OpenCV: cv::VideoCapture Class Reference

| OpenCV 3.4.2 Open Source Computer Vision

Public Member Functions | Protected Attributes | List of all members
cv::VideoCapture Class Reference
Video I/O

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

`#include "videoio.hpp"`

## Public Member Functions
| VideoCapture ()
| Default constructor. More...
| VideoCapture (const String &filename)
| Open video file or a capturing device or a IP video stream for video capturing. More...
| VideoCapture (const String &filename, int apiPreference)
| Open video file or a capturing device or a IP video stream for video capturing with API Preference. More...
| VideoCapture (int index)
| Open a camera for video capturing. More...
virtual | ~VideoCapture ()
| Default destructor. More...
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 or a IP video stream for video capturing. More...
virtual bool | open (int index)
| Open a camera for video capturing. More...
bool | open (int cameraNum, int apiPreference)
| Open a camera for video capturing. More...
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. More...
virtual VideoCapture & | operator>> (Mat &image)
| Stream operator to read the next video frame. More...
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/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:
  camshiftdemo.cpp, facedetect.cpp, hog.cpp, laplace.cpp, lkdemo.cpp, peopledetect.cpp, polar_transforms.cpp, segment_objects.cpp, train_HOG.cpp, and videowriter_basic.cpp.

## Constructor & Destructor Documentation

## ◆ VideoCapture() [1/4]

cv::VideoCapture::VideoCapture | ( |  | ) |

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() [2/4]

cv::VideoCapture::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() [3/4]

cv::VideoCapture::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: 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`). 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
  The list of supported API backends cv::VideoCaptureAPIs

## ◆ VideoCapture() [4/4]

cv::VideoCapture::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

## ◆ ~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

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:
  laplace.cpp.

## ◆ grab()

virtual bool cv::VideoCapture::grab | ( |  | ) |
| virtual

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

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:
  camshiftdemo.cpp, facedetect.cpp, hog.cpp, laplace.cpp, lkdemo.cpp, peopledetect.cpp, polar_transforms.cpp, segment_objects.cpp, train_HOG.cpp, and videowriter_basic.cpp.

## ◆ open() [1/4]

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

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)

Returns
  `true` if the file has been successfully opened

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

Examples:
  camshiftdemo.cpp, facedetect.cpp, hog.cpp, laplace.cpp, lkdemo.cpp, peopledetect.cpp, polar_transforms.cpp, segment_objects.cpp, and train_HOG.cpp.

## ◆ open() [2/4]

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

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)

Returns
  `true` if the camera has been successfully opened.

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

## ◆ open() [3/4]

bool cv::VideoCapture::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.

## ◆ open() [4/4]

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

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)

Returns
  `true` if the file 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

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

Examples:
  videowriter_basic.cpp.

## ◆ release()

virtual void cv::VideoCapture::release | ( |  | ) |
| virtual

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

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

## ◆ set()

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

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:
  laplace.cpp.

## Member Data Documentation

## ◆ cap

Ptr<CvCapture> cv::VideoCapture::cap
| protected

## ◆ icap

Ptr<IVideoCapture> cv::VideoCapture::icap
| protected

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

- videoio/include/opencv2/videoio.hpp