From e08199f06f86f9e25277ea3095be81413fb95005 Mon Sep 17 00:00:00 2001
From: Michael Fogleman <fogleman@gmail.com>
Date: Sat, 20 Feb 2016 14:45:17 -0500
Subject: [PATCH] ScaleAbout

---
 README.md  |  1 +
 context.go |  4 ++++
 matrix.go  | 11 +++++++++++
 3 files changed, 16 insertions(+)

diff --git a/README.md b/README.md
index 8b144a0..9b5715e 100644
--- a/README.md
+++ b/README.md
@@ -101,6 +101,7 @@ SetFillRule(fillRule FillRule)
 Identity()
 Translate(x, y float64)
 Scale(x, y float64)
+ScaleAbout(sx, sy, x, y float64)
 Rotate(angle float64)
 RotateAbout(angle, x, y float64)
 Shear(x, y float64)
diff --git a/context.go b/context.go
index 4f65f71..6abb827 100644
--- a/context.go
+++ b/context.go
@@ -388,6 +388,10 @@ func (dc *Context) Scale(x, y float64) {
 	dc.matrix = dc.matrix.Scale(x, y)
 }
 
+func (dc *Context) ScaleAbout(sx, sy, x, y float64) {
+	dc.matrix = dc.matrix.ScaleAbout(sx, sy, x, y)
+}
+
 func (dc *Context) Rotate(angle float64) {
 	dc.matrix = dc.matrix.Rotate(angle)
 }
diff --git a/matrix.go b/matrix.go
index 6694247..601fb45 100644
--- a/matrix.go
+++ b/matrix.go
@@ -30,6 +30,13 @@ func Scale(x, y float64) Matrix {
 	}
 }
 
+func ScaleAbout(sx, sy, x, y float64) Matrix {
+	a := Translate(-x, -y)
+	b := Scale(sx, sy)
+	c := Translate(x, y)
+	return a.Multiply(b).Multiply(c)
+}
+
 func Rotate(angle float64) Matrix {
 	c := math.Cos(angle)
 	s := math.Sin(angle)
@@ -86,6 +93,10 @@ func (a Matrix) Scale(x, y float64) Matrix {
 	return Scale(x, y).Multiply(a)
 }
 
+func (a Matrix) ScaleAbout(sx, sy, x, y float64) Matrix {
+	return ScaleAbout(sx, sy, x, y).Multiply(a)
+}
+
 func (a Matrix) Rotate(angle float64) Matrix {
 	return Rotate(angle).Multiply(a)
 }