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_SIMPLE); in java i used this syntax Imgproc.findContours(gray, contours, new Mat(), Imgproc.RETR_LIST, Imgproc.CHAIN_APPROX_SIMPLE);.

So now contours should be List<MatOfPoint> contours = new ArrayList<MatOfPoint>(); rather than vector<vector<cv::Point> > contours;.

Then i need implement this approxPolyDP(Mat(contours[i]), approx, arcLength(Mat(contours[i]), true)*0.02, true);. In java api, Imgproc.approxPolyDP accept argument as approxPolyDP(MatOfPoint2f curve, MatOfPoint2f approxCurve, double epsilon, boolean closed). How i can i convert MatOfPoint to MatOfPoint2f?

Or is there a way to use vectors as same as c++ interface to implement this. Any suggestion or sample code is greatly appreciated.

Viktor Latypov · Accepted Answer · 2012-06-30T11:30:08.207

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() );

score 35 · Answer 2 · edited Nov 16 '14 at 19:21

35

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);
}

edited Nov 16 '14 at 19:21

Dan Nuffer

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answered Oct 04 '12 at 01:00

eskimo9

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Where is the mMOP2f2 coming from? – user2534365 Oct 21 '15 at 11:19
`mMOP2f1` and `mMOP2f2` are local variables of the loop which are omitted in this example. My edit should clarify this. – t7tran Aug 06 '18 at 21:56
How do you know that @coolersport? They could be global variables too. – hellow Aug 07 '18 at 06:03
It doesn't hurt to declare them as global but it is a bad practice. Content two objects are corresponding/reset to each contour. And it is proven in my case which shares the same loop. – t7tran Aug 08 '18 at 07:15
Those guys rejected my changes which help clarify this code didn't have a clue. Best of luck guys. Look at the first comment/question with two upvotes. – t7tran Aug 08 '18 at 07:20
Those guys rejected my changes which help clarify this code didn't have a clue. Best of luck guys. Look at the first comment/question with two upvotes. – t7tran Aug 08 '18 at 07:20

score 26 · Answer 3 · answered Nov 06 '16 at 10:56

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.

Not working on open CV anymore. So if ppl can verify I can mark it as the accepted answer. — chAmi, Dec 29 '21 at 07:27

How to convert MatOfPoint to MatOfPoint2f in opencv java api

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