Bug 464811. Snap the anchor point to pixels using a method that guarantees the source rect and the subimage rect intersect. r+sr=dbaron

layout/base/nsLayoutUtils.cpp

@@ -2662,6 +2662,14 @@ nsLayoutUtils::GetClosestLayer(nsIFrame* aFrame)
   return aFrame->PresContext()->PresShell()->FrameManager()->GetRootFrame();
 }
 
+/**
+ * Given an image being drawn into an appunit coordinate system, and
+ * a point in that coordinate system, map the point back into image
+ * pixel space.
+ * @param aSize the size of the image, in pixels
+ * @param aDest the rectangle that the image is being mapped into
+ * @param aPt a point in the same coordinate system as the rectangle
+ */
 static gfxPoint
 MapToFloatImagePixels(const nsIntSize& aSize,
                       const nsRect& aDest, const nsPoint& aPt)
@@ -2670,6 +2678,22 @@ MapToFloatImagePixels(const nsIntSize& aSize,
                   (gfxFloat(aPt.y - aDest.y)*aSize.height)/aDest.height);
 }
 
+/**
+ * Given an image being drawn into an pixel-based coordinate system, and
+ * a point in image space, map the point into the pixel-based coordinate
+ * system.
+ * @param aSize the size of the image, in pixels
+ * @param aDest the rectangle that the image is being mapped into
+ * @param aPt a point in image space
+ */
+static gfxPoint
+MapToFloatUserPixels(const nsIntSize& aSize,
+                     const gfxRect& aDest, const gfxPoint& aPt)
+{
+  return gfxPoint(aPt.x*aDest.size.width/aSize.width + aDest.pos.x,
+                  aPt.y*aDest.size.height/aSize.height + aDest.pos.y);
+}
+
 /* static */ nsresult
 nsLayoutUtils::DrawImage(nsIRenderingContext* aRenderingContext,
                          imgIContainer*       aImage,
@@ -2733,15 +2757,22 @@ nsLayoutUtils::DrawImage(nsIRenderingContext* aRenderingContext,
   // device unit!
   gfxPoint anchorPoint(aAnchor.x/appUnitsPerDevPixel,
                        aAnchor.y/appUnitsPerDevPixel);
+  gfxPoint imageSpaceAnchorPoint =
+    MapToFloatImagePixels(imageSize, aDest, aAnchor);
   gfxMatrix currentMatrix = ctx->CurrentMatrix();
+
   gfxRect finalFillRect = fill;
   if (didSnap) {
     NS_ASSERTION(!currentMatrix.HasNonAxisAlignedTransform(),
                  "How did we snap, then?");
-    anchorPoint.x = fill.pos.x + 
+    imageSpaceAnchorPoint.Round();
-        (anchorPoint.x - devPixelFill.pos.x)*fill.size.width/devPixelFill.size.width;
+    anchorPoint = imageSpaceAnchorPoint;
-    anchorPoint.y = fill.pos.y +
+    gfxRect devPixelDest(aDest.x/appUnitsPerDevPixel,
-        (anchorPoint.y - devPixelFill.pos.y)*fill.size.height/devPixelFill.size.height;
+                         aDest.y/appUnitsPerDevPixel,
+                         aDest.width/appUnitsPerDevPixel,
+                         aDest.height/appUnitsPerDevPixel);
+    anchorPoint = MapToFloatUserPixels(imageSize, devPixelDest, anchorPoint);
+    anchorPoint = currentMatrix.Transform(anchorPoint);
     anchorPoint.Round();
 
     // This form of Transform is safe to call since non-axis-aligned
@@ -2759,9 +2790,6 @@ nsLayoutUtils::DrawImage(nsIRenderingContext* aRenderingContext,
   // to be aligned perfectly with pixel boundaries or the choice of
   // dirty rect will affect the values of rendered pixels.
 
-  gfxPoint imageSpaceAnchorPoint =
-    MapToFloatImagePixels(imageSize, aDest, aAnchor);
-  imageSpaceAnchorPoint.Round();
   gfxFloat scaleX = imageSize.width*appUnitsPerDevPixel/aDest.width;
   gfxFloat scaleY = imageSize.height*appUnitsPerDevPixel/aDest.height;
   if (didSnap) {

layout/reftests/bugs/433640-1-ref.html

@@ -28,8 +28,8 @@
 <div>
 <div class="cell"><p>32    x 32</p><div style="width:32px; background-position:-16px -16px;"  class="image"></div></div>
 <div class="cell"><p>32.1  x 32</p><div style="width:32px; background-position:-16px -16px;"  class="image"></div></div>
-<div class="cell"><p>32.5  x 32</p><div style="width:33px; background-position:-15px -16px;"  class="image"></div></div>
+<div class="cell"><p>32.5  x 32</p><div style="width:33px; background-position:-16px -16px;"  class="image"></div></div>
-<div class="cell"><p>32.8  x 32</p><div style="width:33px; background-position:-15px -16px;"  class="image"></div></div>
+<div class="cell"><p>32.8  x 32</p><div style="width:33px; background-position:-16px -16px;"  class="image"></div></div>
 </div>
 
 <div>
@@ -42,8 +42,8 @@
 <div>
 <div class="cell"><p>32 x 32   </p><div style="height:32px; background-position:-16px -16px;" class="image"></div></div>
 <div class="cell"><p>32 x 32.1 </p><div style="height:32px; background-position:-16px -16px;" class="image"></div></div>
-<div class="cell"><p>32 x 32.5 </p><div style="height:33px; background-position:-16px -15px;" class="image"></div></div>
+<div class="cell"><p>32 x 32.5 </p><div style="height:33px; background-position:-16px -16px;" class="image"></div></div>
-<div class="cell"><p>32 x 32.8 </p><div style="height:33px; background-position:-16px -15px;" class="image"></div></div>
+<div class="cell"><p>32 x 32.8 </p><div style="height:33px; background-position:-16px -16px;" class="image"></div></div>
 </div>
 
 </body></html>

layout/reftests/bugs/464811-1-ref.html

@@ -0,0 +1,7 @@
+<!DOCTYPE HTML>
+<html>
+<body>
+<div style="background:url(mozilla-banner.gif); width:600px; height:58px;"></div>
+<div style="margin-left: 100px; background:url(mozilla-banner.gif) -100px 0px; width:1px; height:58px;"></div>
+</body>
+</html>

layout/reftests/bugs/464811-1.html

@@ -0,0 +1,7 @@
+<!DOCTYPE HTML>
+<html>
+<body>
+<div style="background:url(mozilla-banner.gif); width:600px; height:58px;"></div>
+<div style="margin-left: 100.4px; background:url(mozilla-banner.gif) -100.4px 0px; width:0.2px; height:58px;"></div>
+</body>
+</html>

layout/reftests/bugs/reftest.list

@@ -960,3 +960,4 @@ fails-if(MOZ_WIDGET_TOOLKIT=="cocoa") == 456147.xul 456147-ref.html # bug 456147
 fails == 461512-1.html 461512-1-ref.html # Bug 461512
 == 463204-1.html 463204-1-ref.html
 == 463217-1.xul 463217-1-ref.xul
+== 464811-1.html 464811-1-ref.html