Detecting simple geometric shapes using OpenCV [Java]

Question

I'm programming a Java application that detects simple geometric shapes.

The following Python code is used as a reference: How to detect simple geometric shapes using OpenCV

This is some of the code [Python]:

contours,h = cv2.findContours(thresholdedImage,1,2)

for cnt in contours:
    approx = cv2.approxPolyDP(cnt,0.01*cv2.arcLength(cnt,True),True)
    print len(approx)
    if len(approx)==5:
        print "pentagon"
        cv2.drawContours(img,[cnt],0,255,-1)
    elif len(approx)==3:
        print "triangle"
        cv2.drawContours(img,[cnt],0,(0,255,0),-1)
    elif len(approx)==4:
        print "square"
        cv2.drawContours(img,[cnt],0,(0,0,255),-1)
    elif len(approx) == 9:
        print "half-circle"
        cv2.drawContours(img,[cnt],0,(255,255,0),-1)
    elif len(approx) > 15:
        print "circle"
        cv2.drawContours(img,[cnt],0,(0,255,255),-1)

Using Java's OpenCV methods, I'm unable to extract the "len" (length) property (in order to determine what shape has been detected) of an approximated contour.

Printing out a couple of contours produces [Java]:

[ 4*1*CV_32SC2, isCont=true, isSubmat=false, nativeObj=0x23ac75f0, dataAddr=0x1c7c66c0 ]
[ 5*1*CV_32SC2, isCont=true, isSubmat=false, nativeObj=0x23ac7200, dataAddr=0x1c0e5fc0 ]
etc.

I'd like to get the number of points of a specific contour object -- the 4 and 5 from the code above.

I know I could convert it to a String and then extract the numbers, but there must be a better way, right?

Thanks for your replies.

Note that `len` is simply getting the number of elements in the `approx` array. It's not computing `length` of points/lines or anything. It's like a `.length` in Java. So it's just saying "if there's three points its a triangle, five its a pentagon" and so on. — alkasm, Sep 21 '17 at 10:48

Oscar Martinez · Accepted Answer · 2017-09-21T10:52:05.110

This should do the trick. (Assuming you already have yourImagestored in a MatOfPoint2fobject).

MatOfPoint2f approx = new MatOfPoint2f();
Imgproc.approxPolyDP(yourImage, approx, Imgproc.arcLength(yourImage, true) * 0.02, true);
long count = approx.total();
if (count == 5) { 
    // this is a pentagon
}

Check this to see the use of total()

OpenCV on Java becomes a bit tricky compared with other languages like Python or C++.

Detecting simple geometric shapes using OpenCV [Java]

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