From de3ccecd5bb2874812d40138edecc4d37cfe05fc Mon Sep 17 00:00:00 2001
From: Pietro Gagliardi <pietro10@mac.com>
Date: Mon, 12 Oct 2015 07:58:07 -0400
Subject: [PATCH] Implemented skewing. Will do the test now.

---
 darwin/draw.m |  4 ++--
 matrix.c      | 18 +++++++++++++++++-
 uipriv.h      |  1 +
 unix/draw.c   |  4 ++--
 4 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/darwin/draw.m b/darwin/draw.m
index a8569737..167d3af4 100644
--- a/darwin/draw.m
+++ b/darwin/draw.m
@@ -333,9 +333,9 @@ void uiDrawMatrixRotate(uiDrawMatrix *m, double x, double y, double amount)
 	c2m(&c, m);
 }
 
-void uiDrawMatrixSkew(uiDrawMatrix *m, double x, double y, double amount, double yamount)
+void uiDrawMatrixSkew(uiDrawMatrix *m, double x, double y, double xamount, double yamount)
 {
-	complain("TODO");
+	fallbackSkew(m, x, y, xamount, yamount);
 }
 
 void uiDrawMatrixMultiply(uiDrawMatrix *dest, uiDrawMatrix *src)
diff --git a/matrix.c b/matrix.c
index eda79ed8..937fc136 100644
--- a/matrix.c
+++ b/matrix.c
@@ -12,7 +12,23 @@ void setIdentity(uiDrawMatrix *m)
 	m->M32 = 0;
 }
 
-// TODO skew
+// TODO don't default to fallback functions within the fallback functions
+
+// technique from "Programming with Quartz: 2D and PDF Graphics in Mac OS X"
+// TODO if Windows 7 is ever dropped change this so we can pass in D2D1Tan()
+void fallbackSkew(uiDrawMatrix *m, double x, double y, double xamount, double yamount)
+{
+	uiDrawMatrix t, n;
+
+	setIdentity(&t);
+	fallbackTranslate(&t, x, y);
+	setIdentity(&n);
+	n.M12 = tan(xamount);
+	n.M21 = tan(yamount);
+	fallbackMultiply(&n, &t);
+	// TODO undo the translation?
+	fallbackMultiply(&m, &n);
+}
 
 // see windows/draw.c for more information
 // TODO we don't need to do this if we can bypass the multiplication somehow
diff --git a/uipriv.h b/uipriv.h
index 6c355d2a..247a317f 100644
--- a/uipriv.h
+++ b/uipriv.h
@@ -62,6 +62,7 @@ extern int fromScancode(uintptr_t, uiAreaKeyEvent *);
 
 // matrix.c
 extern void setIdentity(uiDrawMatrix *);
+extern void fallbackSkew(uiDrawMatrix *, double, double, double, double);
 extern void fallbackTranslate(uiDrawMatrix *, double, double);
 extern void scaleCenter(double, double, double *, double *);
 extern void fallbackScale(uiDrawMatrix *, double, double, double, double);
diff --git a/unix/draw.c b/unix/draw.c
index 24c13dda..565a0edc 100644
--- a/unix/draw.c
+++ b/unix/draw.c
@@ -362,9 +362,9 @@ void uiDrawMatrixRotate(uiDrawMatrix *m, double x, double y, double amount)
 	c2m(&c, m);
 }
 
-void uiDrawMatrixSkew(uiDrawMatrix *m, double x, double y, double amount, double yamount)
+void uiDrawMatrixSkew(uiDrawMatrix *m, double x, double y, double xamount, double yamount)
 {
-	complain("TODO");
+	fallbackSkew(m, x, y, xamount, yamount);
 }
 
 void uiDrawMatrixMultiply(uiDrawMatrix *dest, uiDrawMatrix *src)