Fix KNN background subtraction not honoring width step.

modules/video/src/bgfg_KNN.cpp

@@ -458,10 +458,8 @@ CV_INLINE void
         uchar nShadowDetection
         )
 {
-    int size=_src.rows*_src.cols;
     int nchannels = CV_MAT_CN(_src.type());
-    const uchar* pDataCurrent=_src.ptr(0);
-    uchar* pDataOutput=_dst.ptr(0);
+
     //model
     uchar* m_aModel=_bgmodel.ptr(0);
     uchar* m_nNextLongUpdate=_nNextLongUpdate.ptr(0);
@@ -509,10 +507,12 @@ CV_INLINE void
     if (_nLongCounter >= m_nLongUpdate) _nLongCounter = 0;
 
     //go through the image
-    for (long i=0;i<size;i++)
+    long i = 0;
+    for (long y = 0; y < _src.rows; y++)
+    {
+        for (long x = 0; x < _src.cols; x++)
         {
-        const uchar* data=pDataCurrent;
-        pDataCurrent=pDataCurrent+nchannels;
+            const uchar* data = _src.ptr(y, x);
 
             //update model+ background subtract
             uchar include=0;
@@ -539,18 +539,19 @@ CV_INLINE void
             {
                 case 0:
                     //foreground
-                (* pDataOutput)=255;
+                    *_dst.ptr(y, x) = 255;
                     break;
                 case 1:
                     //background
-                (* pDataOutput)=0;
+                    *_dst.ptr(y, x) = 0;
                     break;
                 case 2:
                     //shadow
-                (* pDataOutput)=nShadowDetection;
+                    *_dst.ptr(y, x) = nShadowDetection;
                     break;
             }
-        pDataOutput++;
+            i++;
+        }
     }
 };