Module Opencv.Video_capture

type t
val video_capture1 : unit -> t

Usage: video_capture1 ()

Default constructor Note: In 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.

val video_capture2 : ?⁠api_preference:int -> string -> t

Usage: video_capture2 ?api_preference filename

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

  • Parameter: 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.
  • Parameter: api_preference: 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: cv::VideoCaptureAPIs

val video_capture3 : ?⁠api_preference:int -> int -> t

Usage: video_capture3 ?api_preference index

Opens a camera for video capturing

  • Parameter: 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)
  • Parameter: api_preference: 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 also: cv::VideoCaptureAPIs

val open1 : ?⁠api_preference:int -> t -> string -> bool

Usage: open1 ?api_preference __self 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)

  • Returns: true if the file has been successfully opened

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

val open2 : ?⁠api_preference:int -> t -> int -> bool

Usage: open2 ?api_preference __self index

Opens a camera for video capturing

Parameters are same as the constructor VideoCapture(int index, int apiPreference = CAP_ANY)

  • Returns: true if the camera has been successfully opened.

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

val is_opened : t -> bool

Usage: is_opened __self

Returns true if video capturing has been initialized already.

If the previous call to VideoCapture constructor or VideoCapture::open() succeeded, the method returns true.

val release : t -> unit

Usage: release __self

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.

val grab : t -> bool

Usage: grab __self

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.

tutorial_kinect_openni

val retrieve : ?⁠image:Cvdata.t -> ?⁠flag:int -> t -> Cvdata.t * bool

Usage: retrieve ?image ?flag __self

Decodes and returns the grabbed video frame.

  • Parameter: :return image the video frame is returned here. If no frames has been grabbed the image will be empty.
  • Parameter: 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 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.

val read : ?⁠image:Cvdata.t -> t -> Cvdata.t * bool

Usage: read ?image __self

Grabs, decodes and returns the next video frame.

  • Parameter: :return 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 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.

val set : t -> int -> float -> bool

Usage: set __self prop_id value

Sets a property in the VideoCapture.

  • Parameter: prop_id: Property identifier from cv::VideoCaptureProperties (eg. cv::CAP_PROP_POS_MSEC, cv::CAP_PROP_POS_FRAMES, ...) or one from videoio_flags_others
  • Parameter: 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()
val get : t -> int -> float

Usage: get __self prop_id

Returns the specified VideoCapture property

  • Parameter: prop_id: Property identifier from cv::VideoCaptureProperties (eg. cv::CAP_PROP_POS_MSEC, cv::CAP_PROP_POS_FRAMES, ...) or one from 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

val get_backend_name : t -> string

Usage: get_backend_name __self

Returns used backend API name

Note: Stream should be opened.

val set_exception_mode : t -> bool -> unit

Usage: set_exception_mode __self enable

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

val get_exception_mode : t -> bool

Usage: get_exception_mode __self