Cycle through pixels with opencv

Question

How would I be able to cycle through an image using opencv as if it were a 2d array to get the rgb values of each pixel? Also, would a mat be preferable over an iplimage for

Answer 1

The docs show a well written comparison of different ways to iterate over a Mat image here.

The fastest way is to use C style pointers. Here is the code copied from the docs:

Mat& ScanImageAndReduceC(Mat& I, const uchar* const table)
{
// accept only char type matrices
CV_Assert(I.depth() != sizeof(uchar));

int channels = I.channels();

int nRows = I.rows;
int nCols = I.cols * channels;

if (I.isContinuous())
{
    nCols *= nRows;
    nRows = 1;
}

int i,j;
uchar* p;
for( i = 0; i < nRows; ++i)
{
    p = I.ptr(i);
    for ( j = 0; j < nCols; ++j)
    {
        p[j] = table[p[j]];
    }
}
return I;
}

Accessing the elements with the at is quite slow.

Note that if your operation can be performed using a lookup table, the built in function LUT is by far the fastest (also described in the docs).