Different values in accessing openCV Mat

Question

I'm facing a strange problem.

My Mat is declared as:

cv::Mat matr(src.rows, src.cols, CV_32FC1);

If i print it using cout<<matr I get all the data in range from 0 to 255 and everything is fine.

If I try to access using this cycle:

float* matrF= (float*)matr.data;

for (int r=0; r<matrF.rows; r++)
  {
    for (int c=0; c<matrF.cols; c++)
    {
      std::cout<<matrF[r*matr.cols+c]<<std::endl;
    }
  }

I get many values like: 4.684e-42, 2.223e-22 ecc.

If I use

for (int r=0; r<matrF.rows; r++)
      {
        for (int c=0; c<matrF.cols; c++)
        {
          std::cout<<matr.data[r*matr.cols+c]<<std::endl;
        }
      }

It print some random char and no values.

Ho can I access the element printed using cout<<matr ? I don't want to use function from openCV to access them, I'm trying to loop over the matr.data...

I cant't use mat.at<float> or .step1(). Thanks.

---------- SOLUTION

if instead of

for (int r=0; r<matrF.rows; r++)
  {
    for (int c=0; c<matrF.cols; c++)
    {
      std::cout<<matrF.data[r*matr.cols+c]<<std::endl;
    }
  }

I use

for (int r=0; r<matrF.rows; r++)
  {
    for (int c=0; c<matrF.cols; c++)
    {
      float val = matF.data[r*matr.cols+c];
      std::cout<<val<<std::endl;
    }
  }

the problem disappears.

Just found out the problem. Could someone explain that? I edit my post. — 0abc0cba0, Nov 11 '21 at 10:55
Inszeadt of r*matr.cols use r*matr.step or rowPrt(r) if you want to make it stable for all kind of matrices (which can easily have a padding at the end of each row). .at(row, col) is another fast (in release mode), nice, and well readable way. — Micka, Nov 11 '21 at 12:32

score -1 · Answer 1 · answered Nov 11 '21 at 09:18

-1

It is highly possibly that it is not continuous. Then you cannot use matr.data to see all elements.

Please read this tutorial to see what to do: https://docs.opencv.org/4.x/db/da5/tutorial_how_to_scan_images.html

answered Nov 11 '21 at 09:18

ch271828n

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I want to access data without using functions like `mat.at` – 0abc0cba0 Nov 11 '21 at 09:29
@0abc0cba0 please look at the tutorial. it tells you how to do it. e.g. get a pointer *row by row*. – ch271828n Nov 11 '21 at 09:31
The link does not refer to 32F – l000000l Nov 11 '21 at 09:42
@l000000l Idea is the same. just change `p = I.ptr(i);` to `p = I.ptr(i);`. People in stackoverflow should have basic understanding of programming imho. – ch271828n Nov 11 '21 at 09:47
I said in the post, I cannot use `.ptr` – 0abc0cba0 Nov 11 '21 at 10:00
@0abc0cba0 why? – ch271828n Nov 11 '21 at 10:35
then just clone your mat, and it will be continous so your method works – ch271828n Nov 11 '21 at 10:36
For university project, that function is not allowed – 0abc0cba0 Nov 11 '21 at 10:40
I solved the problem, look at my post. – 0abc0cba0 Nov 11 '21 at 10:58

Different values in accessing openCV Mat

1 Answers1