Calculation of coordinates of point on a scaled Mat image in OpenCv

Question

If I have a Mat image object (OpenCV) whose size is 960*720, on which I have calculated the coordinates of a Point object, and then I scale this Mat image, and its new size is 640*480, how can I find the new coordinates of the Point?

Answer 1

A point (x,y) in the original matrix will be mapped to (x',y') in the new matrix by

(x',y') = 2*(x,y)/3.

Reducing that to an OpenCV function we have:

cv::Point scale_point(cv::Point p) // p is location in 960*720 Mat
{
    return 2 * p / 3;   // return location in 640*480 Mat
}

Answer 2

What I ended-up doing was to create a ScaledPoint object that extended Point. That way, it was less disruptive to the code I'd already been using with just the plain Point object.

public class ScaledPoint extends Point {
    public ScaledPoint (double[] points, double scale) {
        super(points[0] * scale, points[1] * scale);
    }
}

Then, I calculated a scaling factor and used it in the class I extended:

Mat originalObject; 
// TODO: populate the original object
Mat scaledObject; 
// TODO: populate the scaled object
double scaleFactor = scaledObject.getHeight()/(double)originalObject.getHeight();
matOfSomePoints = new Mat(4,1, CvType.CV_23FC2);
// TODO: populate the above matrix with your points
Point aPointForTheUnscaledImage = new Point(matOfSomePoints.get(0,0));
Point aPointForTheScaledImage = new ScaledPoint(matOfSomePoints.get(0,0), scaleFactor);