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: : More...

#include "videoio.hpp"

Public Member Functions
	VideoCapture ()

	VideoCapture (const String &filename)

	VideoCapture (const String &filename, int apiPreference)

	VideoCapture (int index)

virtual	~VideoCapture ()

virtual double	get (int propId) const
	Returns the specified VideoCapture property. More...

virtual bool	grab ()
	Grabs the next frame from video file or capturing device. More...

virtual bool	isOpened () const
	Returns true if video capturing has been initialized already. More...

virtual bool	open (const String &filename)
	Open video file or a capturing device for video capturing. More...

virtual bool	open (int index)

virtual bool	open (const String &filename, int apiPreference)

virtual VideoCapture &	operator>> (Mat &image)

virtual VideoCapture &	operator>> (UMat &image)

virtual bool	read (OutputArray image)
	Grabs, decodes and returns the next video frame. More...

virtual void	release ()
	Closes video file or capturing device. More...

virtual bool	retrieve (OutputArray image, int flag=0)
	Decodes and returns the grabbed video frame. More...

virtual bool	set (int propId, double value)
	Sets a property in the VideoCapture. More...

Protected Attributes
Ptr< CvCapture >	cap

Ptr< IVideoCapture >	icap

Detailed Description

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.

A basic sample on using the VideoCapture interface can be found at opencv_source_code/samples/cpp/starter_video.cpp
Another basic video processing sample can be found at opencv_source_code/samples/cpp/video_dmtx.cpp
(Python) A basic sample on using the VideoCapture interface can be found at opencv_source_code/samples/python/video.py
(Python) Another basic video processing sample can be found at opencv_source_code/samples/python/video_dmtx.py
(Python) A multi threaded video processing sample can be found at opencv_source_code/samples/python/video_threaded.py

Examples:: laplace.cpp, polar_transforms.cpp, and segment_objects.cpp.

Constructor & Destructor Documentation

cv::VideoCapture::VideoCapture ( )

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.

cv::VideoCapture::VideoCapture ( const String & filename )

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

Parameters

filename name of the opened video file (eg. video.avi) or image sequence (eg. img_%02d.jpg, which will read samples like img_00.jpg, img_01.jpg, img_02.jpg, ...)

cv::VideoCapture::VideoCapture	(	const String &	filename,
		int	apiPreference
	)

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

Parameters

filename	name of the opened video file (eg. video.avi) or image sequence (eg. img_%02d.jpg, which will read samples like img_00.jpg, img_01.jpg, img_02.jpg, ...)
apiPreference	preferred Capture API to use. Can be used to enforce a specific reader implementation if multiple are available: e.g. cv::CAP_FFMPEG or cv::CAP_IMAGES

cv::VideoCapture::VideoCapture ( int index )

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

Parameters

index = camera_id + domain_offset (CAP_*). id of the video capturing device to open. If there is a single camera connected, just pass 0. Advanced Usage: to open Camera 1 using the MS Media Foundation API: index = 1 + cv::CAP_MSMF

virtual cv::VideoCapture::~VideoCapture ( )

virtual

Member Function Documentation

virtual double cv::VideoCapture::get ( int propId ) const

virtual

Returns the specified VideoCapture property.

Parameters

propId

Property identifier. It can be one of the following:

cv::CAP_PROP_POS_MSEC Current position of the video file in milliseconds.
cv::CAP_PROP_POS_FRAMES 0-based index of the frame to be decoded/captured next.
cv::CAP_PROP_POS_AVI_RATIO Relative position of the video file: 0=start of the film, 1=end of the film.
cv::CAP_PROP_FRAME_WIDTH Width of the frames in the video stream.
cv::CAP_PROP_FRAME_HEIGHT Height of the frames in the video stream.
cv::CAP_PROP_FPS Frame rate.
cv::CAP_PROP_FOURCC 4-character code of codec. see VideoWriter::fourcc .
cv::CAP_PROP_FRAME_COUNT Number of frames in the video file.
cv::CAP_PROP_FORMAT Format of the Mat objects returned by VideoCapture::retrieve().
cv::CAP_PROP_MODE Backend-specific value indicating the current capture mode.
cv::CAP_PROP_BRIGHTNESS Brightness of the image (only for cameras).
cv::CAP_PROP_CONTRAST Contrast of the image (only for cameras).
cv::CAP_PROP_SATURATION Saturation of the image (only for cameras).
cv::CAP_PROP_HUE Hue of the image (only for cameras).
cv::CAP_PROP_GAIN Gain of the image (only for cameras).
cv::CAP_PROP_EXPOSURE Exposure (only for cameras).
cv::CAP_PROP_CONVERT_RGB Boolean flags indicating whether images should be converted to RGB.
cv::CAP_PROP_WHITE_BALANCE_BLUE_U Currently unsupported.
cv::CAP_PROP_WHITE_BALANCE_RED_V
cv::CAP_PROP_RECTIFICATION Rectification flag for stereo cameras (note: only supported by DC1394 v 2.x backend currently).

Note: When querying a property that is not supported by the backend used by the VideoCapture class, value 0 is returned.

Examples:: laplace.cpp.

virtual bool cv::VideoCapture::grab ( )

virtual

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/opencv/opencv/tree/master/samples/cpp/openni_capture.cpp

virtual bool cv::VideoCapture::isOpened ( ) const

virtual

Returns true if video capturing has been initialized already.

If the previous call to VideoCapture constructor or VideoCapture::open succeeded, the method returns true.

Examples:: laplace.cpp, polar_transforms.cpp, and segment_objects.cpp.

virtual bool cv::VideoCapture::open ( const String & filename )

virtual

Open video file or a capturing device for video capturing.

Parameters

filename name of the opened video file (eg. video.avi) or image sequence (eg. img_%02d.jpg, which will read samples like img_00.jpg, img_01.jpg, img_02.jpg, ...)

The methods first call VideoCapture::release to close the already opened file or camera.

Examples:: laplace.cpp, polar_transforms.cpp, and segment_objects.cpp.

virtual bool cv::VideoCapture::open ( int index )

virtual

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

Parameters

index = camera_id + domain_offset (CAP_*). id of the video capturing device to open. If there is a single camera connected, just pass 0. Advanced Usage: to open Camera 1 using the MS Media Foundation API: index = 1 + cv::CAP_MSMF

virtual bool cv::VideoCapture::open	(	const String &	filename,
		int	apiPreference
	)

virtual

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

Parameters

filename	name of the opened video file (eg. video.avi) or image sequence (eg. img_%02d.jpg, which will read samples like img_00.jpg, img_01.jpg, img_02.jpg, ...)
apiPreference	preferred Capture API to use. Can be used to enforce a specific reader implementation if multiple are available: e.g. cv::CAP_FFMPEG or cv::CAP_IMAGES

The methods first call VideoCapture::release to close the already opened file or camera.

virtual VideoCapture& cv::VideoCapture::operator>> ( Mat & image )

virtual

virtual VideoCapture& cv::VideoCapture::operator>> ( UMat & image )

virtual

virtual bool cv::VideoCapture::read ( OutputArray image )

virtual

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 :ocvcvCloneImage and then do whatever you want with the copy.

virtual void cv::VideoCapture::release ( )

virtual

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.

virtual bool cv::VideoCapture::retrieve	(	OutputArray	image,
		int	flag = `0`
	)

virtual

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 :ocvcvCloneImage and then do whatever you want with the copy.

virtual bool cv::VideoCapture::set	(	int	propId,
		double	value
	)

virtual

Sets a property in the VideoCapture.

Parameters

propId

Property identifier. It can be one of the following:

cv::CAP_PROP_POS_MSEC Current position of the video file in milliseconds.
cv::CAP_PROP_POS_FRAMES 0-based index of the frame to be decoded/captured next.
cv::CAP_PROP_POS_AVI_RATIO Relative position of the video file: 0=start of the film, 1=end of the film.
cv::CAP_PROP_FRAME_WIDTH Width of the frames in the video stream.
cv::CAP_PROP_FRAME_HEIGHT Height of the frames in the video stream.
cv::CAP_PROP_FPS Frame rate.
cv::CAP_PROP_FOURCC 4-character code of codec. see VideoWriter::fourcc .
cv::CAP_PROP_FRAME_COUNT Number of frames in the video file.
cv::CAP_PROP_FORMAT Format of the Mat objects returned by VideoCapture::retrieve().
cv::CAP_PROP_MODE Backend-specific value indicating the current capture mode.
cv::CAP_PROP_BRIGHTNESS Brightness of the image (only for cameras).
cv::CAP_PROP_CONTRAST Contrast of the image (only for cameras).
cv::CAP_PROP_SATURATION Saturation of the image (only for cameras).
cv::CAP_PROP_HUE Hue of the image (only for cameras).
cv::CAP_PROP_GAIN Gain of the image (only for cameras).
cv::CAP_PROP_EXPOSURE Exposure (only for cameras).
cv::CAP_PROP_CONVERT_RGB Boolean flags indicating whether images should be converted to RGB.
cv::CAP_PROP_WHITE_BALANCE_BLUE_U Currently unsupported.
cv::CAP_PROP_WHITE_BALANCE_RED_V
cv::CAP_PROP_RECTIFICATION Rectification flag for stereo cameras (note: only supported by DC1394 v 2.x backend currently).

value Value of the property.

Examples:: laplace.cpp.

Member Data Documentation

Ptr<CvCapture> cv::VideoCapture::cap

protected

Ptr<IVideoCapture> cv::VideoCapture::icap

protected

The documentation for this class was generated from the following file:

videoio/include/opencv2/videoio.hpp

Public Member Functions

Protected Attributes

Detailed Description

Constructor & Destructor Documentation

Member Function Documentation

Member Data Documentation