cvmSet(M,i,j,2.0); // Set M(i,j)
t = cvmGet(M,j); // Get M(i,j)

CvMat* M= cvCreateMat(4,CV_32FC1);
int n= M->cols;
float *data = M->data.fl;

data[i*n+j] = 3.0;

intstep= M->step/sizeof(float);
float *data = M->data.fl;

(data+i*step)[j] = 3.0;

double a[16];
CvMat Ma = cvMat(3,a);
a[i*4+j] = 2.0; // Ma(i,j)=2.0;

CvMat *Ma,*Mb,*Mc;
cvAdd(Ma,Mb,Mc);// Ma+Mb-> Mc
cvSub(Ma,Mc);// Ma-Mb-> Mc
cvMatMul(Ma,Mc);// Ma*Mb-> Mc

cvMul(Ma,Mc);// Ma.*Mb-> Mc
cvDiv(Ma,Mc);// Ma./Mb-> Mc
cvAddS(Ma,cvScalar(-10.0),Mc); // Ma.-10 -> Mc

double va[] = {1,2,3};
double vb[] = {0,1};
double vc[3];

CvMat Va=cvMat(3,va);
CvMat Vb=cvMat(3,vb);
CvMat Vc=cvMat(3,vc);

double res=cvDotProduct(&Va,&Vb); // 点乘:Va . Vb -> res
cvCrossProduct(&Va,&Vb,&Vc);// 向量积: Va x Vb -> Vc
end{verbatim}

注意 Va,Vb,Vc 在向量积中向量元素个数须相同.

cvTranspose(Ma,Mb);// transpose(Ma) -> Mb (不能对自身进行转置)
CvScalar t = cvTrace(Ma); // trace(Ma) -> t.val[0]
double d = cvDet(Ma);// det(Ma) -> d
cvInvert(Ma,Mb);// inv(Ma) -> Mb

CvMat* A= cvCreateMat(3,CV_32FC1);
CvMat* x= cvCreateMat(3,CV_32FC1);
CvMat* b= cvCreateMat(3,CV_32FC1);
cvSolve(&A,&b,&x);// solve (Ax=b) for x

CvMat* E= cvCreateMat(3,CV_32FC1);
CvMat* l= cvCreateMat(3,CV_32FC1);
cvEigenVV(&A,&E,&l);// l = A的特征值 (降序排列)
// E = 对应的特征向量 (每行)

CvMat* U= cvCreateMat(3,CV_32FC1);
CvMat* D= cvCreateMat(3,CV_32FC1);
CvMat* V= cvCreateMat(3,CV_32FC1);
cvSVD(A,D,U,V,CV_SVD_U_T|CV_SVD_V_T); // A = U D V^T

标号使得 U 和 V 返回时被转置(若没有转置标号，则有问题不成功!!!).

CvCapture* capture = cvCaptureFromCAM(0); // capture from video device #0

CvCapture* capture = cvCaptureFromAVI("infile.avi");

IplImage* img = 0;
if(!cvGrabFrame(capture)){// 抓取一帧
printf("Could not grab a frame\n\7");
exit(0);
}
img=cvRetrieveFrame(capture);// 恢复获取的帧图像

要从多个摄像头同时获取图像,首先从每个摄像头抓取一帧. 在抓取动作都结束后再恢复帧图像.

cvReleaseCapture(&capture);

注意由设备抓取的图像是由capture函数自动分配和释放的. 不要试图自己释放它.

cvQueryFrame(capture); // this call is necessary to get correct
// capture properties
int frameH= (int) cvGetCaptureProperty(capture,CV_CAP_PROP_FRAME_HEIGHT);
int frameW= (int) cvGetCaptureProperty(capture,CV_CAP_PROP_FRAME_WIDTH);
int fps= (int) cvGetCaptureProperty(capture,CV_CAP_PROP_FPS);
int numFrames = (int) cvGetCaptureProperty(capture,CV_CAP_PROP_FRAME_COUNT);

所有帧数似乎只与视频文件有关. 用摄像头时不对，奇怪!!!.

float posMsec=cvGetCaptureProperty(capture,CV_CAP_PROP_POS_MSEC);
int posFrames= (int) cvGetCaptureProperty(capture,CV_CAP_PROP_POS_FRAMES);
float posRatio=cvGetCaptureProperty(capture,CV_CAP_PROP_POS_AVI_RATIO);

获取所抓取帧在视频序列中的位置,从首帧开始按[毫秒]算. 或者从首帧开始从0标号,获取所抓取帧的标号. 或者取相对位置，首帧为0，末帧为1,只对视频文件有效.

// 从视频文件相对位置0.9处开始抓取
cvSetCaptureProperty(capture,CV_CAP_PROP_POS_AVI_RATIO,(double)0.9);

只对从视频文件抓取有效. 不过似乎也不成功!!!

CvVideoWriter *writer = 0;
int isColor = 1;
int fps= 25;// or 30
int frameW= 640; // 744 for firewire cameras
int frameH= 480; // 480 for firewire cameras
writer=cvCreateVideoWriter("out.avi",CV_FOURCC('P','I','M','1'),
fps,cvSize(frameW,frameH),isColor);

其他有效编码:

CV_FOURCC('P','1')= MPEG-1 codec
CV_FOURCC('M','J','P','G')= motion-jpeg codec (does not work well)
CV_FOURCC('M','4','2') = MPEG-4.2 codec
CV_FOURCC('D','V','3') = MPEG-4.3 codec
CV_FOURCC('D','X') = MPEG-4 codec
CV_FOURCC('U','2','6','3') = H263 codec
CV_FOURCC('I','3') = H263I codec
CV_FOURCC('F','L','1') = FLV1 codec

若把视频编码设为-1则将打开一个编码选择窗口(windows系统下).

IplImage* img = 0;
int nFrames = 50;
for(i=0;i<nFrames;i++){
cvGrabFrame(capture);// 抓取帧
img=cvRetrieveFrame(capture);// 恢复图像
cvWriteFrame(writer,img);// 将帧添加入视频文件
}

若想在抓取中查看抓取图像,可在循环中加入下列代码:

cvShowImage("mainWin",img);
key=cvWaitKey(20);// wait 20 ms

若没有20[毫秒]延迟，将无法正确显示视频序列.

cvReleaseVideoWriter(&writer);