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?

Note that `data[row * cols + col]` only works if the matrix `isContinuous()`. If you don't have that information, the safest way is to do `matrix.ptr(row)[col]`. — ChronoTrigger, Dec 02 '15 at 12:20

score 12 · Accepted Answer · edited May 23 '17 at 12:17

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

Is a Mat created like this `cv::Mat matF(3,3,CV_32F)` always guaranteed to be continuous? — tisch, Dec 02 '15 at 12:26
Yes, have a look [here](http://stackoverflow.com/a/33674655/5008845) for more details. — Miki, Dec 02 '15 at 12:27
This answers my question completely. Thanks for your effort! — tisch, Dec 02 '15 at 13:07

OpenCV Mat data member access

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