cv::VideoCapture Class Reference

Class for video capturing from video files, image sequences or cameras. More...

#include <opencv2/videoio.hpp>

Collaboration diagram for cv::VideoCapture:

Public Member Functions
	VideoCapture ()
	Default constructor.
	VideoCapture (const String &filename, int apiPreference, const std::vector< int > &params)
	Opens a video file or a capturing device or an IP video stream for video capturing with API Preference and parameters.
	VideoCapture (const String &filename, int apiPreference=CAP_ANY)
	Opens a video file or a capturing device or an IP video stream for video capturing with API Preference.
	VideoCapture (int index, int apiPreference, const std::vector< int > &params)
	Opens a camera for video capturing with API Preference and parameters.
	VideoCapture (int index, int apiPreference=CAP_ANY)
	Opens a camera for video capturing.
virtual	~VideoCapture ()
	Default destructor.
virtual double	get (int propId) const
	Returns the specified VideoCapture property.
String	getBackendName () const
	Returns used backend API name.
bool	getExceptionMode () const
	query if exception mode is active
virtual bool	grab ()
	Grabs the next frame from video file or capturing device.
virtual bool	isOpened () const
	Returns true if video capturing has been initialized already.
virtual bool	open (const String &filename, int apiPreference, const std::vector< int > &params)
	Opens a video file or a capturing device or an IP video stream for video capturing with API Preference and parameters.
virtual bool	open (const String &filename, int apiPreference=CAP_ANY)
	Opens a video file or a capturing device or an IP video stream for video capturing.
virtual bool	open (int index, int apiPreference, const std::vector< int > &params)
	Opens a camera for video capturing with API Preference and parameters.
virtual bool	open (int index, int apiPreference=CAP_ANY)
	Opens a camera for video capturing.
virtual VideoCapture &	operator>> (Mat &image)
	Stream operator to read the next video frame.
virtual VideoCapture &	operator>> (UMat &image)
virtual bool	read (OutputArray image)
	Grabs, decodes and returns the next video frame.
virtual void	release ()
	Closes video file or capturing device.
virtual bool	retrieve (OutputArray image, int flag=0)
	Decodes and returns the grabbed video frame.
virtual bool	set (int propId, double value)
	Sets a property in the VideoCapture.
void	setExceptionMode (bool enable)

Static Public Member Functions
static bool	waitAny (const std::vector< VideoCapture > &streams, std::vector< int > &readyIndex, int64 timeoutNs=0)
	Wait for ready frames from VideoCapture.

Protected Attributes
Ptr< CvCapture >	cap
Ptr< IVideoCapture >	icap
bool	throwOnFail

Friends
class	internal::VideoCapturePrivateAccessor

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/core.hpp>
#include <opencv2/videoio.hpp>
#include <opencv2/highgui.hpp>
#include <iostream>
#include <stdio.h>
 
using namespace cv;
using namespace std;
 
int main(int, char**)
{
    Mat frame;
    //--- INITIALIZE VIDEOCAPTURE
    VideoCapture cap;
    // open the default camera using default API
    // cap.open(0);
    // OR advance usage: select any API backend
    int deviceID = 0;             // 0 = open default camera
    int apiID = cv::CAP_ANY;      // 0 = autodetect default API
    // open selected camera using selected API
    cap.open(deviceID, apiID);
    // check if we succeeded
    if (!cap.isOpened()) {
        cerr << "ERROR! Unable to open camera\n";
        return -1;
    }
 
    //--- GRAB AND WRITE LOOP
    cout << "Start grabbing" << endl
        << "Press any key to terminate" << endl;
    for (;;)
    {
        // wait for a new frame from camera and store it into 'frame'
        cap.read(frame);
        // check if we succeeded
        if (frame.empty()) {
            cerr << "ERROR! blank frame grabbed\n";
            break;
        }
        // show live and wait for a key with timeout long enough to show images
        imshow("Live", frame);
        if (waitKey(5) >= 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.

• (C++) A basic sample on using the VideoCapture interface can be found at OPENCV_SOURCE_CODE/samples/cpp/videocapture_starter.cpp
• (Python) A basic sample on using the VideoCapture interface can be found at OPENCV_SOURCE_CODE/samples/python/video.py
• (Python) A multi threaded video processing sample can be found at OPENCV_SOURCE_CODE/samples/python/video_threaded.py
• (Python) VideoCapture sample showcasing some features of the Video4Linux2 backend OPENCV_SOURCE_CODE/samples/python/video_v4l2.py

Examples

samples/cpp/camshiftdemo.cpp, samples/cpp/facedetect.cpp, samples/cpp/laplace.cpp, samples/cpp/lkdemo.cpp, samples/cpp/peopledetect.cpp, samples/cpp/polar_transforms.cpp, samples/cpp/segment_objects.cpp, samples/cpp/train_HOG.cpp, samples/cpp/tutorial_code/videoio/video-write/video-write.cpp, samples/cpp/videowriter_basic.cpp, samples/dnn/classification.cpp, samples/dnn/object_detection.cpp, samples/dnn/segmentation.cpp, samples/dnn/text_detection.cpp, and samples/tapi/hog.cpp.

Constructor & Destructor Documentation

VideoCapture() [1/5]

cv::VideoCapture::VideoCapture

(

)

Python:
	cv.VideoCapture(		) ->	<VideoCapture object>
	cv.VideoCapture(	filename[, apiPreference]	) ->	<VideoCapture object>
	cv.VideoCapture(	filename, apiPreference, params	) ->	<VideoCapture object>
	cv.VideoCapture(	index[, apiPreference]	) ->	<VideoCapture object>
	cv.VideoCapture(	index, apiPreference, params	) ->	<VideoCapture object>

Default constructor.

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.

VideoCapture() [2/5]

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

explicit

Python:
	cv.VideoCapture(		) ->	<VideoCapture object>
	cv.VideoCapture(	filename[, apiPreference]	) ->	<VideoCapture object>
	cv.VideoCapture(	filename, apiPreference, params	) ->	<VideoCapture object>
	cv.VideoCapture(	index[, apiPreference]	) ->	<VideoCapture object>
	cv.VideoCapture(	index, apiPreference, params	) ->	<VideoCapture object>

Opens a video file or a capturing device or an IP video stream for video capturing with API Preference.

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

Parameters

filename

it can be: name of 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, ...) or URL of video stream (eg. protocol://host:port/script_name?script_params|auth) or GStreamer pipeline string in gst-launch tool format in case if GStreamer is used as backend Note that each video stream or IP camera feed has its own URL scheme. Please refer to the documentation of source stream to know the right URL.

apiPreference

preferred Capture API backends to use. Can be used to enforce a specific reader implementation if multiple are available: e.g. cv::CAP_FFMPEG or cv::CAP_IMAGES or cv::CAP_DSHOW.

See also

cv::VideoCaptureAPIs

VideoCapture() [3/5]

cv::VideoCapture::VideoCapture ( const String & filename, int apiPreference, const std::vector< int > & params )

explicit

Python:
	cv.VideoCapture(		) ->	<VideoCapture object>
	cv.VideoCapture(	filename[, apiPreference]	) ->	<VideoCapture object>
	cv.VideoCapture(	filename, apiPreference, params	) ->	<VideoCapture object>
	cv.VideoCapture(	index[, apiPreference]	) ->	<VideoCapture object>
	cv.VideoCapture(	index, apiPreference, params	) ->	<VideoCapture object>

Opens a video file or a capturing device or an IP video stream for video capturing with API Preference and parameters.

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

The params parameter allows to specify extra parameters encoded as pairs (paramId_1, paramValue_1, paramId_2, paramValue_2, ...). See cv::VideoCaptureProperties

VideoCapture() [4/5]

cv::VideoCapture::VideoCapture ( int index, int apiPreference = CAP_ANY )

explicit

Python:
	cv.VideoCapture(		) ->	<VideoCapture object>
	cv.VideoCapture(	filename[, apiPreference]	) ->	<VideoCapture object>
	cv.VideoCapture(	filename, apiPreference, params	) ->	<VideoCapture object>
	cv.VideoCapture(	index[, apiPreference]	) ->	<VideoCapture object>
	cv.VideoCapture(	index, apiPreference, params	) ->	<VideoCapture object>

Opens a camera for video capturing.

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

Parameters

index	id of the video capturing device to open. To open default camera using default backend just pass 0. (to backward compatibility usage of camera_id + domain_offset (CAP_*) is valid when apiPreference is CAP_ANY)
apiPreference	preferred Capture API backends to use. Can be used to enforce a specific reader implementation if multiple are available: e.g. cv::CAP_DSHOW or cv::CAP_MSMF or cv::CAP_V4L.

See also

cv::VideoCaptureAPIs

VideoCapture() [5/5]

cv::VideoCapture::VideoCapture ( int index, int apiPreference, const std::vector< int > & params )

explicit

Python:
	cv.VideoCapture(		) ->	<VideoCapture object>
	cv.VideoCapture(	filename[, apiPreference]	) ->	<VideoCapture object>
	cv.VideoCapture(	filename, apiPreference, params	) ->	<VideoCapture object>
	cv.VideoCapture(	index[, apiPreference]	) ->	<VideoCapture object>
	cv.VideoCapture(	index, apiPreference, params	) ->	<VideoCapture object>

Opens a camera for video capturing with API Preference and parameters.

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

The params parameter allows to specify extra parameters encoded as pairs (paramId_1, paramValue_1, paramId_2, paramValue_2, ...). See cv::VideoCaptureProperties

~VideoCapture()

virtual cv::VideoCapture::~VideoCapture ( )

virtual

Default destructor.

The method first calls VideoCapture::release to close the already opened file or camera.

Member Function Documentation

get()

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

virtual

Python:
	cv.VideoCapture.get(	propId	) ->	retval

Returns the specified VideoCapture property.

Parameters

propId

Property identifier from cv::VideoCaptureProperties (eg. cv::CAP_PROP_POS_MSEC, cv::CAP_PROP_POS_FRAMES, ...) or one from Additional flags for video I/O API backends

Returns

Value for the specified property. Value 0 is returned when querying a property that is not supported by the backend used by the VideoCapture instance.

Note

Reading / writing properties involves many layers. Some unexpected result might happens along this chain.

VideoCapture -> API Backend -> Operating System -> Device Driver -> Device Hardware

The returned value might be different from what really used by the device or it could be encoded using device dependent rules (eg. steps or percentage). Effective behaviour depends from device driver and API Backend

Examples

samples/cpp/laplace.cpp.

getBackendName()

String cv::VideoCapture::getBackendName

(

)

const

Python:
	cv.VideoCapture.getBackendName(		) ->	retval

Returns used backend API name.

Note

Stream should be opened.

getExceptionMode()

bool cv::VideoCapture::getExceptionMode ( ) const

inline

Python:
	cv.VideoCapture.getExceptionMode(		) ->	retval

query if exception mode is active

grab()

virtual bool cv::VideoCapture::grab ( )

virtual

Python:
	cv.VideoCapture.grab(		) ->	retval

Grabs the next frame from video file or capturing device.

Returns

true (non-zero) in the case of success.

The method/function grabs the next frame from video file or camera and returns 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.

Using Kinect and other OpenNI compatible depth sensors

isOpened()

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

virtual

Python:
	cv.VideoCapture.isOpened(		) ->	retval

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

samples/cpp/camshiftdemo.cpp, samples/cpp/facedetect.cpp, samples/cpp/laplace.cpp, samples/cpp/lkdemo.cpp, samples/cpp/peopledetect.cpp, samples/cpp/polar_transforms.cpp, samples/cpp/segment_objects.cpp, samples/cpp/train_HOG.cpp, samples/cpp/videowriter_basic.cpp, and samples/tapi/hog.cpp.

open() [1/4]

virtual bool cv::VideoCapture::open ( const String & filename, int apiPreference, const std::vector< int > & params )

virtual

Python:
	cv.VideoCapture.open(	filename[, apiPreference]	) ->	retval
	cv.VideoCapture.open(	filename, apiPreference, params	) ->	retval
	cv.VideoCapture.open(	index[, apiPreference]	) ->	retval
	cv.VideoCapture.open(	index, apiPreference, params	) ->	retval

Opens a video file or a capturing device or an IP video stream for video capturing with API Preference and parameters.

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

The params parameter allows to specify extra parameters encoded as pairs (paramId_1, paramValue_1, paramId_2, paramValue_2, ...). See cv::VideoCaptureProperties

Returns

true if the file has been successfully opened

The method first calls VideoCapture::release to close the already opened file or camera.

open() [2/4]

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

virtual

Python:
	cv.VideoCapture.open(	filename[, apiPreference]	) ->	retval
	cv.VideoCapture.open(	filename, apiPreference, params	) ->	retval
	cv.VideoCapture.open(	index[, apiPreference]	) ->	retval
	cv.VideoCapture.open(	index, apiPreference, params	) ->	retval

Opens a video file or a capturing device or an IP video stream for video capturing.

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

Parameters are same as the constructor VideoCapture(const String& filename, int apiPreference = CAP_ANY)

Returns

true if the file has been successfully opened

The method first calls VideoCapture::release to close the already opened file or camera.

Examples

samples/cpp/camshiftdemo.cpp, samples/cpp/facedetect.cpp, samples/cpp/laplace.cpp, samples/cpp/lkdemo.cpp, samples/cpp/peopledetect.cpp, samples/cpp/polar_transforms.cpp, samples/cpp/segment_objects.cpp, samples/cpp/train_HOG.cpp, samples/dnn/classification.cpp, samples/dnn/object_detection.cpp, samples/dnn/segmentation.cpp, samples/dnn/text_detection.cpp, and samples/tapi/hog.cpp.

open() [3/4]

virtual bool cv::VideoCapture::open ( int index, int apiPreference, const std::vector< int > & params )

virtual

Python:
	cv.VideoCapture.open(	filename[, apiPreference]	) ->	retval
	cv.VideoCapture.open(	filename, apiPreference, params	) ->	retval
	cv.VideoCapture.open(	index[, apiPreference]	) ->	retval
	cv.VideoCapture.open(	index, apiPreference, params	) ->	retval

Opens a camera for video capturing with API Preference and parameters.

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

The params parameter allows to specify extra parameters encoded as pairs (paramId_1, paramValue_1, paramId_2, paramValue_2, ...). See cv::VideoCaptureProperties

Returns

true if the camera has been successfully opened.

The method first calls VideoCapture::release to close the already opened file or camera.

open() [4/4]

virtual bool cv::VideoCapture::open ( int index, int apiPreference = CAP_ANY )

virtual

Python:
	cv.VideoCapture.open(	filename[, apiPreference]	) ->	retval
	cv.VideoCapture.open(	filename, apiPreference, params	) ->	retval
	cv.VideoCapture.open(	index[, apiPreference]	) ->	retval
	cv.VideoCapture.open(	index, apiPreference, params	) ->	retval

Opens a camera for video capturing.

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

Parameters are same as the constructor VideoCapture(int index, int apiPreference = CAP_ANY)

Returns

true if the camera has been successfully opened.

The method first calls VideoCapture::release to close the already opened file or camera.

operator>>() [1/2]

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

virtual

Stream operator to read the next video frame.

See also

read()

operator>>() [2/2]

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

virtual

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

See also

read()

read()

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

virtual

Python:
	cv.VideoCapture.read(	[, image]	) ->	retval, image

Grabs, decodes and returns the next video frame.

Parameters

[out]

image

the video frame is returned here. If no frames has been grabbed the image will be empty.

Returns

false if no frames has been grabbed

The method/function combines VideoCapture::grab() and VideoCapture::retrieve() in one call. This is the most convenient method for reading video files or capturing data from decode and returns the just grabbed frame. If no frames has been grabbed (camera has been disconnected, or there are no more frames in video file), the method returns false and the function returns empty image (with cv::Mat, test it with Mat::empty()).

Note

In C API, 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.

Examples

samples/cpp/videowriter_basic.cpp.

release()

virtual void cv::VideoCapture::release ( )

virtual

Python:
	cv.VideoCapture.release(		) ->	None

Closes video file or capturing device.

The method is automatically called by subsequent VideoCapture::open and by VideoCapture destructor.

The C function also deallocates memory and clears *capture pointer.

retrieve()

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

virtual

Python:
	cv.VideoCapture.retrieve(	[, image[, flag]]	) ->	retval, image

Decodes and returns the grabbed video frame.

Parameters

[out]	image	the video frame is returned here. If no frames has been grabbed the image will be empty.
	flag	it could be a frame index or a driver specific flag

Returns

false if no frames has been grabbed

The method decodes and returns the just grabbed frame. If no frames has been grabbed (camera has been disconnected, or there are no more frames in video file), the method returns false and the function returns an empty image (with cv::Mat, test it with Mat::empty()).

See also

read()

Note

In C API, 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.

set()

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

virtual

Python:
	cv.VideoCapture.set(	propId, value	) ->	retval

Sets a property in the VideoCapture.

Parameters

propId	Property identifier from cv::VideoCaptureProperties (eg. cv::CAP_PROP_POS_MSEC, cv::CAP_PROP_POS_FRAMES, ...) or one from Additional flags for video I/O API backends
value	Value of the property.

Returns

true if the property is supported by backend used by the VideoCapture instance.

Note

Even if it returns true this doesn't ensure that the property value has been accepted by the capture device. See note in VideoCapture::get()

Examples

samples/cpp/laplace.cpp.

setExceptionMode()

void cv::VideoCapture::setExceptionMode ( bool enable )

inline

Python:
	cv.VideoCapture.setExceptionMode(	enable	) ->	None

Switches exceptions mode

methods raise exceptions if not successful instead of returning an error code

waitAny()

static bool cv::VideoCapture::waitAny ( const std::vector< VideoCapture > & streams, std::vector< int > & readyIndex, int64 timeoutNs = 0 )

static

Python:
	cv.VideoCapture.waitAny(	streams[, timeoutNs]	) ->	retval, readyIndex
	cv.VideoCapture_waitAny(	streams[, timeoutNs]	) ->	retval, readyIndex

Wait for ready frames from VideoCapture.

Parameters

streams	input video streams
readyIndex	stream indexes with grabbed frames (ready to use .retrieve() to fetch actual frame)
timeoutNs	number of nanoseconds (0 - infinite)

Returns

true if streamReady is not empty

Exceptions

Exception

Exception on stream errors (check .isOpened() to filter out malformed streams) or VideoCapture type is not supported

The primary use of the function is in multi-camera environments. The method fills the ready state vector, grabs video frame, if camera is ready.

After this call use VideoCapture::retrieve() to decode and fetch frame data.

Friends And Related Symbol Documentation

internal::VideoCapturePrivateAccessor

friend class internal::VideoCapturePrivateAccessor

friend

Member Data Documentation

cap

Ptr<CvCapture> cv::VideoCapture::cap

protected

icap

Ptr<IVideoCapture> cv::VideoCapture::icap

protected

throwOnFail

bool cv::VideoCapture::throwOnFail

protected

---

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

• opencv2/videoio.hpp