How to sort contours from top to bottom : Opencv , Android

Question

I have found the contour but the problem is that the findcontours() returns contours in random order like at contour(0) it shows some sontour from middle of the page. How can I sort them vertically? From top to bottom and then left to right? Given the image below I connected the components horizontally and connected every MCQ with their choices and then applied findcontours() and Now I want to sort them in an order that they should be retrieved sequentially

enter image description here

Just an advice, if you trying to detect those little circle locations template matching on normalized images should give you good results. — ivan_a, Apr 23 '15 at 03:51
Useful, but sometimes these algos can throw an exception: https://stackoverflow.com/questions/53538333/java-sorting-a-massive-arraylist-using-a-custom-comparator-sometimes-throws-iil — Riccardo Finazzi, Nov 29 '18 at 11:50

score 2 · Answer 1 · answered May 31 '18 at 15:51

I have the same problem and use the sort method of java.

//sort by y coordinates using the topleft point of every contour's bounding box
    Collections.sort(contourList, new Comparator<MatOfPoint>() {
        @Override
        public int compare(MatOfPoint o1, MatOfPoint o2) {
            Rect rect1 = Imgproc.boundingRect(o1);
            Rect rect2 = Imgproc.boundingRect(o2);
            int result = Double.compare(rect1.tl().y, rect2.tl().y);
            return result;
        }
    } );


//sort by x coordinates
Collections.sort(contourList, new Comparator<MatOfPoint>() {
        @Override
        public int compare(MatOfPoint o1, MatOfPoint o2) {
            Rect rect1 = Imgproc.boundingRect(o1);
            Rect rect2 = Imgproc.boundingRect(o2);
            int result = 0;
            double total = rect1.tl().y/rect2.tl().y;
            if (total>=0.9 && total<=1.4 ){
                result = Double.compare(rect1.tl().x, rect2.tl().x);
            }
            return result;
        }
    });

score 1 · Answer 2 · answered Apr 22 '15 at 21:20

1

You have the function boundingRect: With that you can find the topleft point of a bounding rectangle around the points. Then you can define a sorting function that orders the contours based on the location of this point for each contour.

answered Apr 22 '15 at 21:20

Antonio

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@HelpingDesk It will return a [Rect](http://docs.opencv.org/java/org/opencv/core/Rect.html), which has different methods, like `tl()` (top left point) and `br()` (bottom right point). – Antonio Apr 22 '15 at 21:28
I have used boundingrect() to crop the region but not familiar with how it actually does. Can you please help with some code? – Zain ul abdeen Apr 22 '15 at 21:31
@HelpingDesk I am sorry but I don't use the Java version. It's pretty tricky in fact. Maybe [this](http://stackoverflow.com/questions/29517069/how-are-two-element-vector-represented-in-a-opencv-mat-in-java) can give you a little help. – Antonio Apr 22 '15 at 21:38
I am bit confused with Tl().x and y can you pleaasse explain something? Thanks – Zain ul abdeen Apr 26 '15 at 22:57
@HelpingDesk Can you edit your question to explain better what is the results that you obtain and what is that you do not understand? As I said, I have no experience with the Java interface, so I have no ready made answers. – Antonio Apr 26 '15 at 23:38
What axis value does tl().x and tl().y gives? – Zain ul abdeen Apr 26 '15 at 23:44
@HelpingDesk Take a set of points. Build a containing bounding box. Take the top left corner. It has coordinates (x,y): y is the row number of the image (from top to bottom), x is the column number of the image (from left to right) – Antonio Apr 26 '15 at 23:50
Ok I understand please take a look at my other question. 1 and 16 are on the sane distance from top but their values have drastic difference. http://stackoverflow.com/questions/29884692/sort-bounding-boxes-with-tl-opencv-android – Zain ul abdeen Apr 26 '15 at 23:54

score 0 · Answer 3 · answered Jul 24 '22 at 10:25

@user9876747 solution works great for small contour list but throws the below exception for larger contour list.

java.lang.IllegalArgumentException: Comparison method violates its general contract!

So i was able to tweak the solution to below and it works fine for large contour lists.

 Collections.sort(contours, new Comparator<MatOfPoint>() {
        public int compare(MatOfPoint o1, MatOfPoint o2) {
            org.opencv.core.Rect rect1 = Imgproc.boundingRect(o1);
            org.opencv.core.Rect rect2 = Imgproc.boundingRect(o2);


            int result = Double.compare(rect1.br().y, rect2.br().y);
            double total = rect1.br().y / rect2.br().y;

            if (total < 1.1 && total > 1/1.1) {
                result = Double.compare(rect1.tl().x, rect2.tl().x);
            }
            return result;
        }
    });

How to sort contours from top to bottom : Opencv , Android

3 Answers3