From e554ef45e8bda875037da03b0d3caecbd98297cc Mon Sep 17 00:00:00 2001
From: Michael Fogleman <fogleman@gmail.com>
Date: Fri, 19 Feb 2016 13:34:04 -0500
Subject: [PATCH] Push / Pop

---
 context.go          | 33 +++++++++++++++++++++++++++++++++
 examples/ellipse.go |  5 +++--
 matrix.go           |  6 +++---
 3 files changed, 39 insertions(+), 5 deletions(-)

diff --git a/context.go b/context.go
index 58ca153..3c5f2d9 100644
--- a/context.go
+++ b/context.go
@@ -47,6 +47,7 @@ type Context struct {
 	fillRule  FillRule
 	fontFace  font.Face
 	matrix    Matrix
+	stack     []*Context
 }
 
 func NewContext(width, height int) *Context {
@@ -362,3 +363,35 @@ func (dc *Context) Shear(x, y float64) {
 func (dc *Context) TransformPoint(x, y float64) (tx, ty float64) {
 	return dc.matrix.TransformPoint(x, y)
 }
+
+// Stack
+
+func (dc *Context) Push() {
+	path := make(raster.Path, len(dc.path))
+	copy(path, dc.path)
+	dc.stack = append(dc.stack, &Context{
+		color:     dc.color,
+		path:      path,
+		start:     dc.start,
+		lineWidth: dc.lineWidth,
+		lineCap:   dc.lineCap,
+		lineJoin:  dc.lineJoin,
+		fillRule:  dc.fillRule,
+		fontFace:  dc.fontFace,
+		matrix:    dc.matrix,
+	})
+}
+
+func (dc *Context) Pop() {
+	s := dc.stack
+	x, s := s[len(s)-1], s[:len(s)-1]
+	dc.color = x.color
+	dc.path = x.path
+	dc.start = x.start
+	dc.lineWidth = x.lineWidth
+	dc.lineCap = x.lineCap
+	dc.lineJoin = x.lineJoin
+	dc.fillRule = x.fillRule
+	dc.fontFace = x.fontFace
+	dc.matrix = x.matrix
+}
diff --git a/examples/ellipse.go b/examples/ellipse.go
index 3519bbb..5fefe68 100644
--- a/examples/ellipse.go
+++ b/examples/ellipse.go
@@ -7,12 +7,13 @@ func main() {
 	dc := gg.NewContext(S, S)
 	dc.SetRGB(1, 1, 1)
 	dc.Clear()
+	dc.SetRGBA(0, 0, 0, 0.1)
 	for i := 0; i < 360; i += 15 {
-		dc.Identity()
+		dc.Push()
 		dc.RotateAbout(gg.Radians(float64(i)), S/2, S/2)
 		dc.DrawEllipse(S/2, S/2, S*7/16, S/8)
-		dc.SetRGBA(0, 0, 0, 0.1)
 		dc.Fill()
+		dc.Pop()
 	}
 	dc.WritePNG("out.png")
 }
diff --git a/matrix.go b/matrix.go
index 0066dd6..6694247 100644
--- a/matrix.go
+++ b/matrix.go
@@ -41,10 +41,10 @@ func Rotate(angle float64) Matrix {
 }
 
 func RotateAbout(angle, x, y float64) Matrix {
-	a := Translate(x, y)
+	a := Translate(-x, -y)
 	b := Rotate(angle)
-	c := Translate(-x, -y)
-	return c.Multiply(b).Multiply(a)
+	c := Translate(x, y)
+	return a.Multiply(b).Multiply(c)
 }
 
 func Shear(x, y float64) Matrix {