Bug 641426. Part 2.5: Make gfxBlur bail out in all cases where the blur rect is empty. r=tnikkel

gfx/thebes/gfxBlur.cpp

@@ -66,9 +66,6 @@ gfxAlphaBoxBlur::Init(const gfxRect& aRect,
     rect.Outset(aBlurRadius + aSpreadRadius);
     rect.RoundOut();
 
-    if (rect.IsEmpty())
-        return nsnull;
-
     if (aDirtyRect) {
         // If we get passed a dirty rect from layout, we can minimize the
         // shadow size and make painting faster.
@@ -81,6 +78,12 @@ gfxAlphaBoxBlur::Init(const gfxRect& aRect,
         mHasDirtyRect = PR_FALSE;
     }
 
+    // Check rect empty after accounting for aDirtyRect, since that may have
+    // make the rectangle empty. BoxBlurVertical and BoxBlurHorizontal require
+    // that we have a nonzero number of rows and columns.
+    if (rect.IsEmpty())
+        return nsnull;
+
     if (aSkipRect) {
         // If we get passed a skip rect, we can lower the amount of
         // blurring/spreading we need to do. We convert it to nsIntRect to avoid
@@ -138,6 +141,8 @@ BoxBlurHorizontal(unsigned char* aInput,
                   PRInt32 aRows,
                   const nsIntRect& aSkipRect)
 {
+    NS_ASSERTION(aWidth > 0, "Can't handle zero width here");
+
     PRInt32 boxSize = aLeftLobe + aRightLobe + 1;
     PRBool skipRectCoversWholeRow = 0 >= aSkipRect.x &&
                                     aWidth <= aSkipRect.XMost();
@@ -156,6 +161,8 @@ BoxBlurHorizontal(unsigned char* aInput,
         PRInt32 alphaSum = 0;
         for (PRInt32 i = 0; i < boxSize; i++) {
             PRInt32 pos = i - aLeftLobe;
+            // See assertion above; if aWidth is zero, then we would have no
+            // valid position to clamp to.
             pos = NS_MAX(pos, 0);
             pos = NS_MIN(pos, aWidth - 1);
             alphaSum += aInput[aWidth * y + pos];
@@ -174,6 +181,8 @@ BoxBlurHorizontal(unsigned char* aInput,
                 alphaSum = 0;
                 for (PRInt32 i = 0; i < boxSize; i++) {
                     PRInt32 pos = x + i - aLeftLobe;
+                    // See assertion above; if aWidth is zero, then we would have no
+                    // valid position to clamp to.
                     pos = NS_MAX(pos, 0);
                     pos = NS_MIN(pos, aWidth - 1);
                     alphaSum += aInput[aWidth * y + pos];
@@ -205,6 +214,8 @@ BoxBlurVertical(unsigned char* aInput,
                 PRInt32 aRows,
                 const nsIntRect& aSkipRect)
 {
+    NS_ASSERTION(aRows > 0, "Can't handle zero rows here");
+
     PRInt32 boxSize = aTopLobe + aBottomLobe + 1;
     PRBool skipRectCoversWholeColumn = 0 >= aSkipRect.y &&
                                        aRows <= aSkipRect.YMost();
@@ -220,6 +231,8 @@ BoxBlurVertical(unsigned char* aInput,
         PRInt32 alphaSum = 0;
         for (PRInt32 i = 0; i < boxSize; i++) {
             PRInt32 pos = i - aTopLobe;
+            // See assertion above; if aRows is zero, then we would have no
+            // valid position to clamp to.
             pos = NS_MAX(pos, 0);
             pos = NS_MIN(pos, aRows - 1);
             alphaSum += aInput[aWidth * pos + x];
@@ -234,6 +247,8 @@ BoxBlurVertical(unsigned char* aInput,
                 alphaSum = 0;
                 for (PRInt32 i = 0; i < boxSize; i++) {
                     PRInt32 pos = y + i - aTopLobe;
+                    // See assertion above; if aRows is zero, then we would have no
+                    // valid position to clamp to.
                     pos = NS_MAX(pos, 0);
                     pos = NS_MIN(pos, aRows - 1);
                     alphaSum += aInput[aWidth * pos + x];