【第二部分图像处理】第3章 Opencv图像处理进阶【6角点检测 C】

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6.3亚像素角点检测

6.3.1 亚像素角点检测

前文讲解了利用Harris进行角点检测和利用Shi-Tomasi方法进行角点检测外，如果对角点的精度有更高的要求，可以用cornerSubPix()函数将角点定位到子像素，从而取得亚像素级别的角点检测效果。

6.3.2 亚像素角点检测：cornerSubPix()函数

 cornerSubPix()函数讲解

C++: void cornerSubPix( InputArray image, InputOutputArray corners, Size winSize, Size zeroZone,TermCriteria criteria)

【参数】
第一个参数，image – Input image.
第二个参数，corners – Initial coordinates of the input corners and refined coordinates provided for output.
第三个参数，winSize – Half of the side length of the search window. For example, if winSize=Size(5,5) , then a search window is used.
第四个参数，zeroZone – Half of the size of the dead region in the middle of the search zone over which the summation in the formula below is not done. It is used sometimes to avoid possible singularities of the autocorrelation matrix. The value of (-1,-1) indicates that there is no such a size.
第五个参数，criteria – Criteria for termination of the iterative process of corner refinement. That is, the process of corner position refinement stops either after criteria.maxCount iterations or when the corner position moves by less than criteria.epsilon on some iteration.
 cornerSubPix()函数源代码

/*【cornerSubPix ( )源代码】************************************************************ @Version：OpenCV 3.0.0（Opnencv2和Opnencv3差别不大，Linux和PC的对应版本源码完全一样，均在对应的安装目录下）  * @源码路径：…\opencv\sources\modules\imgproc\src\ cornersubpix.cpp* @起始行数：44行   
********************************************************************************/
void cv::cornerSubPix( InputArray _image, InputOutputArray _corners,Size win, Size zeroZone, TermCriteria criteria )
{const int MAX_ITERS = 100;int win_w = win.width * 2 + 1, win_h = win.height * 2 + 1;int i, j, k;int max_iters = (criteria.type & CV_TERMCRIT_ITER) ? MIN(MAX(criteria.maxCount, 1), MAX_ITERS) : MAX_ITERS;double eps = (criteria.type & CV_TERMCRIT_EPS) ? MAX(criteria.epsilon, 0.) : 0;eps *= eps; // use square of error in comparsion operationscv::Mat src = _image.getMat(), cornersmat = _corners.getMat();int count = cornersmat.checkVector(2, CV_32F);CV_Assert( count >= 0 );Point2f* corners = cornersmat.ptr<Point2f>();if( count == 0 )return;CV_Assert( win.width > 0 && win.height > 0 );CV_Assert( src.cols >= win.width*2 + 5 && src.rows >= win.height*2 + 5 );CV_Assert( src.channels() == 1 );Mat maskm(win_h, win_w, CV_32F), subpix_buf(win_h+2, win_w+2, CV_32F);float* mask = maskm.ptr<float>();for( i = 0; i < win_h; i++ ){float y = (float)(i - win.height)/win.height;float vy = std::exp(-y*y);for( j = 0; j < win_w; j++ ){float x = (float)(j - win.width)/win.width;mask[i * win_w + j] = (float)(vy*std::exp(-x*x));}}// make zero_zoneif( zeroZone.width >= 0 && zeroZone.height >= 0 &&zeroZone.width * 2 + 1 < win_w && zeroZone.height * 2 + 1 < win_h ){for( i = win.height - zeroZone.height; i <= win.height + zeroZone.height; i++ ){for( j = win.width - zeroZone.width; j <= win.width + zeroZone.width; j++ ){mask[i * win_w + j] = 0;}}}// do optimization loop for all the pointsfor( int pt_i = 0; pt_i < count; pt_i++ ){Point2f cT = corners[pt_i], cI = cT;int iter = 0;double err = 0;do{Point2f cI2;double a = 0, b = 0, c = 0, bb1 = 0, bb2 = 0;getRectSubPix(src, Size(win_w+2, win_h+2), cI, subpix_buf, subpix_buf.type());const float* subpix = &subpix_buf.at<float>(1,1);// process gradientfor( i = 0, k = 0; i < win_h; i++, subpix += win_w + 2 ){double py = i - win.height;for( j = 0; j < win_w; j++, k++ ){double m = mask[k];double tgx = subpix[j+1] - subpix[j-1];double tgy = subpix[j+win_w+2] - subpix[j-win_w-2];double gxx = tgx * tgx * m;double gxy = tgx * tgy * m;double gyy = tgy * tgy * m;double px = j - win.width;a += gxx;b += gxy;c += gyy;bb1 += gxx * px + gxy * py;bb2 += gxy * px + gyy * py;}}double det=a*c-b*b;if( fabs( det ) <= DBL_EPSILON*DBL_EPSILON )break;// 2x2 matrix inversiondouble scale=1.0/det;cI2.x = (float)(cI.x + c*scale*bb1 - b*scale*bb2);cI2.y = (float)(cI.y - b*scale*bb1 + a*scale*bb2);err = (cI2.x - cI.x) * (cI2.x - cI.x) + (cI2.y - cI.y) * (cI2.y - cI.y);cI = cI2;if( cI.x < 0 || cI.x >= src.cols || cI.y < 0 || cI.y >= src.rows )break;}while( ++iter < max_iters && err > eps );// if new point is too far from initial, it means poor convergence.// leave initial point as the resultif( fabs( cI.x - cT.x ) > win.width || fabs( cI.y - cT.y ) > win.height )cI = cT;corners[pt_i] = cI;}
}