OpenCV Java convexity defects (Computer Vision)

Question

I have a problem with putting my convexity defects on the frame. To calculate them, I modified c++ source and this is what i archived:

 mConvexityDefectsMatOfInt4 = new MatOfInt4(); if(contours.size() > 0 && convexHullMatOfInt.rows() > 0) Imgproc.convexityDefects(contours.get(0), convexHullMatOfInt, mConvexityDefectsMatOfInt4);

However, the Imgproc.drawContours(...) method requires that convexityDefects passed to it as a parameters will be ArrayList. I don't know how can I make the conversion. I had also similar problem with convex hulls, but I found out a walkaround:

 convexHullMatOfInt = new MatOfInt(); convexHullPointArrayList = new ArrayList<Point>(); convexHullMatOfPoint = new MatOfPoint(); convexHullMatOfPointArrayList = new ArrayList<MatOfPoint>(); //Calculate convex hulls if(contours.size() > 0) { Imgproc.convexHull( contours.get(0), convexHullMatOfInt, false ); for(int j=0; j < convexHullMatOfInt.toList().size(); j++) convexHullPointArrayList.add(contours.get(0).toList().get(convexHullMatOfInt.toList().get(j))); convexHullMatOfPoint.fromList(convexHullPointArrayList); convexHullMatOfPointArrayList.add(convexHullMatOfPoint); }

Similar solution for convexity defects is not working. Does anyone have an idea on how can I solve the problem?

How to convert from MatOfInt4() to ArrayList() to be able to draw convexity defects?

Mahm00d · Accepted Answer · 2013-08-15 12:56:29Z

(I myself was struggling so much with convexityDefect that I wanted to kill whoever has written the Java interface for OpenCV!)

Now the answer:

As stated in docs, MatOfInt4 is basically a 4-element integer array containing the following information:

start_index end_index farthest_pt_index fixpt_depth

You can use the following to convert mConvexityDefectsMatOfInt4 to a list of integers:

List<Integer> cdList = mConvexityDefectsMatOfInt4.toList();

Now, each 4 consecutive elements in cdList holds the info stated above:

cdList 0 : 23 cdList 1 : 30 cdList 2 : 26 cdList 3 : 18101 ----------------- cdList 4 : 30 cdList 5 : 44 cdList 6 : 33 cdList 7 : 43738

So, for example if you want to draw only the farthest point of each concavity, you can simply use the third index of each 4 elements. In this case: 26, 33, ...

Hope it helps.

Pang · Accepted Answer · 2015-11-03 02:36:39Z

Here is an example:

for (int i = 0; i < contours.size(); i++) { convDef.add(new MatOfInt4()); Imgproc.convexityDefects(contours.get(i), hull.get(i), convDef.get(i)); cdList = convDef.get(i).toList(); Point data[] = contours.get(i).toArray(); for (int j = 0; j < cdList.size(); j = j+4) { Point start = data[cdList.get(j)]; Point end = data[cdList.get(j+1)]; Point defect = data[cdList.get(j+2)]; //Point depth = data[cdList.get(j+3)]; Imgproc.circle(mat, start, 5, new Scalar(0, 255, 0), 2); Imgproc.circle(mat, end, 5, new Scalar(0, 255, 0), 2); Imgproc.circle(mat, defect, 5, new Scalar(0, 255, 0), 2); } }

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