Class for video capturing from video files, image sequences or cameras. The class provides C++ API for capturing video from cameras or for reading video files and image sequences. Here is how the class can be used:
#include "opencv2/opencv.hpp"
using namespace cv;
int main(int, char**)
{
VideoCapture cap(0); // open the default camera
if(!cap.isOpened()) // check if we succeeded
return -1;
Mat edges;
namedWindow("edges",1);
for(;;)
{
Mat frame;
cap >> frame; // get a new frame from camera
cvtColor(frame, edges, COLOR_BGR2GRAY);
GaussianBlur(edges, edges, Size(7,7), 1.5, 1.5);
Canny(edges, edges, 0, 30, 3);
imshow("edges", edges);
if(waitKey(30) >= 0) break;
}
// the camera will be deinitialized automatically in VideoCapture destructor
return 0;
}
Note
In C API the black-box structure CvCapture is used instead of VideoCapture.
Note
VideoCapture constructors.
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Note
In C API, when you finished working with video, release CvCapture structure with cvReleaseCapture(), or use Ptr<CvCapture> that calls cvReleaseCapture() automatically in the destructor.
Open video file or a capturing device for video capturing
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The methods first call VideoCapture::release() to close the already opened file or camera.
Returns true if video capturing has been initialized already.
If the previous call to VideoCapture constructor or VideoCapture::open succeeded, the method returns true.
Closes video file or capturing device.
The methods are automatically called by subsequent VideoCapture::open() and by VideoCapture destructor.
The C function also deallocates memory and clears *capture pointer.
Grabs the next frame from video file or capturing device.
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 method VideoCapture::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. See https://github.com/Itseez/opencv/tree/master/samples/cpp/openni_capture.cpp
Decodes and returns the grabbed video frame.
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.
Note
OpenCV 1.x functions cvRetrieveFrame and cv.RetrieveFrame return image stored inside the video capturing structure. It is not allowed to modify or release the image! You can copy the frame using cvCloneImage() and then do whatever you want with the copy.
Grabs, decodes and returns the next video frame.
The methods/functions combine VideoCapture::grab() and VideoCapture::retrieve() in one call. This is the most convenient method for reading video files or capturing data from 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.
Note
OpenCV 1.x functions cvRetrieveFrame and cv.RetrieveFrame return image stored inside the video capturing structure. It is not allowed to modify or release the image! You can copy the frame using cvCloneImage() and then do whatever you want with the copy.
Returns the specified VideoCapture property
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Note: When querying a property that is not supported by the backend used by the VideoCapture class, value 0 is returned.
Sets a property in the VideoCapture.
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VideoWriter constructors
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The constructors/functions initialize video writers. On Linux FFMPEG is used to write videos; on Windows FFMPEG or VFW is used; on MacOSX QTKit is used.
Releases the AVI writer.
The function should be called after you finished using CvVideoWriter opened with CreateVideoWriter().
Initializes or reinitializes video writer.
The method opens video writer. Parameters are the same as in the constructor VideoWriter::VideoWriter().
Returns true if video writer has been successfully initialized.
Writes the next video frame
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The functions/methods write the specified image to video file. It must have the same size as has been specified when opening the video writer.
Concatenates 4 chars to a fourcc code
This static method constructs the fourcc code of the codec to be used in the constructor VideoWriter::VideoWriter() or VideoWriter::open().