cv::VideoCapture Class Reference
视频 I/O

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

#include <opencv2/videoio.hpp>

Public Member Functions

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

Protected Attributes

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

Friends

class internal::VideoCapturePrivateAccessor

详细描述

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 <iostream>
#include <stdio.h>
using namespace cv ;
using namespace std ;
int main( int , char **)
{
Mat frame;
//--- INITIALIZE VIDEOCAPTURE
// 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;
}
注意
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
范例:
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 ,和 samples/tapi/hog.cpp .

Constructor & Destructor Documentation

VideoCapture() [1/3]

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

Default constructor.

注意
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() [2/3]

cv::VideoCapture::VideoCapture ( const 字符串 & filename ,
int apiPreference = CAP_ANY
)
Python:
<VideoCapture object> = cv.VideoCapture( )
<VideoCapture object> = cv.VideoCapture( filename[, apiPreference] )
<VideoCapture object> = cv.VideoCapture( index[, apiPreference] )

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.

参数
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 .
另请参阅
The list of supported API backends cv::VideoCaptureAPIs

VideoCapture() [3/3]

cv::VideoCapture::VideoCapture ( int index ,
int apiPreference = CAP_ANY
)
Python:
<VideoCapture object> = cv.VideoCapture( )
<VideoCapture object> = cv.VideoCapture( filename[, apiPreference] )
<VideoCapture object> = cv.VideoCapture( index[, apiPreference] )

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.

参数
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 .
另请参阅
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.

成员函数文档编制

get()

virtual double cv::VideoCapture::get ( int propId ) const
virtual
Python:
retval = cv.VideoCapture.get( propId )

Returns the specified VideoCapture 特性。

参数
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
返回
Value for the specified property. Value 0 is returned when querying a property that is not supported by the backend used by the VideoCapture 实例。
注意
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
范例:
samples/cpp/laplace.cpp ,和 samples/cpp/tutorial_code/videoio/video-write/video-write.cpp .

getBackendName()

字符串 cv::VideoCapture::getBackendName ( ) const
Python:
retval = cv.VideoCapture.getBackendName( )

Returns used backend API name.

注意
Stream should be opened.

getExceptionMode()

bool cv::VideoCapture::getExceptionMode ( )
inline
Python:
retval = cv.VideoCapture.getExceptionMode( )

query if exception mode is active

grab()

virtual bool cv::VideoCapture::grab ( )
virtual
Python:
retval = cv.VideoCapture.grab( )

Grabs the next frame from video file or capturing device.

返回
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.

使用 Kinect 和其它 OpenNI 兼容的深度传感器

isOpened()

virtual bool cv::VideoCapture::isOpened ( ) const
virtual
Python:
retval = cv.VideoCapture.isOpened( )

open() [1/2]

virtual bool cv::VideoCapture::open ( const 字符串 & filename ,
int apiPreference = CAP_ANY
)
virtual
Python:
retval = cv.VideoCapture.open( filename[, apiPreference] )
retval = cv.VideoCapture.open( index[, apiPreference] )

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)

返回
true if the file has been successfully opened

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

范例:
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 ,和 samples/tapi/hog.cpp .

open() [2/2]

virtual bool cv::VideoCapture::open ( int index ,
int apiPreference = CAP_ANY
)
virtual
Python:
retval = cv.VideoCapture.open( filename[, apiPreference] )
retval = cv.VideoCapture.open( index[, apiPreference] )

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)

返回
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.

另请参阅
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.

另请参阅
read()

read()

virtual bool cv::VideoCapture::read ( OutputArray image )
virtual
Python:
retval, image = cv.VideoCapture.read( [, image] )

Grabs, decodes and returns the next video frame.

参数
[out] image the video frame is returned here. If no frames has been grabbed the image will be empty.
返回
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() ).

注意
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.
范例:
samples/cpp/videowriter_basic.cpp .

release()

virtual void cv::VideoCapture::release ( )
virtual
Python:
None = cv.VideoCapture.release( )

Closes video file or capturing device.

The method is automatically called by subsequent VideoCapture::open and by VideoCapture 析构函数。

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

retrieve()

virtual bool cv::VideoCapture::retrieve ( OutputArray image ,
int flag = 0
)
virtual
Python:
retval, image = cv.VideoCapture.retrieve( [, image[, flag]] )

Decodes and returns the grabbed video frame.

参数
[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
返回
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() ).

另请参阅
read()
注意
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:
retval = cv.VideoCapture.set( propId, value )

Sets a property in the VideoCapture .

参数
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.
返回
true if the property is supported by backend used by the VideoCapture 实例。
注意
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()
范例:
samples/cpp/laplace.cpp .

setExceptionMode()

void cv::VideoCapture::setExceptionMode ( bool enable )
inline
Python:
None = cv.VideoCapture.setExceptionMode( enable )

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

Wait for ready frames from VideoCapture .

参数
streams input video streams
readyIndex stream indexes with grabbed frames (ready to use . retrieve() to fetch actual frame)
timeoutNs number of nanoseconds (0 - infinite)
返回
true if streamReady is not empty
异常
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, grabbs video frame, if camera is ready.

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

Friends And Related Function 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: