Add support for dashed strokes

README.md

@@ -98,6 +98,7 @@ SetHexColor(x string)
 SetLineWidth(lineWidth float64)
 SetLineCap(lineCap LineCap)
 SetLineJoin(lineJoin LineJoin)
+SetDash(dashes ...float64)
 SetFillRule(fillRule FillRule)
 ```
 
@@ -149,9 +150,6 @@ even better, implement it and submit a pull request!
 
 - Clipping Regions
 - Gradients / Patterns
-- Dashed Lines\*
-
-\* *might be implemented soon*
 
 ## How Do it Do?
 

context.go

@@ -43,6 +43,7 @@ type Context struct {
 	start      Point
 	current    Point
 	hasCurrent bool
+	dashes     []float64
 	lineWidth  float64
 	lineCap    LineCap
 	lineJoin   LineJoin
@@ -91,6 +92,10 @@ func (dc *Context) SavePNG(path string) error {
 	return SavePNG(path, dc.im)
 }
 
+func (dc *Context) SetDash(dashes ...float64) {
+	dc.dashes = dashes
+}
+
 func (dc *Context) SetLineWidth(lineWidth float64) {
 	dc.lineWidth = lineWidth
 }
@@ -276,11 +281,15 @@ func (dc *Context) joiner() raster.Joiner {
 }
 
 func (dc *Context) StrokePreserve() {
+	path := dc.strokePath
+	if len(dc.dashes) > 0 {
+		path = dashed(path, dc.dashes)
+	}
 	painter := raster.NewRGBAPainter(dc.im)
 	painter.SetColor(dc.color)
 	r := raster.NewRasterizer(dc.width, dc.height)
 	r.UseNonZeroWinding = true
-	r.AddStroke(dc.strokePath, fi(dc.lineWidth), dc.capper(), dc.joiner())
+	r.AddStroke(path, fi(dc.lineWidth), dc.capper(), dc.joiner())
 	r.Rasterize(painter)
 }
 

examples/cubic.go

@@ -19,9 +19,10 @@ func main() {
 	dc.MoveTo(x0, y0)
 	dc.CubicTo(x1, y1, x2, y2, x3, y3)
 	dc.SetRGBA(0, 0, 0, 0.2)
-	dc.SetLineWidth(10)
+	dc.SetLineWidth(8)
 	dc.FillPreserve()
 	dc.SetRGB(0, 0, 0)
+	dc.SetDash(16, 24)
 	dc.Stroke()
 
 	dc.MoveTo(x0, y0)
@@ -30,6 +31,7 @@ func main() {
 	dc.LineTo(x3, y3)
 	dc.SetRGBA(1, 0, 0, 0.4)
 	dc.SetLineWidth(2)
+	dc.SetDash(4, 8, 1, 8)
 	dc.Stroke()
 
 	dc.SavePNG("out.png")

point.go

@@ -1,11 +1,25 @@
 package gg
 
-import "golang.org/x/image/math/fixed"
+import (
+	"math"
+
+	"golang.org/x/image/math/fixed"
+)
 
 type Point struct {
 	X, Y float64
 }
 
-func (p Point) Fixed() fixed.Point26_6 {
-	return fp(p.X, p.Y)
+func (a Point) Fixed() fixed.Point26_6 {
+	return fp(a.X, a.Y)
+}
+
+func (a Point) Distance(b Point) float64 {
+	return math.Hypot(a.X-b.X, a.Y-b.Y)
+}
+
+func (a Point) Interpolate(b Point, t float64) Point {
+	x := a.X + (b.X-a.X)*t
+	y := a.Y + (b.Y-a.Y)*t
+	return Point{x, y}
 }

util.go

@@ -107,14 +107,14 @@ func loadFontFace(path string, points float64) font.Face {
 
 func flattenPath(p raster.Path) [][]Point {
 	var result [][]Point
-	path := make([]Point, 0, 16)
+	var path []Point
 	var cx, cy float64
 	for i := 0; i < len(p); {
 		switch p[i] {
 		case 0:
 			if len(path) > 0 {
 				result = append(result, path)
-				path = make([]Point, 0, 16)
+				path = nil
 			}
 			x := unfix(p[i+1])
 			y := unfix(p[i+2])
@@ -156,3 +156,85 @@ func flattenPath(p raster.Path) [][]Point {
 	}
 	return result
 }
+
+func dashPath(paths [][]Point, dashes []float64) [][]Point {
+	var result [][]Point
+	if len(dashes) == 0 {
+		return paths
+	}
+	if len(dashes) == 1 {
+		dashes = append(dashes, dashes[0])
+	}
+	for _, path := range paths {
+		if len(path) < 2 {
+			continue
+		}
+		previous := path[0]
+		pathIndex := 1
+		dashIndex := 0
+		segmentLength := 0.0
+		var segment []Point
+		segment = append(segment, previous)
+		for pathIndex < len(path) {
+			dashLength := dashes[dashIndex]
+			point := path[pathIndex]
+			d := previous.Distance(point)
+			maxd := dashLength - segmentLength
+			if d > maxd {
+				t := maxd / d
+				p := previous.Interpolate(point, t)
+				segment = append(segment, p)
+				if dashIndex%2 == 0 && len(segment) > 1 {
+					result = append(result, segment)
+				}
+				segment = nil
+				segment = append(segment, p)
+				segmentLength = 0
+				previous = p
+				dashIndex = (dashIndex + 1) % len(dashes)
+			} else {
+				segment = append(segment, point)
+				previous = point
+				segmentLength += d
+				pathIndex++
+			}
+		}
+		if dashIndex%2 == 0 && len(segment) > 1 {
+			result = append(result, segment)
+		}
+	}
+	return result
+}
+
+func rasterPath(paths [][]Point) raster.Path {
+	var result raster.Path
+	for _, path := range paths {
+		var previous fixed.Point26_6
+		for i, point := range path {
+			f := point.Fixed()
+			if i == 0 {
+				result.Start(f)
+			} else {
+				dx := f.X - previous.X
+				dy := f.Y - previous.Y
+				if dx < 0 {
+					dx = -dx
+				}
+				if dy < 0 {
+					dy = -dy
+				}
+				if dx+dy > 4 {
+					// TODO: this is a hack for cases where two points are
+					// too close - causes rendering issues with joins / caps
+					result.Add1(f)
+				}
+			}
+			previous = f
+		}
+	}
+	return result
+}
+
+func dashed(path raster.Path, dashes []float64) raster.Path {
+	return rasterPath(dashPath(flattenPath(path), dashes))
+}