Accessing value at row,col in a Matrix - opencv

Question

I'm trying to access a specific row in a matrix but am having a hard time doing so.
I want to get the value at row j, column i but I don't think my algorithm is correct. I'm using OpenCV's Mat for my matrix and accessing it through the data member.
Here is how I am attempting to access values:
plane.data[i + j*plane.rows]
Where i = the column, j = the row. Is this correct? The Matrix is 1 plane from a YUV matrix.
Any help would be appreciated! Thanks.

Answer 1

No, your are wrong
plane.data[i + j*plane.rows] is not a good way to access pixel. Your pointer must depend on type of the matrix and its depth.
You should use at() operator of the matrix.
To make it simple here is a code sample which access each pixel of a matrix and prints it. It works almost for every matrix type and for any number of channels:
void printMat(const Mat& M){
switch ( (M.dataend-M.datastart) / (M.cols*M.rows*M.channels())){
case sizeof(char):
printMatTemplate<unsigned char>(M,true);
break;
case sizeof(float):
printMatTemplate<float>(M,false);
break;
case sizeof(double):
printMatTemplate<double>(M,false);
break;
}
}
template <typename T>
void printMatTemplate(const Mat& M, bool isInt = true){
if (M.empty()){
printf("Empty Matrix\n");
return;
}
if ((M.elemSize()/M.channels()) != sizeof(T)){
printf("Wrong matrix type. Cannot print\n");
return;
}
int cols = M.cols;
int rows = M.rows;
int chan = M.channels();
char printf_fmt[20];
if (isInt)
sprintf_s(printf_fmt,"%%d,");
else
sprintf_s(printf_fmt,"%%0.5g,");
if (chan > 1){
// Print multi channel array
for (int i = 0; i < rows; i++){
for (int j = 0; j < cols; j++){
printf("(");
const T* Pix = &M.at<T>(i,j);
for (int c = 0; c < chan; c++){
printf(printf_fmt,Pix[c]);
}
printf(")");
}
printf("\n");
}
printf("-----------------\n");
}
else {
// Single channel
for (int i = 0; i < rows; i++){
const T* Mi = M.ptr<T>(i);
for (int j = 0; j < cols; j++){
printf(printf_fmt,Mi[j]);
}
printf("\n");
}
printf("\n");
}
}

Answer 2

I do not think there is anything different between accessing RGB Mat and YUV Mat. Its just the colorspace different.
Please refer to http://opencv.willowgarage.com/wiki/faq#Howtoaccessmatrixelements.3F on how to access each pixel.