Accessing an element of an Array in OpenCV

Question

I am trying to get the value of an element in an array in order to use it in an if statement but unfortunately the following code is not working for me. The cout of comp is not matching the first element of the array C. I'm new to OpenCV so any help is appreciated.

Mat A = (Mat_<double>(2,1) << u, v); 
Mat B = (Mat_<double>(2,6) << -1/Z,  0 ,  x/Z , x*y , -(x*x+1),y,                                                       
                               0 ,-1/Z,  y/Z ,y*y+1,   -x*y  ,-x);
Mat pinvB = B.inv(DECOMP_SVD);
Mat C=pinvB*A; // 6x1 Array

float comp = C.at<float>(0,0);
cout << "comp " << comp << endl; //This value does not match C[0,0]
cout << "C " << C << endl;

if (comp < 0.0001){
   //process
}

Does it work if you do double comp = C.at<double>(0,0);?

juanchopanza
– juanchopanza

2013-06-08 21:47:36 +00:00
Commented Jun 8, 2013 at 21:47 — juanchopanza
– juanchopanza, Commented Jun 8, 2013 at 21:47

juanchopanza · Accepted Answer · 2013-06-08 21:52:19Z

1

Your Mat_<double> instances internally store doubles. When you do this:

float comp = C.at<float>(0,0);

you are trying to use some of the bits that form a double, and interpret them as a float. Floating point representation means that half of the bits of a double don't translate into a meaningful float (assuming a platform where float has half the size of a double, which is quite common). So, call C.at<double> instead.

answered Jun 8, 2013 at 21:52

juanchopanza

228k35 gold badges421 silver badges493 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

cxyzs7 · Accepted Answer · 2013-06-08 22:13:13Z

0

Actually, if you use the template version of cv::Mat_<_Tp>, you can access pixel value by Mat_<_Tp>::operator ()(int y, int x)

cv::Mat_<double> M(3, 3);
for (int i = 0;i < 3; ++i) {
  for (int j = 0;j < 3; ++j) {
    std::cout<<M(i, j)<<std::endl;
  }
}

so that later if you change the template argument from double to float, you don't need to modify each at().

answered Jun 8, 2013 at 22:13

cxyzs7

1,2278 silver badges15 bronze badges

Collectives™ on Stack Overflow

Accessing an element of an Array in OpenCV

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related