How to convert MatOfPoint to MatOfPoint2f in opencv java api

Question

I\'m trying to implement the example code of the following question by using opencv java api. To implement findContours(gray, contours, CV_RETR_LIST, CV_CHAIN_APPROX_

Answer 1

Although this question has already been answered, I believe that the accepted answer is not the best. Converting a matrix to an array and then back comes with quite the performance penalty, both time-wise and memory-wise.

Instead, OpenCV already has a function that does exactly this: convertTo.

MatOfPoint src;
// initialize src
MatOfPoint2f dst = new MatOfPoint2f();
src.convertTo(dst, CvType.CV_32F);

I have found this to be significantly faster and more memory-friendly.

To convert a MatOfPoint2f to a MatOfPoint, use CvType.CV_32S instead.

Answer 2

I realise this question has already been well answered, but to add an alternative for anyone who finds it in the future -

Imgproc.findContours(gray, contours, new Mat(), Imgproc.RETR_LIST, Imgproc.CHAIN_APPROX_SIMPLE);

for(int i=0;i<contours.size();i++){
    //Convert contours(i) from MatOfPoint to MatOfPoint2f
    contours.get(i).convertTo(mMOP2f1, CvType.CV_32FC2);
    //Processing on mMOP2f1 which is in type MatOfPoint2f
    Imgproc.approxPolyDP(mMOP2f1, mMOP2f2, approxDistance, true); 
    //Convert back to MatOfPoint and put the new values back into the contours list
    mMOP2f2.convertTo(contours.get(i), CvType.CV_32S);
}

Answer 3

MatOfPoint2f differs from MatOfPoint only in the type of the elements (32-bit float and 32-bit int respectively). The viable option (though with a performance penalty) is to create MatOfPoint2f instance and set its elements (in a loop) to be equal to the elements of of the source MatOfPoint.

There are

 public void fromArray(Point... lp);
 public Point[] toArray();

methods in both of the classes.

So you can do just

 /// Source variable
 MatOfPoint SrcMtx;

 /// New variable
 MatOfPoint2f  NewMtx = new MatOfPoint2f( SrcMtx.toArray() );