opencv/javacv: How to iterate over contours for shape identification?

Question

I\'m developing a shape identification project using JavaCV and I have found some OpenCV code to identify U shapes in a particular image. I have tried to convert it into JavaCV

Answer 1

This code work for me and I just put cvSeq=cvSeq.h_next(); line in to the program and remove the for loop add while loop for thet.

    package Beam;
    import com.googlecode.javacpp.Loader;
    import com.googlecode.javacv.CanvasFrame;
    import static com.googlecode.javacpp.Loader.*;
    import static com.googlecode.javacv.cpp.opencv_core.*;
    import static com.googlecode.javacv.cpp.opencv_imgproc.*;
    import static com.googlecode.javacv.cpp.opencv_highgui.*;
    import java.io.File;
    import javax.swing.JFileChooser;

    public class TestBeam2 {
        public static void main(String[] args) {
            JFileChooser f=new JFileChooser();
            int result=f.showOpenDialog(f);//show dialog box to choose files
                File myfile=null;
                String path="";
            if(result==0){
                myfile=f.getSelectedFile();//selected file taken to myfile
                path=myfile.getAbsolutePath();//get the path of the file
            }
            IplImage src = cvLoadImage(path);//hear path is actual path to image
            IplImage grayImage    = IplImage.create(src.width(), src.height(), IPL_DEPTH_8U, 1);
            cvCvtColor(src, grayImage, CV_RGB2GRAY);
            cvThreshold(grayImage, grayImage, 127, 255, CV_THRESH_BINARY);
            CvSeq cvSeq=new CvSeq();
            CvMemStorage memory=CvMemStorage.create();
            cvFindContours(grayImage, memory, cvSeq, Loader.sizeof(CvContour.class), CV_RETR_LIST, CV_CHAIN_APPROX_SIMPLE);

            while (cvSeq != null && !cvSeq.isNull()) {
                CvRect rect=cvBoundingRect(cvSeq, 0);
                int x=rect.x(),y=rect.y(),h=rect.height(),w=rect.width();
                if (10 < w/h || w/h < 0.1){
                    cvRectangle(src, cvPoint(x, y), cvPoint(x+w, y+h), CvScalar.RED, 1, CV_AA, 0);
                }
                cvSeq=cvSeq.h_next();
            }
            CanvasFrame cnvs=new CanvasFrame("Beam");
            cnvs.setDefaultCloseOperation(javax.swing.JFrame.EXIT_ON_CLOSE);
            cnvs.showImage(src);
            //cvShowImage("Final ", src);
        }
    }

Answer 2

Check your type promotions, e.g.:

if (10 < (w/h) || (w/h) < 0.1){

.. is highly suspect. To get a floating point division, one (or both) of the operands must at least be a float (and likewise a double for double division). Otherwise, as in this case, it is an integer division. (Note that the original code has promotion to float as well.)

For instance:

 float ratio = (float)w/h; // (float / int) => (float / float) -> float
 if (10 < ratio || ratio < 0.1 ) { 

(Although I am unsure if this is the issue here.)

Happy coding!

Answer 3

EDIT: Here is the most interesting finding - I think you are not iterating correctly through the contours - you should do something like:

CvRect rect = cvBoundingRect(cvGetSeqElem(cvSeq, i),0); //python default?

Or:

// ... 
CvSeq contours = new CvSeq();
CvSeq ptr = new CvSeq();
CvRect rect = null;
// ...
cvFindContours(..., contours, ...);

for (ptr = contours; ptr != null; ptr = ptr.h_next()) {
    rect =  cvBoundingRect(ptr, 0);
    // ... Draw the box if meets criteria
}

First, I think pst is right regarding the calculation of the ratio - you have to cast the width to float.

Secondly, I see that when you are making the gray image in python you use COLOR_BGR2GRAY and in java you are using CV_RGB2GRAY that could lead to a totally different gray picture. I would add some debug steps on both programs to save the temp gray images and compare them as also print outs for the values of x,y,w and h when (10 < (w/h) || (w/h) < 0.1) is true.

Another thing is that in the java solution you use CV_RETR_CCOMP to get the contours and in the python solution you use CV_RETR_LIST according to the documentation:

CV_RETR_LIST retrieves all of the contours without establishing any hierarchical relationships CV_RETR_CCOMP retrieves all of the contours and organizes them into a two-level hierarchy: on the top level are the external boundaries of the components, on the second level are the boundaries of the holes. If inside a hole of a connected component there is another contour, it will still be put on the top level

So first I would double check that all cv's parameters in both programs are the same, then I would add debug steps to see that the intermediate variables contains the same data.