OpenCV  3.1.0-dev
Open Source Computer Vision
Public Member Functions | Protected Attributes | List of all members
cv::VideoCapture Class Reference

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 VideoCaptureoperator>> (Mat &image)
 
virtual VideoCaptureoperator>> (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< CvCapturecap
 
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
filenamename 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
filenamename 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, ...)
apiPreferencepreferred 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
propIdProperty identifier. It can be one of the following:
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
filenamename 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
filenamename 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, ...)
apiPreferencepreferred 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
propIdProperty identifier. It can be one of the following:
valueValue 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: