OpenCV Mat data member access

Question

I've seen a lot of OpenCV code which accesses the data member of a cv::Mat directly. cv::Mat stores the pointer to the data in a unsigned char* data member. Access to the data member looks like:

cv::Mat matUC(3,3,CV_8U)
int rowIdx = 1;
int colIdx = 1;

unsigned char val = matUC.data[ rowIdx * matUC.cols + colIdx]

I'm wondering if this works for a cv::Mat with pixel type other than unsigned char.

cv::Mat matF(3,3,CV_32F)
int rowIdx = 1;
int colIdx = 1;

float val = matF.data[ rowIdx * matF.cols + colIdx];

My understanding would be that a typecast is needed to access the elements correctly. Something like:

float val = ((float*)matF.data)[ rowIdx * matF.cols + colIdx];

I've seen a lot of code which doesn't use a typecast. So my question is: Is the typecast mandatory to access the correct element?

Answer 1

Mat data is an uchar*. If you have a, say, float matrix CV_32FC1, you need to access data as float.

You can do in different ways, not necessarily using casting:

#include <opencv2\opencv.hpp>
using namespace cv;

int main()
{
    cv::Mat matF(3, 3, CV_32F);
    randu(matF, Scalar(0), Scalar(10));

    int rowIdx = 1;
    int colIdx = 1;

    // 1
    float f1 = matF.at<float>(rowIdx, colIdx);

    // 2
    float* fData2 = (float*)matF.data;
    float f2 = fData2[rowIdx*matF.step1() + colIdx];

    // 3
    float* fData3 = matF.ptr<float>(0);
    float f3 = fData3[rowIdx*matF.step1() + colIdx];

    // 4
    float* fData4 = matF.ptr<float>(rowIdx);
    float f4 = fData4[colIdx];

    // 5
    Mat1f mm(matF); // Or directly create like: Mat1f mm(3, 3);
    float f5 = mm(rowIdx, colIdx);

    // f1 == f2 == f3 == f4 == f5

    return 0;
}

Notes

• it's better to use step1() instead of cols when accessing directly data through pointers, since the image may not be continuous. Check here for more details.
• The snippet is adapted from this my other answer