Fix distance field edge detection.

src/core/SkDistanceFieldGen.cpp

Index: src/core/SkDistanceFieldGen.cpp
diff --git a/src/core/SkDistanceFieldGen.cpp b/src/core/SkDistanceFieldGen.cpp
index cd469a4d953077828fbaf394cd30d000c9557002..056472bd29d03ad1e62c7bb61a83e175a6df5ecd 100755
--- a/src/core/SkDistanceFieldGen.cpp
+++ b/src/core/SkDistanceFieldGen.cpp
@@ -14,16 +14,37 @@ struct DFData {
     SkPoint fDistVector; // distance vector to nearest (so far) edge texel
 };
 

[robertphillips, 2014/03/12 16:40:07]

Name this guy EdgeFlags? NeighborFlags might be clearer.
And shouldn't these be kLeft_EdgeFlag?

+enum {
+    kEdgeFlag_Left        = 0x01,
+    kEdgeFlag_Right       = 0x02,
+    kEdgeFlag_TopLeft     = 0x04,
+    kEdgeFlag_Top         = 0x08,
+    kEdgeFlag_TopRight    = 0x10,
+    kEdgeFlag_BottomLeft  = 0x20,
+    kEdgeFlag_Bottom      = 0x40,
+    kEdgeFlag_BottomRight = 0x80,
+    kEdgeFlag_All         = 0xff
+};
+
 // We treat an "edge" as a place where we cross from a texel >= 128 to a texel < 128,
 // or vice versa. This means we just need to check if the MSBs are different.

[robertphillips, 2014/03/12 16:40:07]

// 'edgeFlags' is used to limit the directions in which we test in order to
prevent indexing off the table.

-static bool found_edge(const unsigned char* imagePtr, int width) {
+static bool found_edge(const unsigned char* imagePtr, int width, int edgeFlags) {
+    // the order of these should match the edge flags above

[robertphillips, 2014/03/12 16:40:07]

swap 8 out for kEdgeFlagCount? Add kNum8ConnectedNeighbors?
Maybe add a static assert somewhere (e.g., kEdgeFlagCount ==
kNum8ConnectedNeighbors)?

     const int offsets[8] = {-1, 1, -width-1, -width, -width+1, width-1, width, width+1 };
 
     // search for an edge
-    int checkVal = *imagePtr >> 7;
+    unsigned char currVal = *imagePtr >> 7;
     for (int i = 0; i < 8; ++i) {
-        const unsigned char* checkPtr = imagePtr + offsets[i];
-        if (checkVal ^ (*checkPtr >> 7)) {
+        unsigned char checkVal;
+        if ((1 << i) & edgeFlags) {
+            const unsigned char* checkPtr = imagePtr + offsets[i];
+            checkVal = *checkPtr >> 7;
+        } else {
+            checkVal = 0;
+        }
+        SkASSERT(checkVal == 0 || checkVal == 1);
+        SkASSERT(currVal == 0 || currVal == 1);
+        if (checkVal != currVal) {
             return true;
         }
     }
@@ -46,8 +67,20 @@ static void init_glyph_data(DFData* data, unsigned char* edges, const unsigned c
             } else {
                 data->fAlpha = (*image)*0.00392156862f;  // 1/255
             }
-            if (i > 0 && i < imageWidth-1 && j > 0 && j < imageHeight-1 &&
-                found_edge(image, imageWidth)) {
+            int checkMask = kEdgeFlag_All;

[robertphillips, 2014/03/12 16:40:07]

Would it be helpful to add helper values like "kAllLeft"? Don't have to though -
it is clear enough as is.

+            if (i == 0) {
+                checkMask &= ~(kEdgeFlag_Left|kEdgeFlag_TopLeft|kEdgeFlag_BottomLeft);
+            }
+            if (i == imageWidth-1) {
+                checkMask &= ~(kEdgeFlag_Right|kEdgeFlag_TopRight|kEdgeFlag_BottomRight);
+            }
+            if (j == 0) {
+                checkMask &= ~(kEdgeFlag_TopLeft|kEdgeFlag_Top|kEdgeFlag_TopRight);
+            }
+            if (j == imageHeight-1) {
+                checkMask &= ~(kEdgeFlag_BottomLeft|kEdgeFlag_Bottom|kEdgeFlag_BottomRight);
+            }
+            if (found_edge(image, imageWidth, checkMask)) {
                 *edges = 255;  // using 255 makes for convenient debug rendering
             }
             ++data;
@@ -268,6 +301,9 @@ static void B2(DFData* curr, int width) {
     }
 }
 

[robertphillips, 2014/03/12 16:40:07]

// Enable this to ...?

+#define DUMP_EDGE 0
+
+#if !DUMP_EDGE
 static unsigned char pack_distance_field_val(float dist, float distanceMagnitude) {
     if (dist <= -distanceMagnitude) {
         return 255;
@@ -277,6 +313,7 @@ static unsigned char pack_distance_field_val(float dist, float distanceMagnitude
         return (unsigned char)((distanceMagnitude-dist)*128.0f/distanceMagnitude);
     }
 }
+#endif
 
 // assumes an 8-bit image and distance field
 bool SkGenerateDistanceFieldFromImage(unsigned char* distanceField,
@@ -382,14 +419,19 @@ bool SkGenerateDistanceFieldFromImage(unsigned char* distanceField,
     unsigned char *dfPtr = distanceField;
     for (int j = 1; j < dataHeight-1; ++j) {
         for (int i = 1; i < dataWidth-1; ++i) {
+#if DUMP_EDGE
+            unsigned char val = (currData->fAlpha >= 0.5f) ? 255 : 0;

[robertphillips, 2014/03/12 16:40:07]

newline after { and before } ?

+            if (*currEdge) { val = 128; }
+            *dfPtr++ = val;
+#else
             float dist;
             if (currData->fAlpha > 0.5f) {
                 dist = -SkScalarSqrt(currData->fDistSq);
             } else {
                 dist = SkScalarSqrt(currData->fDistSq);
             }
-
             *dfPtr++ = pack_distance_field_val(dist, (float)distanceMagnitude);
+#endif
             ++currData;
             ++currEdge;
         }