在Matlab中利用OpenCV裁剪出旋转矩形区域

在OpenCV中有minAreaRect()来获取旋转的最小面积矩形，Matlab中暂时没有对应的函数，但我找到一篇同样功能的函数minBoundingBox.m。利用这个函数可以获得旋转矩形的四个顶角，顺序如下图

如果要将目标区域从原图上裁剪下来，需要计算外包络正矩形，然后裁剪下来，然后旋转正，然后再裁剪到旋转矩形的大小。为了解决这种麻烦，并加快执行速度，本文编写了cv_rotateRect.cpp,利用mex和OpenCV的透视变换warpPerspective()来快速裁剪需要的区域。
需要注意的有三个地方:
一是Matlab储存方式是按列储存，左图，OpenCV是按行储存，右图

进行参数传递时要转置。
二是OpenCV拷贝有两种，深拷贝和浅拷贝
(1) 浅拷贝：
Mat B;
B = image // 第一种方式
Mat C(image); // 第二种方式
这两种方式称为浅copy，是由于它们有不同的矩阵头，但是它们共享内存空间，即指向一个矩阵。当图像矩阵发生变化时，两者相关联，都会变化。
（2）深拷贝
Mat B,C;
B = image.clone(); // 第一种方式
image.copyTo(C); // 第二种方式
深拷贝是真正的copy了一个新的图像矩阵，此时image,B,C三者相互没有影响。
还要注意的是：方式一是全新的深拷贝，会重新分配内存：
Mat a=b.clone();
如果要拷贝到另一个Mat（避免内存重新分配），必须用copyTo()：
Mat a(b.size(),b.type());
b.copyTo(a);
三是minBoundingBox.m对点的表述方式是第一行是x，第二行是y，而不是一般的第一列是x，第二列是y，minBoundingBox的输入是2行n列的点坐标，输出是2行4列的顶角坐标。

minBoundingBox.m

function bb = minBoundingBox(X)% compute the minimum bounding box of a set of 2D points%   Use:   boundingBox = minBoundingBox(point_matrix)%% Input:  2xn matrix containing the [x,y] coordinates of n points%         *** there must be at least 3 points which are not collinear% output: 2x4 matrix containing the coordinates of the bounding box corners%% Example : generate a random set of point in a randomly rotated rectangle%     n = 50000;%     t = pi*rand(1);%     X = [cos(t) -sin(t) ; sin(t) cos(t)]*[7 0; 0 2]*rand(2,n);%     X = [X  20*(rand(2,1)-0.5)];  % add an outlier% %     tic%     c = minBoundingBox(X);%     toc% %     figure(42);%     hold off,  plot(X(1,:),X(2,:),'.')%     hold on,   plot(c(1,[1:end 1]),c(2,[1:end 1]),'r')%     axis equal% compute the convex hull (CH is a 2*k matrix subset of X)k = convhull(X(1,:),X(2,:));CH = X(:,k);% compute the angle to test, which are the angle of the CH edges as:%   "one side of the bounding box contains an edge of the convex hull"E = diff(CH,1,2);           % CH edgesT = atan2(E(2,:),E(1,:));   % angle of CH edges (used for rotation)T = unique(mod(T,pi/2));    % reduced to the unique set of first quadrant angles% create rotation matrix which contains% the 2x2 rotation matrices for *all* angles in T% R is a 2n*2 matrixR = cos( reshape(repmat(T,2,2),2*length(T),2) ... % duplicate angles in T       + repmat([0 -pi ; pi 0]/2,length(T),1));   % shift angle to convert sine in cosine% rotate CH by all anglesRCH = R*CH;% compute border size  [w1;h1;w2;h2;....;wn;hn]% and area of bounding box for all possible edgesbsize = max(RCH,[],2) - min(RCH,[],2);area  = prod(reshape(bsize,2,length(bsize)/2));% find minimal area, thus the index of the angle in T [a,i] = min(area);% compute the bound (min and max) on the rotated frameRf    = R(2*i+[-1 0],:);   % rotated framebound = Rf * CH;           % project CH on the rotated framebmin  = min(bound,[],2);bmax  = max(bound,[],2);% compute the corner of the bounding boxRf = Rf';bb(:,4) = bmax(1)*Rf(:,1) + bmin(2)*Rf(:,2);bb(:,1) = bmin(1)*Rf(:,1) + bmin(2)*Rf(:,2);bb(:,2) = bmin(1)*Rf(:,1) + bmax(2)*Rf(:,2);bb(:,3) = bmax(1)*Rf(:,1) + bmax(2)*Rf(:,2);

cv_rotateRect.cpp

#include "mex.h"#include "matrix.h"#include "math.h"#include "opencv2/opencv.hpp"using namespace cv;void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[]){    int N = mxGetN(prhs[0]);//number of cols    int M = mxGetM(prhs[0]);//number of rows    char* srcPtr = (char*)mxGetData(prhs[0]);     double* pointPtr = mxGetPr(prhs[1]);    double dstSizeW = 0;    double dstSizeH = 0;    double x1 = *(pointPtr+0);    double x2 = *(pointPtr+1);    double x3 = *(pointPtr+2);    double x4 = *(pointPtr+3);    double y1 = *(pointPtr+4);    double y2 = *(pointPtr+5);    double y3 = *(pointPtr+6);    double y4 = *(pointPtr+7);    Point2f src_vertices[4];//Point2f(x,y)    src_vertices[0] = Point2f(x1,y1);    src_vertices[1] = Point2f(x2,y2);    src_vertices[2] = Point2f(x3,y3);    src_vertices[3] = Point2f(x4,y4);    mexPrintf("src_Vertice: x,y\n");    mexPrintf("%f,%f;\n",src_vertices[0].x,src_vertices[0].y);    mexPrintf("%f,%f;\n",src_vertices[1].x,src_vertices[1].y);    mexPrintf("%f,%f;\n",src_vertices[2].x,src_vertices[2].y);    mexPrintf("%f,%f;\n",src_vertices[3].x,src_vertices[3].y);    Point2f dst_vertices[4];    if(nrhs==3)    {        double* dstPointPtr = mxGetPr(prhs[2]);        dst_vertices[0] = Point2f(*(dstPointPtr+0),*(dstPointPtr+4));        dst_vertices[1] = Point2f(*(dstPointPtr+1),*(dstPointPtr+5));        dst_vertices[2] = Point2f(*(dstPointPtr+2),*(dstPointPtr+6));        dst_vertices[3] = Point2f(*(dstPointPtr+3),*(dstPointPtr+7));    }    else     {        mexPrintf("Auto Dst Point:\n");           double dstSize1 = sqrt(pow((x2-x1),2)+pow((y2-y1),2));        double dstSize2 = sqrt(pow((x3-x2),2)+pow((y3-y2),2));        if(dstSize1 > dstSize2)        {            dstSizeW = dstSize1;            dstSizeH = dstSize2;            dst_vertices[0] = Point2f(dstSizeW,0);            dst_vertices[1] = Point2f(0,0);            dst_vertices[2] = Point2f(0,dstSizeH);            dst_vertices[3] = Point2f(dstSizeW,dstSizeH);         }        else        {            dstSizeW = dstSize2;            dstSizeH = dstSize1;            dst_vertices[0] = Point2f(0,0);            dst_vertices[1] = Point2f(0,dstSizeH);            dst_vertices[2] = Point2f(dstSizeW,dstSizeH);            dst_vertices[3] = Point2f(dstSizeW,0);         }        mexPrintf("dstSizeW,dstSizeH:%f,%f\n",dstSizeW,dstSizeH);        mexPrintf("dst_Vertice: x,y\n");        mexPrintf("%f,%f;\n",dst_vertices[0].x,dst_vertices[0].y);        mexPrintf("%f,%f;\n",dst_vertices[1].x,dst_vertices[1].y);        mexPrintf("%f,%f;\n",dst_vertices[2].x,dst_vertices[2].y);        mexPrintf("%f,%f;\n",dst_vertices[3].x,dst_vertices[3].y);    }    //Acturally it should be Mat(rows,cols,...);    //but Matlab Array is store by cols    Mat src = Mat(N,M,CV_8UC1,srcPtr);    //transposition because Matlab Array store by cols    src = src.t();    //imwrite("cvSrc.bmp",src);    Mat warpMatrix = getPerspectiveTransform(src_vertices, dst_vertices);    plhs[0] = mxCreateNumericMatrix(dstSizeH,dstSizeW,mxUINT8_CLASS,mxREAL);    char* dstPtr = (char*)mxGetData(plhs[0]);     Mat dst = Mat(dstSizeH,dstSizeW,CV_8UC1);    warpPerspective(src, dst, warpMatrix, dst.size(), INTER_LINEAR, BORDER_CONSTANT);     dst = dst.t();    Mat dstMx = Mat(dstSizeW,dstSizeH,CV_8UC1,dstPtr);    dst.copyTo(dstMx);    //imwrite("cvDst.bmp",dst);}

mex -I.\ -I'C:\Program Files\OpenCV249\opencv\build\include' -L'C:\Program Files\OpenCV249\opencv\build\x64\vc10\lib' -lopencv_core249 -lopencv_highgui249 -lopencv_video249 -lopencv_imgproc249 cv_rotateRect.cpp

使用示例：

I = imread('m.bmp');BW = im2bw(I,0.5);L = bwlabel(BW);s = regionprops(L,'Area');if ~isempty(s)    [maxarea,maxi] = max([s.Area]);    [r, c] = find(L==maxi);    cr = [c';r'];    rRect = minBoundingBox(cr);    figure(1);imshow(L);    imline(gca,[rRect(1,1),rRect(2,1);rRect(1,2),rRect(2,2)]);    imline(gca,[rRect(1,2),rRect(2,2);rRect(1,3),rRect(2,3)]);    imline(gca,[rRect(1,3),rRect(2,3);rRect(1,4),rRect(2,4)]);    imline(gca,[rRect(1,4),rRect(2,4);rRect(1,1),rRect(2,1)]);    rRect = [rRect(1,:)',rRect(2,:)'];    I2 = cv_rotateRect(I,rRect);    figure(2);imshow(I2);end

原图m.bmp

实验结果

cv_rotatedRect.mexw64:
http://download.csdn.net/detail/sunflower_boy/9740877

1 http://www.mathworks.com/matlabcentral/fileexchange/31126-2d-minimal-bounding-box
2 http://gis.stackexchange.com/questions/22895/how-to-find-the-minimum-area-rectangle-for-given-points
3 http://www.bubuko.com/infodetail-493981.html
4 http://stackoverflow.com/questions/21659496/deep-copy-of-opencv-cvmat