linear gradient

examples/gradient-linear.go

@@ -0,0 +1,39 @@
+package main
+
+import (
+	"image/color"
+
+	"github.com/fogleman/gg"
+)
+
+func main() {
+	dc := gg.NewContext(500, 400)
+
+	grad := gg.NewLinearGradient(20, 320, 400, 20)
+	grad.AddColorStop(0, color.RGBA{0, 255, 0, 255})
+	grad.AddColorStop(1, color.RGBA{0, 0, 255, 255})
+	grad.AddColorStop(0.5, color.RGBA{255, 0, 0, 255})
+
+	dc.SetColor(color.White)
+	dc.DrawRectangle(20, 20, 400-20, 300)
+	dc.Stroke()
+
+	dc.SetStrokeStyle(grad)
+	dc.SetLineWidth(4)
+	dc.MoveTo(10, 10)
+	dc.LineTo(410, 10)
+	dc.LineTo(410, 100)
+	dc.LineTo(10, 100)
+	dc.ClosePath()
+	dc.Stroke()
+
+	dc.SetFillStyle(grad)
+	dc.MoveTo(10, 120)
+	dc.LineTo(410, 120)
+	dc.LineTo(410, 300)
+	dc.LineTo(10, 300)
+	dc.ClosePath()
+	dc.Fill()
+
+	dc.SavePNG("out.png")
+}

examples/pattern-fill.go

@@ -1,12 +1,27 @@
 package main
 
-import "github.com/fogleman/gg"
+import (
+	"fmt"
+	"math"
+
+	"github.com/fogleman/gg"
+)
 
 func main() {
 	im, err := gg.LoadPNG("examples/lenna.png")
 	if err != nil {
 		panic(err)
 	}
+	var x0, y0, x1, y1 float64 = 5, 5, 15, 15
+	a := math.Atan2(y1-y0, x1-x0)
+	d := a * 180 / math.Pi
+	fmt.Println(a, d)
+	a = (2*math.Pi - a)
+	d = a * 180 / math.Pi
+	fmt.Println(a, d)
+	m := gg.Translate(-x0, -y0).Rotate(a)
+	tx, ty := m.TransformPoint(5, 5)
+	fmt.Println(tx, ty)
 	pattern := gg.NewSurfacePattern(im, gg.RepeatBoth)
 	dc := gg.NewContext(600, 600)
 	dc.MoveTo(20, 20)

gradient.go

@@ -0,0 +1,131 @@
+package gg
+
+import (
+	"image/color"
+	"math"
+	"sort"
+)
+
+type stop struct {
+	pos   float64
+	color color.Color
+}
+
+type stops []stop
+
+// Len satisfies the Sort interface.
+func (s stops) Len() int {
+	return len(s)
+}
+
+// Less satisfies the Sort interface.
+func (s stops) Less(i, j int) bool {
+	return s[i].pos < s[j].pos
+}
+
+// Swap satisfies the Sort interface.
+func (s stops) Swap(i, j int) {
+	s[i], s[j] = s[j], s[i]
+}
+
+type Gradient interface {
+	Pattern
+	AddColorStop(offset float64, color color.Color)
+}
+
+// Linear Gradient
+type linearGradient struct {
+	x0, y0, x1, y1 float64
+	dx, dy         float64
+	mag            float64
+	px0, py0       float64
+	stops          stops
+}
+
+func (g *linearGradient) ColorAt(x, y int) color.Color {
+	if len(g.stops) == 0 {
+		return color.Transparent
+	}
+
+	fx, fy := float64(x), float64(y)
+	x0, y0 := g.x0, g.y0
+	dx, dy := g.dx, g.dy
+
+	// Horizontal
+	if dy == 0 && dx != 0 {
+		return getColor((fx-x0)/dx, g.stops)
+	}
+
+	// Vertical
+	if dx == 0 && dy != 0 {
+		return getColor((fy-y0)/dy, g.stops)
+	}
+
+	px0, py0 := g.px0, g.py0
+	mag := g.mag
+
+	s0 := (px0-x0)*(fy-y0) - (py0-y0)*(fx-x0)
+	if s0 > 0 {
+		return g.stops[0].color
+	}
+	u := ((fx-x0)*(px0-x0) + (fy-y0)*(py0-y0)) / (mag * mag)
+	x2, y2 := x0+u*(px0-x0), y0+u*(py0-y0)
+	d := math.Hypot(fx-x2, fy-y2) / mag
+	return getColor(d, g.stops)
+}
+
+func (g *linearGradient) AddColorStop(offset float64, color color.Color) {
+	g.stops = append(g.stops, stop{pos: offset, color: color})
+	sort.Sort(g.stops)
+}
+
+func NewLinearGradient(x0, y0, x1, y1 float64) Gradient {
+	dx, dy := x1-x0, y1-y0
+	mag := math.Hypot(dx, dy)
+	px0, py0 := x0-dy, y0+dx
+	g := &linearGradient{
+		x0: x0, y0: y0,
+		x1: x1, y1: y1,
+		dx: dx, dy: dy,
+		px0: px0, py0: py0,
+		mag: mag,
+	}
+	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.NRGBA{
+		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)
+}