Create Mat from vector

Question

I am extremely new to computer vision and the opencv library.

I've done some googling around to try to find how to make a new image from a vector of Point2fs and haven't found any examples that work. I've seen vector<Point> to Mat but when I use those examples I always get errors.

I'm working from this example and any help would be appreciated.

Code: I pass in occludedSquare.

   resize(occludedSquare, occludedSquare, Size(0, 0), 0.5, 0.5);

   Mat occludedSquare8u;
   cvtColor(occludedSquare, occludedSquare8u, CV_BGR2GRAY);

   //convert to a binary image. pixel values greater than 200 turn to white. otherwize black
   Mat thresh;
   threshold(occludedSquare8u, thresh, 170.0, 255.0, THRESH_BINARY);



   GaussianBlur(thresh, thresh, Size(7, 7), 2.0, 2.0);

   //Do edge detection
   Mat edges;
   Canny(thresh, edges, 45.0, 160.0, 3);

   //Do straight line detection
   vector<Vec2f> lines;
   HoughLines( edges, lines, 1.5, CV_PI/180, 50, 0, 0 );

   //imshow("thresholded", edges);


   cout << "Detected " << lines.size() << " lines." << endl;

   // compute the intersection from the lines detected...
   vector<Point2f> intersections;
   for( size_t i = 0; i < lines.size(); i++ )
   {
       for(size_t j = 0; j < lines.size(); j++)
       {
           Vec2f line1 = lines[i];
           Vec2f line2 = lines[j];
           if(acceptLinePair(line1, line2, CV_PI / 32))
           {
               Point2f intersection = computeIntersect(line1, line2);
               intersections.push_back(intersection);
           }
       }

   }

   if(intersections.size() > 0)
   {
       vector<Point2f>::iterator i;
       for(i = intersections.begin(); i != intersections.end(); ++i)
       {
           cout << "Intersection is " << i->x << ", " << i->y << endl;
           circle(occludedSquare8u, *i, 1, Scalar(0, 255, 0), 3);
       }
   }

//Make new matrix bounded by the intersections
...
imshow("localized", localized);

I want to take the points I have to create an ROI and from there make a new matrix. Is that more clear? I'm not really sure what the difference between making a Mat of points and drawing points into a matrix is. — myselfesteem, May 10 '14 at 19:12

score 16 · Accepted Answer · edited Oct 11 '16 at 16:18

16

Should be as simple as

std::vector<cv::Point2f> points;
cv::Mat image(points);
//or
cv::Mat image = cv::Mat(points)

The probably confusion is that a cv::Mat is an image width*height*number of channels but it also a mathematical matrix , rows*columns*other dimension.

If you make a Mat from a vector of 'n' 2D points it will create a 2 column by 'n' rows matrix. You are passing this to a function which expects an image.

If you just have a scattered set of 2D points and want to display them as an image you need to make an empty cv::Mat of large enough size (whatever your maximum x,y point is) and then draw the dots using the drawing functions http://docs.opencv.org/doc/tutorials/core/basic_geometric_drawing/basic_geometric_drawing.html

If you just want to set the pixel values at those point coordinates search SO for opencv setting pixel values, there are lots of answers

edited Oct 11 '16 at 16:18

Kevin Katzke

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answered May 10 '14 at 19:15

Martin Beckett

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I get a runtime error "Bad number of channels (Source image must have 1, 3 or 4 channels) in cvConvertImage, ... )" for both of these options. – myselfesteem May 10 '14 at 19:23
I may have been unclear in the problem statement, but I fixed it by creating an ROI then cropping my matrix with that. I made the ROI by finding the smallest x & y values. Then I could compute the height and width. I'm marking you as correct answer because you led me to the right path. – myselfesteem May 11 '14 at 19:43
1

`cv::Mat(points).reshape(1)` give you the exactly (rows, cols) by setting channel to 1. – Zhengfang Xin Nov 05 '21 at 14:30

Create Mat from vector

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