wasm: implementing radial gradient

draw/ellipse.go

@@ -3,6 +3,7 @@ package draw
 import (
 	"image"
 	"image/color"
+	"math"
 )
 
 // ellipse defines the struct components required to apply the ellipse's formula.
@@ -40,7 +41,15 @@ func (e *ellipse) At(x, y int) color.Color {
 	eqn := p1 + p2
 
 	if eqn <= 1 {
-		return color.Alpha{255}
+		rMin := math.Min(float64(e.rx), float64(e.ry))
+		rMax := math.Max(float64(e.rx), float64(e.ry))
+
+		grad := &NewRadialGradient(float64(x), float64(y), rMin*0.7, float64(x), float64(y), rMax-(rMin/4))
+		grad.AddColorStop(0, color.RGBA{255, 255, 255, 255})
+		grad.AddColorStop(0.6, color.RGBA{127, 127, 127, 127})
+		grad.AddColorStop(1, color.RGBA{0, 0, 0, 0})
+
+		return grad.ColorAt(e.cx, e.cy)
 	}
 	return color.Alpha{0}
 }

draw/gradient.go

@@ -0,0 +1,139 @@
+package draw
+
+import (
+	"image/color"
+	"math"
+	"sort"
+)
+
+type Gradient interface {
+	AddColorStop(offset float64, color color.Color)
+	ColorAt(x, y int) color.Color
+}
+
+type stop struct {
+	pos   float64
+	color color.Color
+}
+
+type stops []stop
+
+// Len satisfies the Sort interface.
+func (s stops) Len() int           { return len(s) }
+func (s stops) Less(i, j int) bool { return s[i].pos < s[j].pos }
+func (s stops) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }
+
+type circle struct {
+	x, y, r float64
+}
+
+type radialGradient struct {
+	c0, c1, cd circle
+	a, inva    float64
+	mindr      float64
+	stops      stops
+}
+
+func dot3(x0, y0, z0, x1, y1, z1 float64) float64 {
+	return x0*x1 + y0*y1 + z0*z1
+}
+
+func (g *radialGradient) ColorAt(x, y int) color.Color {
+	if len(g.stops) == 0 {
+		return color.Transparent
+	}
+
+	// copy from pixman's pixman-radial-gradient.c
+	dx, dy := float64(x)+0.5-g.c0.x, float64(y)+0.5-g.c0.y
+	b := dot3(dx, dy, g.c0.r, g.cd.x, g.cd.y, g.cd.r)
+	c := dot3(dx, dy, -g.c0.r, dx, dy, g.c0.r)
+
+	if g.a == 0 {
+		if b == 0 {
+			return color.Transparent
+		}
+		t := 0.5 * c / b
+		if t*g.cd.r >= g.mindr {
+			return getColor(t, g.stops)
+		}
+		return color.Transparent
+	}
+
+	discr := dot3(b, g.a, 0, b, -c, 0)
+	if discr >= 0 {
+		sqrtdiscr := math.Sqrt(discr)
+		t0 := (b + sqrtdiscr) * g.inva
+		t1 := (b - sqrtdiscr) * g.inva
+
+		if t0*g.cd.r >= g.mindr {
+			return getColor(t0, g.stops)
+		} else if t1*g.cd.r >= g.mindr {
+			return getColor(t1, g.stops)
+		}
+	}
+
+	return color.Transparent
+}
+
+func (g *radialGradient) AddColorStop(offset float64, color color.Color) {
+	g.stops = append(g.stops, stop{pos: offset, color: color})
+	sort.Sort(g.stops)
+}
+
+func NewRadialGradient(x0, y0, r0, x1, y1, r1 float64) Gradient {
+	c0 := circle{x0, y0, r0}
+	c1 := circle{x1, y1, r1}
+	cd := circle{x1 - x0, y1 - y0, r1 - r0}
+	a := dot3(cd.x, cd.y, -cd.r, cd.x, cd.y, cd.r)
+	var inva float64
+	if a != 0 {
+		inva = 1.0 / a
+	}
+	mindr := -c0.r
+	g := &radialGradient{
+		c0:    c0,
+		c1:    c1,
+		cd:    cd,
+		a:     a,
+		inva:  inva,
+		mindr: mindr,
+	}
+	return g
+}
+
+func getColor(pos float64, stops stops) color.Color {
+	if pos <= 0.0 || len(stops) == 1 {
+		return stops[0].color
+	}
+
+	last := stops[len(stops)-1]
+
+	if pos >= last.pos {
+		return last.color
+	}
+
+	for i, stop := range stops[1:] {
+		if pos < stop.pos {
+			pos = (pos - stops[i].pos) / (stop.pos - stops[i].pos)
+			return colorLerp(stops[i].color, stop.color, pos)
+		}
+	}
+
+	return last.color
+}
+
+func colorLerp(c0, c1 color.Color, t float64) color.Color {
+	r0, g0, b0, a0 := c0.RGBA()
+	r1, g1, b1, a1 := c1.RGBA()
+
+	return color.RGBA{
+		lerp(r0, r1, t),
+		lerp(g0, g1, t),
+		lerp(b0, b1, t),
+		lerp(a0, a1, t),
+	}
+}
+
+func lerp(a, b uint32, t float64) uint8 {
+	return uint8(int32(float64(a)*(1.0-t)+float64(b)*t) >> 8)
+}