Fix border radius glitch.

Source/core/rendering/RenderBoxModelObject.cpp

Index: Source/core/rendering/RenderBoxModelObject.cpp
diff --git a/Source/core/rendering/RenderBoxModelObject.cpp b/Source/core/rendering/RenderBoxModelObject.cpp
index 98d59b66f675a16693ed04c97fbb793bffd13d93..26ae05575d8ca6ce1c6bd1e994d27c3226cab677 100644
--- a/Source/core/rendering/RenderBoxModelObject.cpp
+++ b/Source/core/rendering/RenderBoxModelObject.cpp
@@ -1993,21 +1993,6 @@ void RenderBoxModelObject::drawBoxSideFromPath(GraphicsContext* graphicsContext,
     graphicsContext->drawRect(pixelSnappedIntRect(borderRect));
 }
 
-static void findInnerVertex(const FloatPoint& outerCorner, const FloatPoint& innerCorner, const FloatPoint& centerPoint, FloatPoint& result)
-{
-    // If the line between outer and inner corner is towards the horizontal, intersect with a vertical line through the center,
-    // otherwise with a horizontal line through the center. The points that form this line are arbitrary (we use 0, 100).
-    // Note that if findIntersection fails, it will leave result untouched.
-    float diffInnerOuterX = fabs(innerCorner.x() - outerCorner.x());
-    float diffInnerOuterY = fabs(innerCorner.y() - outerCorner.y());
-    float diffCenterOuterX = fabs(centerPoint.x() - outerCorner.x());
-    float diffCenterOuterY = fabs(centerPoint.y() - outerCorner.y());
-    if (diffInnerOuterY * diffCenterOuterX < diffCenterOuterY * diffInnerOuterX)
-        findIntersection(outerCorner, innerCorner, FloatPoint(centerPoint.x(), 0), FloatPoint(centerPoint.x(), 100), result);
-    else
-        findIntersection(outerCorner, innerCorner, FloatPoint(0, centerPoint.y()), FloatPoint(100, centerPoint.y()), result);
-}
-
 void RenderBoxModelObject::clipBorderSidePolygon(GraphicsContext* graphicsContext, const RoundedRect& outerBorder, const RoundedRect& innerBorder,
                                                  BoxSide side, bool firstEdgeMatches, bool secondEdgeMatches)
 {
@@ -2040,10 +2025,24 @@ void RenderBoxModelObject::clipBorderSidePolygon(GraphicsContext* graphicsContex
         quad[3] = outerRect.maxXMinYCorner();
 
         if (!innerBorder.radii().topLeft().isZero())
-            findInnerVertex(outerRect.minXMinYCorner(), innerRect.minXMinYCorner(), centerPoint, quad[1]);
+            findIntersection(quad[0], quad[1],
+                FloatPoint(
+                    quad[1].x() + innerBorder.radii().topLeft().width(),
+                    quad[1].y()),
+                FloatPoint(
+                    quad[1].x(),
+                    quad[1].y() + innerBorder.radii().topLeft().height()),
+                quad[1]);
 
         if (!innerBorder.radii().topRight().isZero())
-            findInnerVertex(outerRect.maxXMinYCorner(), innerRect.maxXMinYCorner(), centerPoint, quad[2]);
+            findIntersection(quad[3], quad[2],
+                FloatPoint(
+                    quad[2].x() - innerBorder.radii().topRight().width(),
+                    quad[2].y()),
+                FloatPoint(
+                    quad[2].x(),
+                    quad[2].y() + innerBorder.radii().topRight().height()),
+                quad[2]);
         break;
 
     case BSLeft:
@@ -2053,10 +2052,24 @@ void RenderBoxModelObject::clipBorderSidePolygon(GraphicsContext* graphicsContex
         quad[3] = outerRect.minXMaxYCorner();
 
         if (!innerBorder.radii().topLeft().isZero())
-            findInnerVertex(outerRect.minXMinYCorner(), innerRect.minXMinYCorner(), centerPoint, quad[1]);
+            findIntersection(quad[0], quad[1],
+                FloatPoint(
+                    quad[1].x() + innerBorder.radii().topLeft().width(),
+                    quad[1].y()),
+                FloatPoint(
+                    quad[1].x(),
+                    quad[1].y() + innerBorder.radii().topLeft().height()),
+                quad[1]);
 
         if (!innerBorder.radii().bottomLeft().isZero())
-            findInnerVertex(outerRect.minXMaxYCorner(), innerRect.minXMaxYCorner(), centerPoint, quad[2]);
+            findIntersection(quad[3], quad[2],
+                FloatPoint(
+                    quad[2].x() + innerBorder.radii().bottomLeft().width(),
+                    quad[2].y()),
+                FloatPoint(
+                    quad[2].x(),
+                    quad[2].y() - innerBorder.radii().bottomLeft().height()),
+                quad[2]);
         break;
 
     case BSBottom:
@@ -2066,10 +2079,24 @@ void RenderBoxModelObject::clipBorderSidePolygon(GraphicsContext* graphicsContex
         quad[3] = outerRect.maxXMaxYCorner();
 
         if (!innerBorder.radii().bottomLeft().isZero())
-            findInnerVertex(outerRect.minXMaxYCorner(), innerRect.minXMaxYCorner(), centerPoint, quad[1]);
+            findIntersection(quad[0], quad[1],
+                FloatPoint(
+                    quad[1].x() + innerBorder.radii().bottomLeft().width(),
+                    quad[1].y()),
+                FloatPoint(
+                    quad[1].x(),
+                    quad[1].y() - innerBorder.radii().bottomLeft().height()),
+                quad[1]);
 
         if (!innerBorder.radii().bottomRight().isZero())
-            findInnerVertex(outerRect.maxXMaxYCorner(), innerRect.maxXMaxYCorner(), centerPoint, quad[2]);
+            findIntersection(quad[3], quad[2],
+                FloatPoint(
+                    quad[2].x() - innerBorder.radii().bottomRight().width(),
+                    quad[2].y()),
+                FloatPoint(
+                    quad[2].x(),
+                    quad[2].y() - innerBorder.radii().bottomRight().height()),
+                quad[2]);
         break;
 
     case BSRight:
@@ -2079,10 +2106,24 @@ void RenderBoxModelObject::clipBorderSidePolygon(GraphicsContext* graphicsContex
         quad[3] = outerRect.maxXMaxYCorner();
 
         if (!innerBorder.radii().topRight().isZero())
-            findInnerVertex(outerRect.maxXMinYCorner(), innerRect.maxXMinYCorner(), centerPoint, quad[1]);
+            findIntersection(quad[0], quad[1],
+                FloatPoint(
+                    quad[1].x() - innerBorder.radii().topRight().width(),
+                    quad[1].y()),
+                FloatPoint(
+                    quad[1].x(),
+                    quad[1].y() + innerBorder.radii().topRight().height()),
+                quad[1]);
 
         if (!innerBorder.radii().bottomRight().isZero())
-            findInnerVertex(outerRect.maxXMaxYCorner(), innerRect.maxXMaxYCorner(), centerPoint, quad[2]);
+            findIntersection(quad[3], quad[2],
+                FloatPoint(
+                    quad[2].x() - innerBorder.radii().bottomRight().width(),
+                    quad[2].y()),
+                FloatPoint(
+                    quad[2].x(),
+                    quad[2].y() - innerBorder.radii().bottomRight().height()),
+                quad[2]);
         break;
     }
 
@@ -2093,22 +2134,43 @@ void RenderBoxModelObject::clipBorderSidePolygon(GraphicsContext* graphicsContex
         return;
     }
 
-    // Square off the end which shouldn't be affected by antialiasing, and clip.
+    // If antialiasing settings for the first edge and second edge is different,
+    // they have to be addressed separately. We do this by breaking the quad into
+    // two parallelograms, made by moving quad[1] and quad[2].
+    float ax = quad[1].x() - quad[0].x();
+    float ay = quad[1].y() - quad[0].y();
+    float bx = quad[2].x() - quad[1].x();
+    float by = quad[2].y() - quad[1].y();
+    float cx = quad[3].x() - quad[2].x();
+    float cy = quad[3].y() - quad[2].y();
+    float dx = quad[0].x() - quad[3].x();
+    float dy = quad[0].y() - quad[3].y();

[Julien - ping for review, 2013/06/10 19:50:31]

dx / dy are unused.

[kouhei (in TOK), 2013/06/11 21:02:53]

Done.

+
+    const static float kEpsilon = 1e-2f;

[Julien - ping for review, 2013/06/10 19:50:31]

Maybe worth sending an email to say that Blink should match Chrome for constants
as there is no entry in the style guide. It's our default policy but it would be
nice to get it written for future reviews.

This shouldn't block this patch though.

+    float r1, r2;
+    if (fabsf(bx) < kEpsilon && fabsf(by) < kEpsilon) {
+        // The quad was actually a triangle.
+        r1 = r2 = 1.0f;
+    } else {
+        // Extend parallelogram a bit to hide calculation error
+        const static float kExtendFill = 1e-2f;
+
+        r1 = (-ax*by + ay*bx) / (cx*by - cy*bx) + kExtendFill;
+        r2 = (-cx*by + cy*bx) / (ax*by - ay*bx) + kExtendFill;

[Julien - ping for review, 2013/06/10 19:50:31]

This is a style violation: there should be a space before and after '*'.

[kouhei (in TOK), 2013/06/11 21:02:53]

Done.

+    }
+
     FloatPoint firstQuad[4];
     firstQuad[0] = quad[0];
     firstQuad[1] = quad[1];
-    firstQuad[2] = side == BSTop || side == BSBottom ? FloatPoint(quad[3].x(), quad[2].y())
-        : FloatPoint(quad[2].x(), quad[3].y());
+    firstQuad[2] = FloatPoint(quad[3].x() + r2*ax, quad[3].y() + r2*ay);

[Julien - ping for review, 2013/06/10 19:50:31]

Ditto.

[kouhei (in TOK), 2013/06/11 21:02:53]

Done.

     firstQuad[3] = quad[3];
     graphicsContext->clipConvexPolygon(4, firstQuad, !firstEdgeMatches);
 
     FloatPoint secondQuad[4];
     secondQuad[0] = quad[0];
-    secondQuad[1] = side == BSTop || side == BSBottom ? FloatPoint(quad[0].x(), quad[1].y())
-        : FloatPoint(quad[1].x(), quad[0].y());
+    secondQuad[1] = FloatPoint(quad[0].x() - r1*cx, quad[0].y() - r1*cy);

[Julien - ping for review, 2013/06/10 19:50:31]

Ditto.

[kouhei (in TOK), 2013/06/11 21:02:53]

Done.

     secondQuad[2] = quad[2];
     secondQuad[3] = quad[3];
-    // Antialiasing affects the second side.
     graphicsContext->clipConvexPolygon(4, secondQuad, !secondEdgeMatches);
 }