Fixes rounding bug in transform. This is needed to avoid rounding

ui/gfx/transform.cc

Index: ui/gfx/transform.cc
diff --git a/ui/gfx/transform.cc b/ui/gfx/transform.cc
index d335166779f4b2bac3e9fe3104a97669640403af..55edb4c3f47a782ec1adef0bfcf4cb4187780ef4 100644
--- a/ui/gfx/transform.cc
+++ b/ui/gfx/transform.cc
@@ -4,6 +4,8 @@
 
 #include "ui/gfx/transform.h"
 
+#include <cmath>
+
 #include "ui/gfx/point.h"
 #include "ui/gfx/rect.h"
 #include "ui/gfx/skia_util.h"
@@ -71,7 +73,8 @@ bool Transform::HasChange() const {
 bool Transform::TransformPoint(gfx::Point* point) {
   SkPoint skp;
   matrix_.mapXY(SkIntToScalar(point->x()), SkIntToScalar(point->y()), &skp);
-  point->SetPoint(static_cast<int>(skp.fX), static_cast<int>(skp.fY));
+  point->SetPoint(static_cast<int>(std::floor(skp.fX)),
+                  static_cast<int>(std::floor(skp.fY)));

[wjmaclean, 2011/05/24 12:11:40]

Just curious ... for positive values, wouldn't this run the danger of converting
0.998 to 0 instead of 1? Do we want rounding or floor/ceil here?

The comment in the unit test mentions only negative numbers, so perhaps I'm
missing how this is used.

[sky, 2011/05/24 15:51:49]

I'm sure we'll have rounding problems until we use doubles every where.

The current code uses static cast, which rounds .998 to 0 too, so that behavior
isn't any different. What is different is that negative numbers are now rounded
down, not up (as static_cast does). Here's an example to show the problems with
the current code. Lets say the root view has one child view at 50,50 with a
scale 5. Because the current code rounds down, anything in the range 46-49 of
the parent gets mapped to 0x0 in the child. This means that any mouse movements
from 46-49 get forwarded to the child even though it doesn't contain the point.
By rounding down instead of up for negative numbers we ensure 46-49 are treated
as -1 and don't get forwarded to the child. Does that makes sense?

   return true;
 }
 
@@ -81,7 +84,8 @@ bool Transform::TransformPointReverse(gfx::Point* point) {
   if (matrix_.invert(&inverse)) {
     SkPoint skp;
     inverse.mapXY(SkIntToScalar(point->x()), SkIntToScalar(point->y()), &skp);
-    point->SetPoint(static_cast<int>(skp.fX), static_cast<int>(skp.fY));
+    point->SetPoint(static_cast<int>(std::floor(skp.fX)),
+                    static_cast<int>(std::floor(skp.fY)));
     return true;
   }
   return false;