Introduction to programming with OpenCV
2013-12-02 16:33
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Introduction to programming with OpenCV
Gady AgamDepartment of Computer Science
January 27, 2006
Illinois Institute of Technology
Abstract:
The purpose of this document is to get you started quickly with OpenCVwithout having to go through lengthy reference manuals. Once youunderstand these basics you will be able to consult the OpenCVmanuals on a need basis.http://www.cs.iit.edu/~agam/cs512/lect-notes/opencv-intro/opencv-intro.html
Contents
IntroductionDescription of OpenCV
Resources
OpenCV naming conventions
Compilation instructions
Example C Program
GUI commands
Window management
Input handling
Basic OpenCV data structures
Image data structure
Matrices and vectors
Other data structures
Working with images
Allocating and releasing images
Reading and writing images
Accessing image elements
Image conversion
Drawing commands
Working with matrices
Allocating and releasing matrices
Accessing matrix elements
Matrix/vector operations
Working with video sequences
Capturing a frame from a video sequence
Getting/setting frame information
Saving a video file
Introduction
Description of OpenCV
General descriptionOpen source computer vision library in C/C++.
Optimized and intended for real-time applications.
OS/hardware/window-manager independent.
Generic image/video loading, saving, and acquisition.
Both low and high level API.
Provides interface to Intel's Integrated Performance Primitives (IPP) with processor specific optimization (Intel processors).
Features:
Image data manipulation (allocation, release, copying, setting, conversion).
Image and video I/O (file and camera based input, image/video file output).
Matrix and vector manipulation and linear algebra routines (products, solvers, eigenvalues, SVD).
Various dynamic data structures (lists, queues, sets, trees, graphs).
Basic image processing (filtering, edge detection, corner detection, sampling and interpolation, color conversion, morphological operations, histograms, image pyramids).
Structural analysis (connected components, contour processing, distance transform, various moments, template matching, Hough transform, polygonal approximation, line fitting, ellipse fitting, Delaunay triangulation).
Camera calibration (finding and tracking calibration patterns, calibration, fundamental matrix estimation, homography estimation, stereo correspondence).
Motion analysis (optical flow, motion segmentation, tracking).
Object recognition (eigen-methods, HMM).
Basic GUI (display image/video, keyboard and mouse handling, scroll-bars).
Image labeling (line, conic, polygon, text drawing)
OpenCV modules:
cv - Main OpenCV functions.
cvaux - Auxiliary (experimental) OpenCV functions.
cxcore - Data structures and linear algebra support.
highgui - GUI functions.
Resources
Reference manuals:<opencv-root>/docs/index.htm
Web resources:
Official webpage: http://www.intel.com/technology/computing/opencv/
Software download: http://sourceforge.net/projects/opencvlibrary/
Books:
Open Source Computer Vision Library by Gary R. Bradski, Vadim Pisarevsky, and Jean-Yves Bouguet, Springer, 1st ed. (June, 2006).
Sample programs for video processing (in <opencv-root>/samples/c/):
color tracking: camshiftdemo
point tracking: lkdemo
motion segmentation: motempl
edge detection: laplace
Sample programs for image processing (in <opencv-root>/samples/c/):
edge detection: edge
segmentation: pyramid_segmentation
morphology: morphology
histogram: demhist
distance transform: distrans
ellipse fitting: fitellipse
OpenCV naming conventions
Function naming conventions:cvActionTargetMod(...) Action = the core functionality (e.g. set, create) Target = the target image area (e.g. contour, polygon) Mod = optional modifiers (e.g. argument type)
Matrix data types:
CV_<bit_depth>(S|U|F)C<number_of_channels> S = Signed integer U = Unsigned integer F = Float E.g.: CV_8UC1 means an 8-bit unsigned single-channel matrix, CV_32FC2 means a 32-bit float matrix with two channels.
Image data types:
IPL_DEPTH_<bit_depth>(S|U|F) E.g.: IPL_DEPTH_8U means an 8-bit unsigned image. IPL_DEPTH_32F means a 32-bit float image.
Header files:
#include <cv.h> #include <cvaux.h> #include <highgui.h> #include <cxcore.h> // unnecessary - included in cv.h
Compilation instructions
Linux:g++ hello-world.cpp -o hello-world \ -I /usr/local/include/opencv -L /usr/local/lib \ -lm -lcv -lhighgui -lcvaux
Windows:
In the project preferences set the path to the OpenCV header files and the path to the OpenCV library files.
Example C Program
//////////////////////////////////////////////////////////////////////// // // hello-world.cpp // // This is a simple, introductory OpenCV program. The program reads an // image from a file, inverts it, and displays the result. // //////////////////////////////////////////////////////////////////////// #include <stdlib.h> #include <stdio.h> #include <math.h> #include <cv.h> #include <highgui.h> int main(int argc, char *argv[]) { IplImage* img = 0; int height,width,step,channels; uchar *data; int i,j,k; if(argc<2){ printf("Usage: main <image-file-name>\n\7"); exit(0); } // load an image img=cvLoadImage(argv[1]); if(!img){ printf("Could not load image file: %s\n",argv[1]); exit(0); } // get the image data height = img->height; width = img->width; step = img->widthStep; channels = img->nChannels; data = (uchar *)img->imageData; printf("Processing a %dx%d image with %d channels\n",height,width,channels); // create a window cvNamedWindow("mainWin", CV_WINDOW_AUTOSIZE); cvMoveWindow("mainWin", 100, 100); // invert the image for(i=0;i<height;i++) for(j=0;j<width;j++) for(k=0;k<channels;k++) data[i*step+j*channels+k]=255-data[i*step+j*channels+k]; // show the image cvShowImage("mainWin", img ); // wait for a key cvWaitKey(0); // release the image cvReleaseImage(&img ); return 0; }
GUI commands
Window management
Create and position a window:cvNamedWindow("win1", CV_WINDOW_AUTOSIZE); cvMoveWindow("win1", 100, 100); // offset from the UL corner of the screen
Load an image:
IplImage* img=0; img=cvLoadImage(fileName); if(!img) printf("Could not load image file: %s\n",fileName);
Display an image:
cvShowImage("win1",img);
Can display a color or grayscale byte/float-image. A byte image isassumed to have values in the range
. A float image is assumedto have values in the range
. A color image is assumed to havedata in BGR order.
Close a window:
cvDestroyWindow("win1");
Resize a window:
cvResizeWindow("win1",100,100); // new width/heigh in pixels
Input handling
Handle mouse events:Define a mouse handler:
void mouseHandler(int event, int x, int y, int flags, void* param) { switch(event){ case CV_EVENT_LBUTTONDOWN: if(flags & CV_EVENT_FLAG_CTRLKEY) printf("Left button down with CTRL pressed\n"); break; case CV_EVENT_LBUTTONUP: printf("Left button up\n"); break; } } x,y: pixel coordinates with respect to the UL corner event: CV_EVENT_LBUTTONDOWN, CV_EVENT_RBUTTONDOWN, CV_EVENT_MBUTTONDOWN, CV_EVENT_LBUTTONUP, CV_EVENT_RBUTTONUP, CV_EVENT_MBUTTONUP, CV_EVENT_LBUTTONDBLCLK, CV_EVENT_RBUTTONDBLCLK, CV_EVENT_MBUTTONDBLCLK, CV_EVENT_MOUSEMOVE: flags: CV_EVENT_FLAG_CTRLKEY, CV_EVENT_FLAG_SHIFTKEY, CV_EVENT_FLAG_ALTKEY, CV_EVENT_FLAG_LBUTTON, CV_EVENT_FLAG_RBUTTON, CV_EVENT_FLAG_MBUTTON
Register the handler:
mouseParam=5; cvSetMouseCallback("win1",mouseHandler,&mouseParam);
Handle keyboard events:
The keyboard does not have an event handler.
Get keyboard input without blocking:
int key; key=cvWaitKey(10); // wait 10ms for input
Get keyboard input with blocking:
int key; key=cvWaitKey(0); // wait indefinitely for input
The main keyboard event loop:
while(1){ key=cvWaitKey(10); if(key==27) break; switch(key){ case 'h': ... break; case 'i': ... break; } }
Handle trackbar events:
Define a trackbar handler:
void trackbarHandler(int pos) { printf("Trackbar position: %d\n",pos); }
Register the handler:
int trackbarVal=25; int maxVal=100; cvCreateTrackbar("bar1", "win1", &trackbarVal ,maxVal , trackbarHandler);
Get the current trackbar position:
int pos = cvGetTrackbarPos("bar1","win1");
Set the trackbar position:
cvSetTrackbarPos("bar1", "win1", 25);
Basic OpenCV data structures
Image data structure
IPL image:IplImage |-- int nChannels; // Number of color channels (1,2,3,4) |-- int depth; // Pixel depth in bits: | // IPL_DEPTH_8U, IPL_DEPTH_8S, | // IPL_DEPTH_16U,IPL_DEPTH_16S, | // IPL_DEPTH_32S,IPL_DEPTH_32F, | // IPL_DEPTH_64F |-- int width; // image width in pixels |-- int height; // image height in pixels |-- char* imageData; // pointer to aligned image data | // Note that color images are stored in BGR order |-- int dataOrder; // 0 - interleaved color channels, | // 1 - separate color channels | // cvCreateImage can only create interleaved images |-- int origin; // 0 - top-left origin, | // 1 - bottom-left origin (Windows bitmaps style) |-- int widthStep; // size of aligned image row in bytes |-- int imageSize; // image data size in bytes = height*widthStep |-- struct _IplROI *roi;// image ROI. when not NULL specifies image | // region to be processed. |-- char *imageDataOrigin; // pointer to the unaligned origin of image data | // (needed for correct image deallocation) | |-- int align; // Alignment of image rows: 4 or 8 byte alignment | // OpenCV ignores this and uses widthStep instead |-- char colorModel[4]; // Color model - ignored by OpenCV
Matrices and vectors
Matrices:CvMat // 2D array |-- int type; // elements type (uchar,short,int,float,double) and flags |-- int step; // full row length in bytes |-- int rows, cols; // dimensions |-- int height, width; // alternative dimensions reference |-- union data; |-- uchar* ptr; // data pointer for an unsigned char matrix |-- short* s; // data pointer for a short matrix |-- int* i; // data pointer for an integer matrix |-- float* fl; // data pointer for a float matrix |-- double* db; // data pointer for a double matrix CvMatND // N-dimensional array |-- int type; // elements type (uchar,short,int,float,double) and flags |-- int dims; // number of array dimensions |-- union data; | |-- uchar* ptr; // data pointer for an unsigned char matrix | |-- short* s; // data pointer for a short matrix | |-- int* i; // data pointer for an integer matrix | |-- float* fl; // data pointer for a float matrix | |-- double* db; // data pointer for a double matrix | |-- struct dim[]; // information for each dimension |-- size; // number of elements in a given dimension |-- step; // distance between elements in a given dimension CvSparseMat // SPARSE N-dimensional array
Generic arrays:
CvArr* // Used only as a function parameter to specify that the // function accepts arrays of more than a single type, such // as: IplImage*, CvMat* or even CvSeq*. The particular array // type is determined at runtime by analyzing the first 4 // bytes of the header of the actual array.
Scalars:
CvScalar |-- double val[4]; //4D vector
Initializer function:
CvScalar s = cvScalar(double val0, double val1=0, double val2=0, double val3=0);
Example:
CvScalar s = cvScalar(20.0); s.val[0]=10.0;
Note that the initializer function has the same name as the datastructure only starting with a lower case character. It is not a C++constructor.
Other data structures
Points:CvPoint p = cvPoint(int x, int y); CvPoint2D32f p = cvPoint2D32f(float x, float y); CvPoint3D32f p = cvPoint3D32f(float x, float y, float z); E.g.: p.x=5.0; p.y=5.0;
Rectangular dimensions:
CvSize r = cvSize(int width, int height); CvSize2D32f r = cvSize2D32f(float width, float height);
Rectangular dimensions with offset:
CvRect r = cvRect(int x, int y, int width, int height);
Working with images
Allocating and releasing images
Allocate an image:IplImage* cvCreateImage(CvSize size, int depth, int channels); size: cvSize(width,height); depth: pixel depth in bits: IPL_DEPTH_8U, IPL_DEPTH_8S, IPL_DEPTH_16U, IPL_DEPTH_16S, IPL_DEPTH_32S, IPL_DEPTH_32F, IPL_DEPTH_64F channels: Number of channels per pixel. Can be 1, 2, 3 or 4. The channels are interleaved. The usual data layout of a color image is b0 g0 r0 b1 g1 r1 ...
Examples:
// Allocate a 1-channel byte image IplImage* img1=cvCreateImage(cvSize(640,480),IPL_DEPTH_8U,1); // Allocate a 3-channel float image IplImage* img2=cvCreateImage(cvSize(640,480),IPL_DEPTH_32F,3);
Release an image:
IplImage* img=cvCreateImage(cvSize(640,480),IPL_DEPTH_8U,1); cvReleaseImage(&img);
Clone an image:
IplImage* img1=cvCreateImage(cvSize(640,480),IPL_DEPTH_8U,1); IplImage* img2; img2=cvCloneImage(img1);
Set/get the region of interest:
void cvSetImageROI(IplImage* image, CvRect rect); void cvResetImageROI(IplImage* image); vRect cvGetImageROI(const IplImage* image); The majority of OpenCV functions support ROI.
Set/get the channel of interest:
void cvSetImageCOI(IplImage* image, int coi); // 0=all int cvGetImageCOI(const IplImage* image); The majority of OpenCV functions do NOT support COI.
Reading and writing images
Reading an image from a file:IplImage* img=0; img=cvLoadImage(fileName); if(!img) printf("Could not load image file: %s\n",fileName);
Supported image formats: BMP, DIB, JPEG, JPG, JPE, PNG, PBM, PGM, PPM,
SR, RAS, TIFF, TIF
By default, the loaded image is forced to be a 3-channel colorimage. This default can be modified by using:
img=cvLoadImage(fileName,flag); flag: >0 the loaded image is forced to be a 3-channel color image =0 the loaded image is forced to be a 1 channel grayscale image <0 the loaded image is loaded as is (with number of channels in the file).
Writing an image to a file:
if(!cvSaveImage(outFileName,img)) printf("Could not save: %s\n",outFileName);
The output file format is determined based on the file name extension.
Accessing image elements
Assume that you need to access the-th channel of the pixel atthe
-row and
-th column. The row index
is in the range
.
The column index
is in the range
.
The channel index
is in the range
.
Indirect access: (General, but inefficient, access to any type image)
For a single-channel byte image:
IplImage* img=cvCreateImage(cvSize(640,480),IPL_DEPTH_8U,1); CvScalar s; s=cvGet2D(img,i,j); // get the (i,j) pixel value printf("intensity=%f\n",s.val[0]); s.val[0]=111; cvSet2D(img,i,j,s); // set the (i,j) pixel value
For a multi-channel float (or byte) image:
IplImage* img=cvCreateImage(cvSize(640,480),IPL_DEPTH_32F,3); CvScalar s; s=cvGet2D(img,i,j); // get the (i,j) pixel value printf("B=%f, G=%f, R=%f\n",s.val[0],s.val[1],s.val[2]); s.val[0]=111; s.val[1]=111; s.val[2]=111; cvSet2D(img,i,j,s); // set the (i,j) pixel value
Direct access:(Efficient access, but error prone)
For a single-channel byte image:
IplImage* img=cvCreateImage(cvSize(640,480),IPL_DEPTH_8U,1); ((uchar *)(img->imageData + i*img->widthStep))[j]=111;
For a multi-channel byte image:
IplImage* img=cvCreateImage(cvSize(640,480),IPL_DEPTH_8U,3); ((uchar *)(img->imageData + i*img->widthStep))[j*img->nChannels + 0]=111; // B ((uchar *)(img->imageData + i*img->widthStep))[j*img->nChannels + 1]=112; // G ((uchar *)(img->imageData + i*img->widthStep))[j*img->nChannels + 2]=113; // R
For a multi-channel float image:
IplImage* img=cvCreateImage(cvSize(640,480),IPL_DEPTH_32F,3); ((float *)(img->imageData + i*img->widthStep))[j*img->nChannels + 0]=111; // B ((float *)(img->imageData + i*img->widthStep))[j*img->nChannels + 1]=112; // G ((float *)(img->imageData + i*img->widthStep))[j*img->nChannels + 2]=113; // R
Direct access using a pointer:(Simplified and efficient access under limiting assumptions)
For a single-channel byte image:
IplImage* img = cvCreateImage(cvSize(640,480),IPL_DEPTH_8U,1); int height = img->height; int width = img->width; int step = img->widthStep/sizeof(uchar); uchar* data = (uchar *)img->imageData; data[i*step+j] = 111;
For a multi-channel byte image:
IplImage* img = cvCreateImage(cvSize(640,480),IPL_DEPTH_8U,3); int height = img->height; int width = img->width; int step = img->widthStep/sizeof(uchar); int channels = img->nChannels; uchar* data = (uchar *)img->imageData; data[i*step+j*channels+k] = 111;
For a multi-channel float image (assuming a 4-byte alignment):
IplImage* img = cvCreateImage(cvSize(640,480),IPL_DEPTH_32F,3); int height = img->height; int width = img->width; int step = img->widthStep/sizeof(float); int channels = img->nChannels; float * data = (float *)img->imageData; data[i*step+j*channels+k] = 111;
Direct access using a c++ wrapper:(Simple and efficient access)
Define a c++ wrapper for single-channel byte images, multi-channelbyte images, and multi-channel float images:
template<class T> class Image { private: IplImage* imgp; public: Image(IplImage* img=0) {imgp=img;} ~Image(){imgp=0;} void operator=(IplImage* img) {imgp=img;} inline T* operator[](const int rowIndx) { return ((T *)(imgp->imageData + rowIndx*imgp->widthStep));} }; typedef struct{ unsigned char b,g,r; } RgbPixel; typedef struct{ float b,g,r; } RgbPixelFloat; typedef Image<RgbPixel> RgbImage; typedef Image<RgbPixelFloat> RgbImageFloat; typedef Image<unsigned char> BwImage; typedef Image<float> BwImageFloat;
For a single-channel byte image:
IplImage* img=cvCreateImage(cvSize(640,480),IPL_DEPTH_8U,1); BwImage imgA(img); imgA[i][j] = 111;
For a multi-channel byte image:
IplImage* img=cvCreateImage(cvSize(640,480),IPL_DEPTH_8U,3); RgbImage imgA(img); imgA[i][j].b = 111; imgA[i][j].g = 111; imgA[i][j].r = 111;
For a multi-channel float image:
IplImage* img=cvCreateImage(cvSize(640,480),IPL_DEPTH_32F,3); RgbImageFloat imgA(img); imgA[i][j].b = 111; imgA[i][j].g = 111; imgA[i][j].r = 111;
Image conversion
Convert to a grayscale or color byte-image:cvConvertImage(src, dst, flags=0); src = float/byte grayscale/color image dst = byte grayscale/color image flags = CV_CVTIMG_FLIP (flip vertically) CV_CVTIMG_SWAP_RB (swap the R and B channels)
Convert a color image to grayscale:
Using the OpenCV conversion:
cvCvtColor(cimg,gimg,CV_BGR2GRAY); // cimg -> gimg
Using a direct conversion:
for(i=0;i<cimg->height;i++) for(j=0;j<cimg->width;j++) gimgA[i][j]= (uchar)(cimgA[i][j].b*0.114 + cimgA[i][j].g*0.587 + cimgA[i][j].r*0.299);
Convert between color spaces:
cvCvtColor(src,dst,code); // src -> dst code = CV_<X>2<Y> <X>/<Y> = RGB, BGR, GRAY, HSV, YCrCb, XYZ, Lab, Luv, HLS e.g.: CV_BGR2GRAY, CV_BGR2HSV, CV_BGR2Lab
Drawing commands
Draw a box:// draw a box with red lines of width 1 between (100,100) and (200,200) cvRectangle(img, cvPoint(100,100), cvPoint(200,200), cvScalar(255,0,0), 1);
Draw a circle:
// draw a circle at (100,100) with a radius of 20. Use green lines of width 1 cvCircle(img, cvPoint(100,100), 20, cvScalar(0,255,0), 1);
Draw a line segment:
// draw a green line of width 1 between (100,100) and (200,200) cvLine(img, cvPoint(100,100), cvPoint(200,200), cvScalar(0,255,0), 1);
Draw a set of polylines:
CvPoint curve1[]={10,10, 10,100, 100,100, 100,10}; CvPoint curve2[]={30,30, 30,130, 130,130, 130,30, 150,10}; CvPoint* curveArr[2]={curve1, curve2}; int nCurvePts[2]={4,5}; int nCurves=2; int isCurveClosed=1; int lineWidth=1; cvPolyLine(img,curveArr,nCurvePts,nCurves,isCurveClosed,cvScalar(0,255,255),lineWidth);
Draw a set of filled polygons:
cvFillPoly(img,curveArr,nCurvePts,nCurves,cvScalar(0,255,255));
Add text:
CvFont font; double hScale=1.0; double vScale=1.0; int lineWidth=1; cvInitFont(&font,CV_FONT_HERSHEY_SIMPLEX|CV_FONT_ITALIC, hScale,vScale,0,lineWidth); cvPutText (img,"My comment",cvPoint(200,400), &font, cvScalar(255,255,0));
Other possible fonts:
CV_FONT_HERSHEY_SIMPLEX, CV_FONT_HERSHEY_PLAIN, CV_FONT_HERSHEY_DUPLEX, CV_FONT_HERSHEY_COMPLEX, CV_FONT_HERSHEY_TRIPLEX, CV_FONT_HERSHEY_COMPLEX_SMALL, CV_FONT_HERSHEY_SCRIPT_SIMPLEX, CV_FONT_HERSHEY_SCRIPT_COMPLEX,
Working with matrices
Allocating and releasing matrices
General:OpenCV has a C interface to matrix operations. There are many alternatives that have a C++ interface (which is more convenient) and are as efficient as OpenCV.
Vectors are obtained in OpenCV as matrices having one of their dimensions as 1.
Matrices are stored row by row where each row has a 4 byte alignment.
Allocate a matrix:
CvMat* cvCreateMat(int rows, int cols, int type); type: Type of the matrix elements. Specified in form CV_<bit_depth>(S|U|F)C<number_of_channels>. E.g.: CV_8UC1 means an 8-bit unsigned single-channel matrix, CV_32SC2 means a 32-bit signed matrix with two channels. Example: CvMat* M = cvCreateMat(4,4,CV_32FC1);
Release a matrix:
CvMat* M = cvCreateMat(4,4,CV_32FC1); cvReleaseMat(&M);
Clone a matrix:
CvMat* M1 = cvCreateMat(4,4,CV_32FC1); CvMat* M2; M2=cvCloneMat(M1);
Initialize a matrix:
double a[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 }; CvMat Ma=cvMat(3, 4, CV_64FC1, a);
Alternatively:
CvMat Ma; cvInitMatHeader(&Ma, 3, 4, CV_64FC1, a);
Initialize a matrix to identity:
CvMat* M = cvCreateMat(4,4,CV_32FC1); cvSetIdentity(M); // does not seem to be working properly
Accessing matrix elements
Assume that you need to access thecell of a 2D floatmatrix.
Indirect matrix element access:
cvmSet(M,i,j,2.0); // Set M(i,j) t = cvmGet(M,i,j); // Get M(i,j)
Direct matrix element access assuming a 4-byte alignment:
CvMat* M = cvCreateMat(4,4,CV_32FC1); int n = M->cols; float *data = M->data.fl; data[i*n+j] = 3.0;
Direct matrix element access assuming possible alignment gaps:
CvMat* M = cvCreateMat(4,4,CV_32FC1); int step = M->step/sizeof(float); float *data = M->data.fl; (data+i*step)[j] = 3.0;
Direct matrix element access of an initialized matrix:
double a[16]; CvMat Ma = cvMat(3, 4, CV_64FC1, a); a[i*4+j] = 2.0; // Ma(i,j)=2.0;
Matrix/vector operations
Matrix-matrix operations:CvMat *Ma, *Mb, *Mc; cvAdd(Ma, Mb, Mc); // Ma+Mb -> Mc cvSub(Ma, Mb, Mc); // Ma-Mb -> Mc cvMatMul(Ma, Mb, Mc); // Ma*Mb -> Mc
Elementwise matrix operations:
CvMat *Ma, *Mb, *Mc; cvMul(Ma, Mb, Mc); // Ma.*Mb -> Mc cvDiv(Ma, Mb, Mc); // Ma./Mb -> Mc cvAddS(Ma, cvScalar(-10.0), Mc); // Ma.-10 -> Mc
Vector products:
double va[] = {1, 2, 3}; double vb[] = {0, 0, 1}; double vc[3]; CvMat Va=cvMat(3, 1, CV_64FC1, va); CvMat Vb=cvMat(3, 1, CV_64FC1, vb); CvMat Vc=cvMat(3, 1, CV_64FC1, vc); double res=cvDotProduct(&Va,&Vb); // dot product: Va . Vb -> res cvCrossProduct(&Va, &Vb, &Vc); // cross product: Va x Vb -> Vc end{verbatim}
Note that Va, Vb, Vc, must be 3 element vectors in a cross product.
Single matrix operations:
CvMat *Ma, *Mb; cvTranspose(Ma, Mb); // transpose(Ma) -> Mb (cannot transpose onto self) CvScalar t = cvTrace(Ma); // trace(Ma) -> t.val[0] double d = cvDet(Ma); // det(Ma) -> d cvInvert(Ma, Mb); // inv(Ma) -> Mb
Inhomogeneous linear system solver:
CvMat* A = cvCreateMat(3,3,CV_32FC1); CvMat* x = cvCreateMat(3,1,CV_32FC1); CvMat* b = cvCreateMat(3,1,CV_32FC1); cvSolve(&A, &b, &x); // solve (Ax=b) for x
Eigen analysis (of a symmetric matrix):
CvMat* A = cvCreateMat(3,3,CV_32FC1); CvMat* E = cvCreateMat(3,3,CV_32FC1); CvMat* l = cvCreateMat(3,1,CV_32FC1); cvEigenVV(&A, &E, &l); // l = eigenvalues of A (descending order) // E = corresponding eigenvectors (rows)
Singular value decomposition:
CvMat* A = cvCreateMat(3,3,CV_32FC1); CvMat* U = cvCreateMat(3,3,CV_32FC1); CvMat* D = cvCreateMat(3,3,CV_32FC1); CvMat* V = cvCreateMat(3,3,CV_32FC1); cvSVD(A, D, U, V, CV_SVD_U_T|CV_SVD_V_T); // A = U D V^T
The flags cause U and V to be returned transposed (does not work wellwithout the transpose flags).
Working with video sequences
Capturing a frame from a video sequence
OpenCV supports capturing images from a camera or a video file (AVI).Initializing capture from a camera:
CvCapture* capture = cvCaptureFromCAM(0); // capture from video device #0
Initializing capture from a file:
CvCapture* capture = cvCaptureFromAVI("infile.avi");
Capturing a frame:
IplImage* img = 0; if(!cvGrabFrame(capture)){ // capture a frame printf("Could not grab a frame\n\7"); exit(0); } img=cvRetrieveFrame(capture); // retrieve the captured frame
To obtain images from several cameras simultaneously, first grab animage from each camera. Retrieve the captured images after the grabbing iscomplete.
Releasing the capture source:
cvReleaseCapture(&capture);
Note that the image captured by the device is allocated/released by thecapture function. There is no need to release it explicitly.
Getting/setting frame information
Get capture device properties: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);
The total frame count is relevant for video files only. It does not seemto be working properly.
Get frame information:
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);
Get the position of the captured frame in [msec] with respect to thefirst frame, or get its index where the first frame starts with an indexof 0. The relative position (ratio) is 0 in the first frame and 1 inthe last frame. This ratio is valid only for capturing
images from afile.
Set the index of the first frame to capture:
// start capturing from a relative position of 0.9 of a video file cvSetCaptureProperty(capture, CV_CAP_PROP_POS_AVI_RATIO, (double)0.9);
This only applies for capturing from a file. It does not seem to beworking properly.
Saving a video file
Initializing a video writer: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);
Other possible codec codes:
CV_FOURCC('P','I','M','1') = MPEG-1 codec CV_FOURCC('M','J','P','G') = motion-jpeg codec (does not work well) CV_FOURCC('M', 'P', '4', '2') = MPEG-4.2 codec CV_FOURCC('D', 'I', 'V', '3') = MPEG-4.3 codec CV_FOURCC('D', 'I', 'V', 'X') = MPEG-4 codec CV_FOURCC('U', '2', '6', '3') = H263 codec CV_FOURCC('I', '2', '6', '3') = H263I codec CV_FOURCC('F', 'L', 'V', '1') = FLV1 codec
A codec code of -1 will open a codec selection window (in windows).
Writing the video file:
IplImage* img = 0; int nFrames = 50; for(i=0;i<nFrames;i++){ cvGrabFrame(capture); // capture a frame img=cvRetrieveFrame(capture); // retrieve the captured frame cvWriteFrame(writer,img); // add the frame to the file }
To view the captured frames during capture, add the following in the loop:
cvShowImage("mainWin", img); key=cvWaitKey(20); // wait 20 ms
Note that without the 20[msec] delay the captured sequence is notdisplayed properly.
Releasing the video writer:
cvReleaseVideoWriter(&writer);
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