Bug 633627 Create shadow bitmap in device space r=joe

layout/base/nsCSSRendering.cpp

@@ -4199,13 +4199,22 @@ nsContextBoxBlur::Init(const nsRect& aRect, nscoord aSpreadRadius,
     return nsnull;
   }
 
+  // We convert our radii and our buffer to device space coordinates. When
+  // device space is smaller than our content space, we would otherwise be
+  // oversampling.
   gfxIntSize blurRadius = ComputeBlurRadius(aBlurRadius, aAppUnitsPerDevPixel);
-  PRInt32 spreadRadius = NS_MIN(PRInt32(aSpreadRadius / aAppUnitsPerDevPixel),
-                                PRInt32(MAX_SPREAD_RADIUS));
+  aDestinationCtx->UserToDevice(blurRadius);
+
+  PRInt32 spreadRadiusScalar = NS_MIN(
+    PRInt32(aSpreadRadius / aAppUnitsPerDevPixel),
+    PRInt32(MAX_SPREAD_RADIUS));
+  gfxIntSize spreadRadius(spreadRadiusScalar, spreadRadiusScalar);
+  aDestinationCtx->UserToDevice(spreadRadius);
+
   mDestinationCtx = aDestinationCtx;
 
   // If not blurring, draw directly onto the destination device
-  if (blurRadius.width <= 0 && blurRadius.height <= 0 && spreadRadius <= 0 &&
+  if (blurRadius.width <= 0 && blurRadius.height <= 0 && spreadRadiusScalar <= 0 &&
       !(aFlags & FORCE_MASK)) {
     mContext = aDestinationCtx;
     return mContext;
@@ -4214,13 +4223,29 @@ nsContextBoxBlur::Init(const nsRect& aRect, nscoord aSpreadRadius,
   // Convert from app units to device pixels
   gfxRect rect = nsLayoutUtils::RectToGfxRect(aRect, aAppUnitsPerDevPixel);
 
+  gfxRect deviceRect = mDestinationCtx->UserToDevice(rect);
+
   gfxRect dirtyRect =
     nsLayoutUtils::RectToGfxRect(aDirtyRect, aAppUnitsPerDevPixel);
   dirtyRect.RoundOut();
 
+  gfxRect deviceDirtyRect = mDestinationCtx->UserToDevice(dirtyRect);
+
+  gfxRect deviceSkipRect;
+  gfxRect* deviceSkipRectPtr = NULL;
+  if (aSkipRect) {
+    deviceSkipRect = mDestinationCtx->UserToDevice(*aSkipRect);
+    deviceSkipRectPtr = &deviceSkipRect;
+  }
+
   // Create the temporary surface for blurring
-  mContext = blur.Init(rect, gfxIntSize(spreadRadius, spreadRadius),
-                       blurRadius, &dirtyRect, aSkipRect);
+  mContext = blur.Init(deviceRect, spreadRadius, blurRadius,
+                       &deviceDirtyRect, deviceSkipRectPtr);
+
+  gfxMatrix matrix = aDestinationCtx->CurrentMatrix();
+  if (mContext) {
+    mContext->Scale(matrix.xx, matrix.yy);
+  }
   return mContext;
 }
 
@@ -4230,6 +4255,11 @@ nsContextBoxBlur::DoPaint()
   if (mContext == mDestinationCtx)
     return;
 
+  // Since blurred mask is in device coordinates, the scales for painting the
+  // mask should be identity.
+  gfxContextMatrixAutoSaveRestore save(mDestinationCtx);
+  gfxMatrix matrix = mDestinationCtx->CurrentMatrix();
+  mDestinationCtx->Scale(1 / matrix.xx, 1 / matrix.yy);
   blur.Paint(mDestinationCtx);
 }