtWhen(x, y, width, height)
+	}
+}