Android + OpenCV

原文链接：http://answers.opencv.org/question/134783/android-opencv-finding-extreme-points-in-contours/

导读：本例子使用轮廓分析，寻找到轮廓的极点；使用了STD的SORT特性。

提出问题：

Good Evening,

I have a trouble with finding extreme points in frames. I am detecting all contours, but I want find there one extreme point, which is lowest from others (southernmost contour). Here is a preview of screen, what I have.

image description

And here is a preview of screen, what I want ! For example, lowest point will bounded by red rectangle.

image description .

And here is my code, what I have now.

   contours = new ArrayList<MatOfPoint>();
   Rect rect;
   Point c1 = new Point(); Point c2 = new Point();

   Imgproc.cvtColor(mRgba, mGray, Imgproc.COLOR_RGB2GRAY);
   subtractorMOG2.apply(mGray, mFGMask, 0.005);

   Imgproc.threshold(mFGMask, mFGMask, 45, 255, Imgproc.THRESH_BINARY);
   Imgproc.erode(mFGMask, mFGMask, new Mat());
   Imgproc.dilate(mFGMask, mFGMask, new Mat());

   Imgproc.findContours(mFGMask, contours, new Mat(), Imgproc.RETR_EXTERNAL , Imgproc.CHAIN_APPROX_NONE);
   Imgproc.drawContours(mRgba, contours, -1, new Scalar(255, 255, 0), 3);

        /*for (int idx = 0; idx < contours.size(); idx++) {

            double contourarea = Imgproc.contourArea(contours.get(idx));
            rect = boundingRect(contours.get(idx));

            c1.x = rect.x; c1.y = rect.y;
            c2.x = rect.x + rect.width; c2.y = rect.y + rect.height;
            if (contourarea > 1024 && contourarea < 30000) {
                Imgproc.rectangle(mRgba, c1, c2, new Scalar(255, 0, 0), 3);
            }
        }*/

  return mRgba;

Thanks everybody for help and comments !

优质解答：

bool SortbyXaxis(const Point & a, const Point &b) 
{
    return a.x > b.x;
}
bool SortbyYaxis(const Point & a, const Point &b) 
{
    return a.y > b.y;
}
int main (int argc, char * const argv[]) 
{
    Mat mSrc,mDst;
    mSrc = imread("e:/sandbox/aline.png", 0);
    if (mSrc.empty())
    {
        cerr << "No image supplied ..." << endl;
        return -1;
    }
    cvtColor(mSrc,mDst,COLOR_GRAY2BGR);
    /// Create Window
    const char* source_window = "Output Image";
    namedWindow( source_window, WINDOW_AUTOSIZE );
    vector<vector<Point> > contours;
    vector<Vec4i> hierarchy;
    /// Find contours
    findContours( mSrc.clone(), contours, hierarchy, RETR_EXTERNAL, CHAIN_APPROX_SIMPLE, Point(0, 0) );
    vector<Point> ptsContour;
    for( size_t i = 0; i< contours.size(); i++ )
    {
        drawContours( mDst, contours, (int)i, Scalar(0,255,0), 1);
        ptsContour= contours[i];
        sort( ptsContour.begin(), ptsContour.end(), SortbyYaxis );
        sort( ptsContour.begin(), ptsContour.end(), SortbyXaxis );
        circle(mDst, ptsContour[0],3,Scalar(0,255,255),-1);
    }
    imshow( source_window, mDst );
    waitKey();
    return 0;
}