Pointer access to opencv Mat

cvMat_pointer_access.cpp

Mat img = imread("filename.jpg",CV_LOAD_IMAGE_COLOR);
unsigned char *input = (unsigned char*)(img.data);

int r,g,b;
for(int i = 0;i < img.cols;i++){
    for(int j = 0;j < img.rows;j++){
        b = input[img.cols * j + i ] ;
        g = input[img.cols * j + i + 1];
        r = input[img.cols * j + i + 2];
        }
  }


 if (I.isContinuous()){
  uchar* p = I.data;

  // grayscale
  for( unsigned int i =0; i < ncol*nrows; ++i){
      *p++ = table[*p];
  }
   
  
  // BGR
   for(unsigned int i=0; i<I.rows; i++){
     for(unsigned int j=0; j<I.cols; j++){
       uint8_t b = *p++;
       uint8_t g = *p++;
       uint8_t r = *p++;
     }
   }
   
 }