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。

 

 

 

 

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