class cv::VideoCapture
Overview
Class for video capturing from video files, image sequences or cameras. More…
#include <videoio.hpp> class VideoCapture { public: // construction VideoCapture(); VideoCapture(const String& filename); VideoCapture( const String& filename, int apiPreference ); VideoCapture(int index); // methods virtual double get(int propId) const; virtual bool grab(); virtual bool isOpened() const; virtual bool open(const String& filename); virtual bool open(int index); bool open( int cameraNum, int apiPreference ); virtual bool open( const String& filename, int apiPreference ); virtual VideoCapture& operator>>(Mat& image); virtual VideoCapture& operator>>(UMat& image); virtual bool read(OutputArray image); virtual void release(); virtual bool retrieve( OutputArray image, int flag = 0 ); virtual bool set( int propId, double value ); protected: // fields Ptr<CvCapture> cap; Ptr<IVideoCapture> icap; };
Detailed Documentation
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> #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; }
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
Construction
VideoCapture()
Default constructor.
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(const String& filename)
Open video file or a capturing device or a 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. Same as VideoCapture(const String& filename, int apiPreference) but using default Capture API backends
VideoCapture( const String& filename, int apiPreference )
Open video file or a capturing device or a 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:
|
| 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:
The list of supported API backends cv::VideoCaptureAPIs
VideoCapture(int index)
Open 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 | camera_id + domain_offset (CAP_*) id of the video capturing device to open. To open default camera using default backend just pass 0. Use a domain_offset to enforce a specific reader implementation if multiple are available like cv::CAP_FFMPEG or cv::CAP_IMAGES or cv::CAP_DSHOW. e.g. to open Camera 1 using the MS Media Foundation API use index = 1 + cv::CAP_MSMF |
See also:
The list of supported API backends cv::VideoCaptureAPIs
Methods
virtual double get(int propId) const
Returns the specified VideoCapture property.
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 dependant rules (eg. steps or percentage). Effective behaviour depends from device driver and API Backend
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.
virtual bool grab()
Grabs the next frame from video file or capturing device.
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
Returns:
true (non-zero) in the case of success.
virtual bool isOpened() const
Returns true if video capturing has been initialized already.
If the previous call to VideoCapture constructor or VideoCapture::open() succeeded, the method returns true.
virtual bool open(const String& filename)
Open video file or a capturing device or a 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) The method first calls VideoCapture::release to close the already opened file or camera.
Returns:
true if the file has been successfully opened
virtual bool open(int index)
Open 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) The method first calls VideoCapture::release to close the already opened file or camera.
Returns:
true if the camera has been successfully opened.
bool open( int cameraNum, int apiPreference )
Open 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 similar as the constructor VideoCapture(int index),except it takes an additional argument apiPreference. Definitely, is same as open(int index) where index=cameraNum + apiPreference
Returns:
true if the camera has been successfully opened.
virtual bool open( const String& filename, int apiPreference )
Open video file or a capturing device or a 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 are same as the constructor VideoCapture(const String& filename, int apiPreference) The method first calls VideoCapture::release to close the already opened file or camera.
Returns:
true if the file has been successfully opened
virtual VideoCapture& operator>>(Mat& image)
Stream operator to read the next video frame.
See also:
virtual VideoCapture& operator>>(UMat& image)
This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.
See also:
virtual bool read(OutputArray image)
Grabs, decodes and returns the next video frame.
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()).
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 :ocvcvCloneImage and then do whatever you want with the copy.
Parameters:
| 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
virtual void release()
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.
virtual bool retrieve( OutputArray image, int flag = 0 )
Decodes and returns the grabbed video frame.
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()).
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 :ocvcvCloneImage and then do whatever you want with the copy.
Parameters:
| 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
See also:
virtual bool set( int propId, double value )
Sets a property in the VideoCapture.
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()
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.