Android + OpenCV - Finding extreme points in contours

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.

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

.

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;
     
}

 程序解读：本程序的基本思路是很简答的,就是遍历轮廓，找到轮廓中x和y最大的点，这个点就是极值点。但是有两个地方需要注意：

1、是使用了std的sort特性，应该说简化了程序设计；

2、是findcontours的第二个参数，使用的是“只寻找第一层轮廓”。那么对于本例来说，这样做是合理的。