Update thinning.cpp

Author: sturkmen72

modules/ximgproc/src/thinning.cpp

@@ -106,17 +106,19 @@ static void thinningIteration(Mat &img, Mat &marker, const uint8_t* const lut) {
         if (i == 0 || j == 0 || i == rows - 1 || j == cols - 1) { return; }
 
         auto ptr = img.ptr(i, j); // p1
-        uchar p2 = ptr[-cols];
-        uchar p3 = ptr[-cols + 1];
-        uchar p4 = ptr[1];
-        uchar p5 = ptr[cols + 1];
-        uchar p6 = ptr[cols];
-        uchar p7 = ptr[cols - 1];
-        uchar p8 = ptr[-1];
-        uchar p9 = ptr[-cols - 1];
+        if (ptr[0]) {
+            uchar p2 = ptr[-cols];
+            uchar p3 = ptr[-cols + 1];
+            uchar p4 = ptr[1];
+            uchar p5 = ptr[cols + 1];
+            uchar p6 = ptr[cols];
+            uchar p7 = ptr[cols - 1];
+            uchar p8 = ptr[-1];
+            uchar p9 = ptr[-cols - 1];
 
-        int neighbors = p9 | (p2 << 1) | (p3 << 2) | (p4 << 3) | (p5 << 4) | (p6 << 5) | (p7 << 6) | (p8 << 7);
-        value = lut[neighbors];
+            int neighbors = p9 | (p2 << 1) | (p3 << 2) | (p4 << 3) | (p5 << 4) | (p6 << 5) | (p7 << 6) | (p8 << 7);
+            value = lut[neighbors];
+        }        
     });
 
     img &= marker;