hierarchy of findContours

[Joram]

After searching a lot with Google I finally found how to use the
contours from the findContours function. But what i can't find is how
to use the hierarchy mat. Has anyone got an example? I want to detect
if a contour has any inner contours.

[Hardik]

Hi Joram,

pls look this code...hope it will help you...        

            List<Mat> contour = new ArrayList<Mat>();

            Mat matObject = new Mat(new Size(576,648),CvType.CV_8UC1,new Scalar(0));

            //find contours and store into list as Mat
            Imgproc.findContours(diff,contour,matObject,Imgproc.CHAIN_APPROX_SIMPLE, Imgproc.RETR_LIST);


Thanks

--
Regards,

Pansuria Hardik

Sourcebits  Inc.

[Joram]

Hi,

Ok, my code is very similar only I didn't had the size and type of your matObject set. But how to use that matObject, how can I see if a contour is a child of an other contour or if a contour has a child?

My current code:

[code]

Imgproc.threshold(mGray, mExtra, 100, 255, Imgproc.THRESH_BINARY);

List<Mat> contours = new ArrayList<Mat>();

Mat hierarchy = new Mat(new Size(320,240),CvType.CV_8UC1,new Scalar(0));

Imgproc.findContours(mExtra, contours, hierarchy, 3, 2);

Imgproc.cvtColor(mExtra, mRgba, Imgproc.COLOR_GRAY2RGBA, 4);

if (contours.size() > 2 && contours.size()  < 20)

{

for (int i = 0; i < contours.size(); i++)

{

Mat c = contours.get(i); 

int num = (int) c.total(); 

int buff[] = new int[num*2]; // [x1, y1, x2, y2, ...] 

c.get(0, 0, buff);

List<Point> points = new ArrayList<Point>(num);

for (int p = 0; p < num*2; p+=2)

{

points.add(new Point(buff[p], buff[p+1]));

}

Rect rect = Imgproc.boundingRect(points);

Scalar lcolor;

//if (hierarchy == -1)

// lcolor = new Scalar(255,0,0);

//else

lcolor = new Scalar(0,0,255);

Core.line(mRgba, new Point(rect.x, rect.y), new Point(rect.x+rect.width, rect.y+rect.height), lcolor);

Log.i("vision", "draw line through contour: "+num+" points, ("+rect.x+","+rect.y+"), "+rect.width+"x"+rect.height);

}

}

[/code]

Joram

[Rafa Ortega]

In order to draw the contours found you can use the function 

public static void drawContours(Mat image, java.util.List<Mat> contours, int contourIdx, Scalar color, int thickness, int lineType, Mat hierarchy, int maxLevel, Point offset)

in the Imgproc class, if you want all the contours to be drawn just pass a negative number in the contourIdx parameter.

2011/11/30 Joram <mic...@gmail.com>

[Joram]

I understand that is possible, but not what I was looking for. 

In the c++ opencv you have:

"hiararchy – Optional output vector containing information about the image topology. It has as many elements as the number of contours. For each contour contours[i] , the elements hierarchy[i][0] , hiearchy[i][1] , hiearchy[i][2] , and hiearchy[i][3] are set to 0-based indices in contours of the next and previous contours at the same hierarchical level: the first child contour and the parent contour, respectively. If for a contour i there are no next, previous, parent, or nested contours, the corresponding elements of hierarchy[i] will be negative."

http://opencv.itseez.com/modules/imgproc/doc/structural_analysis_and_shape_descriptors.html?highlight=findcontours#cv.FindContours

That is what I want, the information so I can check the for child or parent contours myself.

As I discovered the hierarchy.total() is the exact length as the number of contours but when I try to retreive the information as i do with the contours( hierarchy.get(0, 0, buff_h); ) it gives an error, UnsupportedOperationException.

Joram

[Andrey Pavlenko]

The hierarchy is a Mat (1xN) of type CV_32SC4.

So to get its values you need an int[]:

    int iBuff[] = new int[ (int) (h.total() * h.channels()) ]; // [ Contour0 (next sibling num, previous sibling num, 1st child num, parent num), Contour1(...), ...

    h.get(0, 0, iBuff);

[Joram]

Thanks that works!