org.opencv.videoio

Class VideoCapture

java.lang.Object

org.opencv.videoio.VideoCapture

public class VideoCapture
extends Object

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: samples/cpp/videocapture_basic.cpp Note: In REF: videoio_c "C API" the black-box structure CvCapture is used instead of %VideoCapture. Note:

• (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

Field Summary

Fields

Modifier and Type	Field and Description
protected long	nativeObj

Constructor Summary

Constructors

Modifier	Constructor and Description
	VideoCapture() Default constructor Note: In REF: videoio_c "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(int index) Opens a camera for video capturing
	VideoCapture(int index, int apiPreference) Opens a camera for video capturing
	VideoCapture(int index, int apiPreference, MatOfInt params) Opens a camera for video capturing with API Preference and parameters The params parameter allows to specify extra parameters encoded as pairs (paramId_1, paramValue_1, paramId_2, paramValue_2, ...).
protected	VideoCapture(long addr)
	VideoCapture(String filename) Opens a video file or a capturing device or an IP video stream for video capturing with API Preference
	VideoCapture(String filename, int apiPreference) Opens a video file or a capturing device or an IP video stream for video capturing with API Preference
	VideoCapture(String filename, int apiPreference, MatOfInt params) Opens a video file or a capturing device or an IP video stream for video capturing with API Preference and parameters The params parameter allows to specify extra parameters encoded as pairs (paramId_1, paramValue_1, paramId_2, paramValue_2, ...).

Method Summary

Modifier and Type	Method and Description
static VideoCapture	__fromPtr__(long addr)
protected void	finalize()
double	get(int propId) Returns the specified VideoCapture property
String	getBackendName() Returns used backend API name Note: Stream should be opened.
boolean	getExceptionMode()
long	getNativeObjAddr()
boolean	grab() Grabs the next frame from video file or capturing device.
boolean	isOpened() Returns true if video capturing has been initialized already.
boolean	open(int index) Opens a camera for video capturing Parameters are same as the constructor VideoCapture(int index, int apiPreference = CAP_ANY)
boolean	open(int index, int apiPreference) Opens a camera for video capturing Parameters are same as the constructor VideoCapture(int index, int apiPreference = CAP_ANY)
boolean	open(int index, int apiPreference, MatOfInt params) Opens a camera for video capturing with API Preference and parameters The params parameter allows to specify extra parameters encoded as pairs (paramId_1, paramValue_1, paramId_2, paramValue_2, ...).
boolean	open(String filename) Opens a video file or a capturing device or an IP video stream for video capturing.
boolean	open(String filename, int apiPreference) Opens a video file or a capturing device or an IP video stream for video capturing.
boolean	open(String filename, int apiPreference, MatOfInt params) Opens a video file or a capturing device or an IP video stream for video capturing with API Preference and parameters The params parameter allows to specify extra parameters encoded as pairs (paramId_1, paramValue_1, paramId_2, paramValue_2, ...).
boolean	read(Mat image) Grabs, decodes and returns the next video frame.
void	release() Closes video file or capturing device.
boolean	retrieve(Mat image) Decodes and returns the grabbed video frame.
boolean	retrieve(Mat image, int flag) Decodes and returns the grabbed video frame.
boolean	set(int propId, double value) Sets a property in the VideoCapture.
void	setExceptionMode(boolean enable) Switches exceptions mode methods raise exceptions if not successful instead of returning an error code

Methods inherited from class java.lang.Object

clone, equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

nativeObj

protected final long nativeObj

Constructor Detail

VideoCapture

protected VideoCapture(long addr)

VideoCapture

public VideoCapture()

Default constructor Note: In REF: videoio_c "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

public VideoCapture(String filename,
                    int apiPreference)

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

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: cv::VideoCaptureAPIs

VideoCapture

public VideoCapture(String filename)

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

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. implementation if multiple are available: e.g. cv::CAP_FFMPEG or cv::CAP_IMAGES or cv::CAP_DSHOW.

SEE: cv::VideoCaptureAPIs

VideoCapture

public VideoCapture(String filename,
                    int apiPreference,
                    MatOfInt params)

Opens a video file or a capturing device or an IP video stream for video capturing with API Preference and parameters The params parameter allows to specify extra parameters encoded as pairs (paramId_1, paramValue_1, paramId_2, paramValue_2, ...). See cv::VideoCaptureProperties

Parameters:

filename - automatically generated

apiPreference - automatically generated

params - automatically generated

VideoCapture

public VideoCapture(int index,
                    int apiPreference)

Opens a camera for video capturing

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: cv::VideoCaptureAPIs

VideoCapture

public VideoCapture(int index)

Opens a camera for video capturing

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) implementation if multiple are available: e.g. cv::CAP_DSHOW or cv::CAP_MSMF or cv::CAP_V4L. SEE: cv::VideoCaptureAPIs

VideoCapture

public VideoCapture(int index,
                    int apiPreference,
                    MatOfInt params)

Opens a camera for video capturing with API Preference and parameters The params parameter allows to specify extra parameters encoded as pairs (paramId_1, paramValue_1, paramId_2, paramValue_2, ...). See cv::VideoCaptureProperties

Parameters:

index - automatically generated

apiPreference - automatically generated

params - automatically generated

Method Detail

getNativeObjAddr

public long getNativeObjAddr()

__fromPtr__

public static VideoCapture __fromPtr__(long addr)

open

public boolean open(String filename,
                    int apiPreference)

Opens a video file or a capturing device or an IP video stream for video capturing. Parameters are same as the constructor VideoCapture(const String& filename, int apiPreference = CAP_ANY)

Parameters:

filename - automatically generated

apiPreference - automatically generated

Returns:

true if the file has been successfully opened The method first calls VideoCapture::release to close the already opened file or camera.

open

public boolean open(String filename)

Opens a video file or a capturing device or an IP video stream for video capturing. Parameters are same as the constructor VideoCapture(const String& filename, int apiPreference = CAP_ANY)

Parameters:

filename - automatically generated

Returns:

true if the file has been successfully opened The method first calls VideoCapture::release to close the already opened file or camera.

open

public boolean open(String filename,
                    int apiPreference,
                    MatOfInt params)

Opens a video file or a capturing device or an IP video stream for video capturing with API Preference and parameters The params parameter allows to specify extra parameters encoded as pairs (paramId_1, paramValue_1, paramId_2, paramValue_2, ...). See cv::VideoCaptureProperties

Parameters:

filename - automatically generated

apiPreference - automatically generated

params - automatically generated

Returns:

true if the file has been successfully opened The method first calls VideoCapture::release to close the already opened file or camera.

open

public boolean open(int index,
                    int apiPreference)

Opens a camera for video capturing Parameters are same as the constructor VideoCapture(int index, int apiPreference = CAP_ANY)

Parameters:

index - automatically generated

apiPreference - automatically generated

Returns:

true if the camera has been successfully opened. The method first calls VideoCapture::release to close the already opened file or camera.

open

public boolean open(int index)

Opens a camera for video capturing Parameters are same as the constructor VideoCapture(int index, int apiPreference = CAP_ANY)

Parameters:

index - automatically generated

Returns:

true if the camera has been successfully opened. The method first calls VideoCapture::release to close the already opened file or camera.

open

public boolean open(int index,
                    int apiPreference,
                    MatOfInt params)

Opens a camera for video capturing with API Preference and parameters The params parameter allows to specify extra parameters encoded as pairs (paramId_1, paramValue_1, paramId_2, paramValue_2, ...). See cv::VideoCaptureProperties

Parameters:

index - automatically generated

apiPreference - automatically generated

params - automatically generated

Returns:

true if the camera has been successfully opened. The method first calls VideoCapture::release to close the already opened file or camera.

isOpened

public boolean isOpened()

Returns true if video capturing has been initialized already. If the previous call to VideoCapture constructor or VideoCapture::open() succeeded, the method returns true.

Returns:

automatically generated

release

public 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.

grab

public boolean grab()

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. REF: tutorial_kinect_openni

retrieve

public boolean retrieve(Mat image,
                        int flag)

Decodes and returns the grabbed video frame.

Parameters:

flag - it could be a frame index or a driver specific flag

image - automatically generated

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: read() Note: In REF: videoio_c "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.

retrieve

public boolean retrieve(Mat image)

Decodes and returns the grabbed video frame.

Parameters:

image - automatically generated

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: read() Note: In REF: videoio_c "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.

read

public boolean read(Mat image)

Grabs, decodes and returns the next video frame.

Parameters:

image - automatically generated

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 REF: videoio_c "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

public boolean set(int propId,
                   double value)

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 REF: videoio_flags_others

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()

get

public double get(int propId)

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 REF: videoio_flags_others

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

getBackendName

public String getBackendName()

Returns used backend API name Note: Stream should be opened.

Returns:

automatically generated

setExceptionMode

public void setExceptionMode(boolean enable)

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

Parameters:

enable - automatically generated

getExceptionMode

public boolean getExceptionMode()

finalize

protected void finalize()
                 throws Throwable

Overrides:

finalize in class Object

Throws:

Throwable