From b77c5f2881a00643f491a7bad138520bb2b5ec73 Mon Sep 17 00:00:00 2001
From: Michael Fogleman <fogleman@gmail.com>
Date: Sat, 20 Feb 2016 14:53:33 -0500
Subject: [PATCH] ShearAbout

---
 README.md  |  5 +++--
 context.go |  4 ++++
 matrix.go  | 15 +++++++++++++--
 3 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/README.md b/README.md
index 1e8b921..52a935a 100644
--- a/README.md
+++ b/README.md
@@ -101,10 +101,11 @@ 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)
+ScaleAbout(sx, sy, x, y float64)
+RotateAbout(angle, x, y float64)
+ShearAbout(sx, sy, x, y float64)
 TransformPoint(x, y float64) (tx, ty float64)
 InvertY()
 Push()
diff --git a/context.go b/context.go
index 6abb827..3d39527 100644
--- a/context.go
+++ b/context.go
@@ -404,6 +404,10 @@ func (dc *Context) Shear(x, y float64) {
 	dc.matrix = dc.matrix.Shear(x, y)
 }
 
+func (dc *Context) ShearAbout(sx, sy, x, y float64) {
+	dc.matrix = dc.matrix.ShearAbout(sx, sy, x, y)
+}
+
 func (dc *Context) TransformPoint(x, y float64) (tx, ty float64) {
 	return dc.matrix.TransformPoint(x, y)
 }
diff --git a/matrix.go b/matrix.go
index 601fb45..d517839 100644
--- a/matrix.go
+++ b/matrix.go
@@ -56,12 +56,19 @@ func RotateAbout(angle, x, y float64) Matrix {
 
 func Shear(x, y float64) Matrix {
 	return Matrix{
-		1, x,
-		y, 1,
+		1, y,
+		x, 1,
 		0, 0,
 	}
 }
 
+func ShearAbout(sx, sy, x, y float64) Matrix {
+	a := Translate(-x, -y)
+	b := Shear(sx, sy)
+	c := Translate(x, y)
+	return a.Multiply(b).Multiply(c)
+}
+
 func (a Matrix) Multiply(b Matrix) Matrix {
 	return Matrix{
 		a.XX*b.XX + a.YX*b.XY,
@@ -108,3 +115,7 @@ func (a Matrix) RotateAbout(angle, x, y float64) Matrix {
 func (a Matrix) Shear(x, y float64) Matrix {
 	return Shear(x, y).Multiply(a)
 }
+
+func (a Matrix) ShearAbout(sx, sy, x, y float64) Matrix {
+	return ShearAbout(sx, sy, x, y).Multiply(a)
+}