Question

最近，我试图了解方向角（theta）的Camshift代码。

在Camshift的参考文献中：“用于感知用户界面的计算机视觉脸部跟踪”，论文中的方向角与OpenCV中的代码不同。

The code in the OpenCV is as follows:

cvCamShift( const void* imgProb, CvRect windowIn,CvTermCriteria criteria, CvConnectedComp* _comp,CvBox2D* box )
{
    const int TOLERANCE = 10;
    CvMoments moments;
    double m00 = 0, m10, m01, mu20, mu11, mu02, inv_m00;
    double a, b, c, xc, yc;
    double rotate_a, rotate_c;
    double theta = 0, square;
    double cs, sn;
    double length = 0, width = 0;
    int itersUsed = 0;
    CvConnectedComp comp;
    CvMat  cur_win, stub, *mat = (CvMat*)imgProb;
comp.rect = windowIn;

mat = cvGetMat( mat, &stub );

itersUsed = cvMeanShift( mat, windowIn, criteria, &comp );
windowIn = comp.rect;

windowIn.x -= TOLERANCE;
if( windowIn.x < 0 )
    windowIn.x = 0;

windowIn.y -= TOLERANCE;
if( windowIn.y < 0 )
    windowIn.y = 0;

windowIn.width += 2 * TOLERANCE;
if( windowIn.x + windowIn.width > mat->width )
    windowIn.width = mat->width - windowIn.x;

windowIn.height += 2 * TOLERANCE;
if( windowIn.y + windowIn.height > mat->height )
    windowIn.height = mat->height - windowIn.y;

cvGetSubRect( mat, &cur_win, windowIn );

/* Calculating moments in new center mass */
cvMoments( &cur_win, &moments );

m00 = moments.m00;
m10 = moments.m10;
m01 = moments.m01;
mu11 = moments.mu11;
mu20 = moments.mu20;
mu02 = moments.mu02;

if( fabs(m00) < DBL_EPSILON )
    return -1;

inv_m00 = 1. / m00;
xc = cvRound( m10 * inv_m00 + windowIn.x );
yc = cvRound( m01 * inv_m00 + windowIn.y );
a = mu20 * inv_m00;
b = mu11 * inv_m00;
c = mu02 * inv_m00;

/* Calculating width & height */
square = sqrt( 4 * b * b + (a - c) * (a - c) );

/* Calculating orientation */
theta = atan2( 2 * b, a - c + square );//Here, i confused!!!!!
/* Calculating width & length of figure */
cs = cos( theta );
sn = sin( theta );

rotate_a = cs * cs * mu20 + 2 * cs * sn * mu11 + sn * sn * mu02;
rotate_c = sn * sn * mu20 - 2 * cs * sn * mu11 + cs * cs * mu02;
length = sqrt( rotate_a * inv_m00 ) * 4;
width = sqrt( rotate_c * inv_m00 ) * 4;

/* In case, when tetta is 0 or 1.57... the Length & Width may be exchanged */
if( length < width )
{
    double t;

    CV_SWAP( length, width, t );
    CV_SWAP( cs, sn, t );
    theta = CV_PI*0.5 - theta;
}

/* Saving results */
if( _comp || box )
{
    int t0, t1;
    int _xc = cvRound( xc );
    int _yc = cvRound( yc );

    t0 = cvRound( fabs( length * cs ));
    t1 = cvRound( fabs( width * sn ));

    t0 = MAX( t0, t1 ) + 2;
    comp.rect.width = MIN( t0, (mat->width - _xc) * 2 );

    t0 = cvRound( fabs( length * sn ));
    t1 = cvRound( fabs( width * cs ));

    t0 = MAX( t0, t1 ) + 2;
    comp.rect.height = MIN( t0, (mat->height - _yc) * 2 );

    comp.rect.x = MAX( 0, _xc - comp.rect.width / 2 );
    comp.rect.y = MAX( 0, _yc - comp.rect.height / 2 );

    comp.rect.width = MIN( mat->width - comp.rect.x, comp.rect.width );
    comp.rect.height = MIN( mat->height - comp.rect.y, comp.rect.height );
    comp.area = (float) m00;
}

if( _comp )
    *_comp = comp;

if( box )
{
    box->size.height = (float)length;
    box->size.width = (float)width;
    box->angle = (float)((CV_PI*0.5+theta)*180./CV_PI);
    while(box->angle < 0)
        box->angle += 360;
    while(box->angle >= 360)
        box->angle -= 360;
    if(box->angle >= 180)
        box->angle -= 180;
    box->center = cvPoint2D32f( comp.rect.x + comp.rect.width*0.5f,
        comp.rect.y + comp.rect.height*0.5f);
}

return itersUsed;

}

我的目标是根据三角函数理论计算长轴上的两个点，所以我需要知道方向角和质心，现在我可以得到质心。

根据“机器人视觉”一文，该方向角被视为x轴与半长轴之间的角度（namaly，length），该机器视觉是“用于感知用户界面的计算机视觉面部跟踪”的参考。。但是，在OpenCV中，x轴和y轴的方向分别指向右侧和下方，这与纸张不同，那么如何理解方向角？它是如何变化的？

感谢您阅读我的问题。祝你有愉快的一天！

OpenCV：Camshift的方向角

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