minAreaRect-OpenCV

由于要将rRect利用仿射变换裁剪下来并旋正，需要知道rRect.point()的四个点的位置，查了一下资料，记在下面：

简介：Finds a circumscribed rectangle of the minimal area for 2D point set by building convex hull for the set and applying rotating calipers4 technique to the hull.1 建立外凸包，应用旋转卡尺技术来求2维点的最小面积包围矩形。
API reference
OpenCV官方例程

源码：
OpenCV249\opencv\sources\modules\imgproc\src\contours.cpp 1910行-1916行

cv::RotatedRect cv::minAreaRect( InputArray _points )
{
    Mat points = _points.getMat();
    CV_Assert(points.checkVector(2) >= 0 && (points.depth() == CV_32F || points.depth() == CV_32S));
    CvMat _cpoints = points;
    return cvMinAreaRect2(&_cpoints, 0);
}

cvMinAreaRect2： 2

http://fossies.org/dox/OpenCV-2.4.4a/rotcalipers_8cpp_source.html
截取自第347行

 347 CV_IMPL  CvBox2D
  348 cvMinAreaRect2( const CvArr* array, CvMemStorage* storage )
  349 {
  350     cv::Ptr temp_storage;
  351     CvBox2D box;
  352     cv::AutoBuffer _points;
  353     CvPoint2D32f* points;
  354 
  355     memset(&box, 0, sizeof(box));
  356 
  357     int i, n;
  358     CvSeqReader reader;
  359     CvContour contour_header;
  360     CvSeqBlock block;
  361     CvSeq* ptseq = (CvSeq*)array;
  362     CvPoint2D32f out[3];
  363 
  364     if( CV_IS_SEQ(ptseq) )
  365     {
  366         if( !CV_IS_SEQ_POINT_SET(ptseq) &&
  367             (CV_SEQ_KIND(ptseq) != CV_SEQ_KIND_CURVE ||
  368             CV_SEQ_ELTYPE(ptseq) != CV_SEQ_ELTYPE_PPOINT ))
  369             CV_Error( CV_StsUnsupportedFormat,
  370                 "Input sequence must consist of 2d points or pointers to 2d points" );
  371         if( !storage )
  372             storage = ptseq->storage;
  373     }
  374     else
  375     {
  376         ptseq = cvPointSeqFromMat( CV_SEQ_KIND_GENERIC, array, &contour_header, &block );
  377     }
  378 
  379     if( storage )
  380     {
  381         temp_storage = cvCreateChildMemStorage( storage );
  382     }
  383     else
  384     {
  385         temp_storage = cvCreateMemStorage(1 << 10);
  386     }
  387 
  388     ptseq = cvConvexHull2( ptseq, temp_storage, CV_CLOCKWISE, 1 );
  389     n = ptseq->total;
  390 
  391     _points.allocate(n);
  392     points = _points;
  393     cvStartReadSeq( ptseq, &reader );
  394 
  395     if( CV_SEQ_ELTYPE( ptseq ) == CV_32SC2 )
  396     {
  397         for( i = 0; i < n; i++ )
  398         {
  399             CvPoint pt;
  400             CV_READ_SEQ_ELEM( pt, reader );
  401             points[i].x = (float)pt.x;
  402             points[i].y = (float)pt.y;
  403         }
  404     }
  405     else
  406     {
  407         for( i = 0; i < n; i++ )
  408         {
  409             CV_READ_SEQ_ELEM( points[i], reader );
  410         }
  411     }
  412 
  413     if( n > 2 )
  414     {
  415         icvRotatingCalipers( points, n, CV_CALIPERS_MINAREARECT, (float*)out );
  416         box.center.x = out[0].x + (out[1].x + out[2].x)*0.5f;
  417         box.center.y = out[0].y + (out[1].y + out[2].y)*0.5f;
  418         box.size.width = (float)sqrt((double)out[1].x*out[1].x + (double)out[1].y*out[1].y);
  419         box.size.height = (float)sqrt((double)out[2].x*out[2].x + (double)out[2].y*out[2].y);
  420         box.angle = (float)atan2( (double)out[1].y, (double)out[1].x );
  421     }
  422     else if( n == 2 )
  423     {
  424         box.center.x = (points[0].x + points[1].x)*0.5f;
  425         box.center.y = (points[0].y + points[1].y)*0.5f;
  426         double dx = points[1].x - points[0].x;
  427         double dy = points[1].y - points[0].y;
  428         box.size.width = (float)sqrt(dx*dx + dy*dy);
  429         box.size.height = 0;
  430         box.angle = (float)atan2( dy, dx );
  431     }
  432     else
  433     {
  434         if( n == 1 )
  435             box.center = points[0];
  436     }
  437 
  438     box.angle = (float)(box.angle*180/CV_PI);
  439     return box;
  440 }
  441

OpenCV以图像的左上角为(0,0)，Point(x,y)，x对应列，y对应行，

RotatedRect rRect;
rRect = minAreaRect(point);
Point2f vertices[4];
rRect.points(vertices);

rRect.size.width，rRect.size.height，rRect.angle，vertices[0]，vertices[1]，vertices[2]，vertices[3]如下图所示，和mine1024博客3里Cvbox2D好像不太一样,改天验证下。

利用仿射变换，将该区域裁剪下来，并旋正：

    vector point;
    vector<vector> contours ;
    findContours( Itwe, contours, CV_RETR_LIST, CV_CHAIN_APPROX_SIMPLE );
    if(!contours.size())
    {
        printf("No contours!\n");
        return 0;
    }
    else
    {
        double maxarea = 0;
        int maxareai = 0;
        for (int i = 0; idouble tmparea = fabs(contourArea(contours[i]));
            if(tmparea > maxarea)
            {tmparea = tmparea;
            maxareai = i;}  
        }
        vector contourspoint = contours[maxareai];
    }
    RotatedRect rRect;
    rRect = minAreaRect(point);
    int dstw,dsth;
    Point2f vertices[4];
    Point2f verdst[4];

    if(rRect.size.width>rRect.size.height)
    {
        //rRect.size.width += 100;  //宽度增加100，以防定位偏差
        rRect.points(vertices);
        dstw = rRect.size.width;
        dsth = rRect.size.height;
        verdst[0] = Point2f(0,dsth);
        verdst[1] = Point2f(0,0);
        verdst[2] = Point2f(dstw,0);
        verdst[3] = Point2f(dstw,dsth); 
    }
    else 
    {
        //rRect.size.height += 100;
        rRect.points(vertices);
        dstw = rRect.size.height;
        dsth = rRect.size.width;
        verdst[0] = Point2f(dstw,dsth);
        verdst[1] = Point2f(0,dsth);
        verdst[2] = Point2f(0,0);
        verdst[3] = Point2f(dstw,0);

    }
    Mat Idst = Mat(dsth,dstw,CV_8UC1);
    Mat warpMatrix = getPerspectiveTransform(vertices, verdst);
    warpPerspective(Isrc, Idst, warpMatrix, Idst.size(), INTER_LINEAR, BORDER_CONSTANT);