Learning OpenCV

Working with Video

When working with video we must consider several functions, including (of course) how to read and write video files. We must also think about how to actually play back such files on the screen.

The first thing we need is the CvCapture device. This structure contains the information needed for reading frames from a camera or video file. Depending on the source, we use one of two different calls to create and initialize a CvCapture structure.

CvCapture* cvCreateFileCapture( const char* filename );
CvCapture* cvCreateCameraCapture( int index );

In the case of cvCreateFileCapture(), we can simply give a filename for an MPG or AVI file and OpenCV will open the file and prepare to read it. If the open is successful and we are able to start reading frames, a pointer to an initialized CvCapture structure will be returned.

A lot of people don't always check these sorts of things, thinking that nothing will go wrong. Don't do that here. The returned pointer will be NULL if for some reason the file could not be opened (e.g., if the file does not exist), but cvCreateFileCapture() will also return a NULL pointer if the codec with which the video is compressed is not known. The subtleties of compression codecs are beyond the scope of this book, but in general you will need to have the appropriate library already resident on your computer in order to successfully read the video file. For example, if you want to read a file encoded with DIVX or MPG4 compression on a Windows machine, ...