Fix size of rotated text with FreeType.

This generalizes and shares the solution found for a similar issue with
GDI. The issue is that the text size is applied early, and the rest of
the transformation late. This allows us to isolate and independently
control the text size from the rest of the transformation.

R=reed@google.com

Review URL: https://codereview.chromium.org/213153006

git-svn-id: http://skia.googlecode.com/svn/trunk@14201 2bbb7eff-a529-9590-31e7-b0007b416f81

1 files changed, 26 insertions, 0 deletions

diff --git a/src/utils/SkMatrix22.h b/src/utils/SkMatrix22.h
new file mode 100644
index 0000000000..d0f4ed3155
--- /dev/null
+++ b/src/utils/SkMatrix22.h
@@ -0,0 +1,26 @@
+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkPoint.h"
+
+class SkMatrix;
+
+/** Find the Givens matrix G, which is the rotational matrix
+ *  that rotates the vector h to the positive hoizontal axis.
+ *  G * h = [hypot(h), 0]
+ *
+ *  This is equivalent to
+ *
+ *  SkScalar r = h.length();
+ *  SkScalar r_inv = r ? SkScalarInvert(r) : 0;
+ *  h.scale(r_inv);
+ *  G->setSinCos(-h.fY, h.fX);
+ *
+ *  but has better numerical stability by using (partial) hypot,
+ *  and saves a multiply by not computing r.
+ */
+void SkComputeGivensRotation(const SkVector& h, SkMatrix* G);