Add conic gradient

README.md

@@ -128,7 +128,7 @@ SetFillRule(fillRule FillRule)
 
 ## Gradients & Patterns
 
-`gg` supports linear and radial gradients and surface patterns. You can also implement your own patterns.
+`gg` supports linear, radial and conic gradients and surface patterns. You can also implement your own patterns.
 
 ```go
 SetFillStyle(pattern Pattern)
@@ -136,6 +136,7 @@ SetStrokeStyle(pattern Pattern)
 NewSolidPattern(color color.Color)
 NewLinearGradient(x0, y0, x1, y1 float64)
 NewRadialGradient(x0, y0, r0, x1, y1, r1 float64)
+NewConicGradient(cx, cy, deg float64)
 NewSurfacePattern(im image.Image, op RepeatOp)
 ```
 

examples/gradient-conic.go

@@ -0,0 +1,38 @@
+// +build ignore
+
+package main
+
+import (
+	"image/color"
+
+	"github.com/fogleman/gg"
+)
+
+func main() {
+	dc := gg.NewContext(400, 400)
+
+	grad1 := gg.NewConicGradient(200, 200, 0)
+	grad1.AddColorStop(0.0, color.Black)
+	grad1.AddColorStop(0.5, color.RGBA{255, 215, 0, 255})
+	grad1.AddColorStop(1.0, color.RGBA{255, 0, 0, 255})
+
+	grad2 := gg.NewConicGradient(200, 200, 90)
+	grad2.AddColorStop(0.00, color.RGBA{255, 0, 0, 255})
+	grad2.AddColorStop(0.16, color.RGBA{255, 255, 0, 255})
+	grad2.AddColorStop(0.33, color.RGBA{0, 255, 0, 255})
+	grad2.AddColorStop(0.50, color.RGBA{0, 255, 255, 255})
+	grad2.AddColorStop(0.66, color.RGBA{0, 0, 255, 255})
+	grad2.AddColorStop(0.83, color.RGBA{255, 0, 255, 255})
+	grad2.AddColorStop(1.00, color.RGBA{255, 0, 0, 255})
+
+	dc.SetStrokeStyle(grad1)
+	dc.SetLineWidth(20)
+	dc.DrawCircle(200, 200, 180)
+	dc.Stroke()
+
+	dc.SetFillStyle(grad2)
+	dc.DrawCircle(200, 200, 150)
+	dc.Fill()
+
+	dc.SavePNG("gradient-conic.png")
+}

gradient.go

@@ -164,6 +164,52 @@ func NewRadialGradient(x0, y0, r0, x1, y1, r1 float64) Gradient {
 	return g
 }
 
+// Conic Gradient
+type conicGradient struct {
+	cx, cy   float64
+	rotation float64
+	stops    stops
+}
+
+func (g *conicGradient) ColorAt(x, y int) color.Color {
+	if len(g.stops) == 0 {
+		return color.Transparent
+	}
+	a := math.Atan2(float64(y)-g.cy, float64(x)-g.cx)
+	t := norm(a, -math.Pi, math.Pi) - g.rotation
+	if t < 0 {
+		t += 1
+	}
+	return getColor(t, g.stops)
+}
+
+func (g *conicGradient) AddColorStop(offset float64, color color.Color) {
+	g.stops = append(g.stops, stop{pos: offset, color: color})
+	sort.Sort(g.stops)
+}
+
+func NewConicGradient(cx, cy, deg float64) Gradient {
+	g := &conicGradient{
+		cx:       cx,
+		cy:       cy,
+		rotation: normalizeAngle(deg) / 360,
+	}
+	return g
+}
+
+func normalizeAngle(t float64) float64 {
+	t = math.Mod(t, 360)
+	if t < 0 {
+		t += 360
+	}
+	return t
+}
+
+// Map value which is in range [a..b] to range [0..1]
+func norm(value, a, b float64) float64 {
+	return (value - a) * (1.0 / (b - a))
+}
+
 func getColor(pos float64, stops stops) color.Color {
 	if pos <= 0.0 || len(stops) == 1 {
 		return stops[0].color