VideoCapture之grab()和retrieve（）

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本文转自： http://blog.csdn.net/weixiaomm/article/details/8725498

http://blog.csdn.net/weixiaomm/article/details/8725511

Grabs the next frame from video file or capturing device.

C++:  bool  VideoCapture:: grab ( )

Python:   cv2.VideoCapture. grab ( ) → retval

C:  int  cvGrabFrame (CvCapture*  capture )

Python:   cv. GrabFrame (capture ) → int

The methods/functions grab the next frame from video file or camera and return true (non-zero) in the case of success.

The primary use of the function is in multi-camera environments, especially when the cameras do not have hardware synchronization. That is, you call VideoCapture::grab() for each camera and after that call the slower methodVideoCapture::retrieve() to decode and get frame from each camera. This way the overhead on demosaicing or motion jpeg decompression etc. is eliminated and the retrieved frames from different cameras will be closer in time.

Also, when a connected camera is multi-head (for example, a stereo camera or a Kinect device), the correct way of retrieving data from it is to call VideoCapture::grab first and then call VideoCapture::retrieve() one or more times with different values of the channel parameter.

Decodes and returns the grabbed video frame.

C++:  bool  VideoCapture:: retrieve (Mat&  image, int  channel=0 ) ¶

Python:   cv2.VideoCapture. retrieve ( [image [, channel ] ] ) → retval, image

C:  IplImage*  cvRetrieveFrame (CvCapture*  capture, int  streamIdx=0  )

Python:   cv. RetrieveFrame (capture ) → image

The methods/functions decode and return the just grabbed frame. If no frames has been grabbed (camera has been disconnected, or there are no more frames in video file), the methods return false and the functions return NULL pointer.

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