how do I calculate the stride for a cv::Mat

Question

I would like to know how would I calculate the stride of a cv:Mat.

I have updated the code, where I need to calculate the stride, I don't know what's wrong to calculate the projective transformation. I get a cv::Mat then copy it to unsigned int array, then do the transformation on it, then get back a cv::Mat to be shown.

cv::Mat3b  srcIm;
srcIm = imread("15016889798859437.jpg");
cv::Mat3b  srcIm, edges;
srcIm = imread("Lenna.png");
   
image_t src, dst;
int n_bytes_for_each_row = srcIm.step;

src.width = srcIm.rows;
src.height = srcIm.cols;
 
src.stride = n_bytes_for_each_row;
 
dst.width = 350;
dst.height = 350;
dst.stride = n_bytes_for_each_row;
dst.pixels = new unsigned int[350*350];
   
std::unique_ptr<unsigned int[]> videoFrame(new unsigned int[srcIm.rows * srcIm.cols]);

std::transform(srcIm.begin(), srcIm.end(), videoFrame.get()
    , [](cv::Vec3b const& v) {
    return v[0] | (v[1] << 8) | (v[2] << 16);
});

vertex_t vert[4];
vert[0].u = 0;
vert[0].v = 0;
vert[0].x = 0;
vert[0].y = 0;

vert[1].u = 50;
vert[1].v = 0;
vert[1].x = 350;
vert[1].y = 0;
//
vert[2].u = 150;
vert[2].v = 350;
vert[2].x = 350;
vert[2].y = 350;
//
vert[3].u = 0;
vert[3].v = 50;
vert[3].x = 0;
vert[3].y = 350;

src.pixels = videoFrame.get();
perspective_transform(&src, &dst, vert);

cv::Mat videoFrameMat(350, 350, CV_32S, dst.pixels);

double min;
double max;
cv::minMaxIdx(videoFrameMat, &min, &max);
cv::Mat adjMap;
cv::convertScaleAbs(videoFrameMat, adjMap, 255 / max);
cv::imshow("Out", adjMap);

cv::waitKey();

Answer 1

You can use step:

step – Number of bytes each matrix row occupies

int n_bytes_for_each_row = mat.step;