Merge pull request #547 from Hixie/matrix-math

Avoid using transforms when simple offsets will do.

packages/flutter/lib/painting.dart

@@ -18,3 +18,4 @@ export 'src/painting/colors.dart';
 export 'src/painting/shadows.dart';
 export 'src/painting/text_painter.dart';
 export 'src/painting/text_style.dart';
+export 'src/painting/transforms.dart';

packages/flutter/lib/src/painting/transforms.dart

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:typed_data';
+
+import 'package:vector_math/vector_math_64.dart';
+
+import 'basic_types.dart';
+
+class MatrixUtils {
+  MatrixUtils._();
+
+  /// If the given transform is nothing but a 2D translation, then returns that
+  /// translation as an Offset.
+  ///
+  /// Otherwise, returns null.
+  static Offset getAsTranslation(Matrix4 transform) {
+    Float64List values = transform.storage;
+    // values are stored in column-major order
+    if (values[0] == 1.0 &&
+        values[1] == 0.0 &&
+        values[2] == 0.0 &&
+        values[3] == 0.0 &&
+        values[4] == 0.0 &&
+        values[5] == 1.0 &&
+        values[6] == 0.0 &&
+        values[7] == 0.0 &&
+        values[8] == 0.0 &&
+        values[9] == 0.0 &&
+        values[10] == 1.0 &&
+        values[11] == 0.0 &&
+        values[14] == 0.0 &&
+        values[15] == 1.0) {
+      return new Offset(values[12], values[13]);
+    }
+    return null;
+  }
+
+}
\ No newline at end of file

packages/flutter/lib/src/rendering/proxy_box.dart

@@ -865,8 +865,14 @@ class RenderTransform extends RenderProxyBox {
   }
 
   void paint(PaintingContext context, Offset offset) {
-    if (child != null)
-      context.pushTransform(needsCompositing, offset, _effectiveTransform, super.paint);
+    if (child != null) {
+      Matrix4 transform = _effectiveTransform;
+      Offset childOffset = MatrixUtils.getAsTranslation(transform);
+      if (childOffset == null)
+        context.pushTransform(needsCompositing, offset, transform, super.paint);
+      else
+        super.paint(context, offset + childOffset);
+    }
   }
 
   void applyPaintTransform(Matrix4 transform) {

packages/unit/test/painting/transform_utilities_test.dart

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/painting.dart';
+import 'package:test/test.dart';
+import 'package:vector_math/vector_math_64.dart';
+
+void main() {
+  test("MatrixUtils.getAsTranslation()", () {
+    Matrix4 test;
+    test = new Matrix4.identity();
+    expect(MatrixUtils.getAsTranslation(test), equals(Offset.zero));
+    test = new Matrix4.zero();
+    expect(MatrixUtils.getAsTranslation(test), isNull);
+    test = new Matrix4.rotationX(1.0);
+    expect(MatrixUtils.getAsTranslation(test), isNull);
+    test = new Matrix4.rotationZ(1.0);
+    expect(MatrixUtils.getAsTranslation(test), isNull);
+    test = new Matrix4.translationValues(1.0, 2.0, 0.0);
+    expect(MatrixUtils.getAsTranslation(test), equals(const Offset(1.0, 2.0)));
+    test = new Matrix4.translationValues(1.0, 2.0, 3.0);
+    expect(MatrixUtils.getAsTranslation(test), isNull);
+
+    test = new Matrix4.identity();
+    expect(MatrixUtils.getAsTranslation(test), equals(Offset.zero));
+    test.rotateX(2.0);
+    expect(MatrixUtils.getAsTranslation(test), isNull);
+
+    test = new Matrix4.identity();
+    expect(MatrixUtils.getAsTranslation(test), equals(Offset.zero));
+    test.scale(2.0);
+    expect(MatrixUtils.getAsTranslation(test), isNull);
+
+    test = new Matrix4.identity();
+    expect(MatrixUtils.getAsTranslation(test), equals(Offset.zero));
+    test.translate(2.0, -2.0);
+    expect(MatrixUtils.getAsTranslation(test), equals(const Offset(2.0, -2.0)));
+    test.translate(4.0, 8.0);
+    expect(MatrixUtils.getAsTranslation(test), equals(const Offset(6.0, 6.0)));
+  });
+}