Capture Video from Camera

class VideoCapture - Class for video capturing from video files or cameras.

bool VideoCapture::open(const string& filename) // filename – name of the opened video file
bool VideoCapture::open(int device) // device – id of the opened video capturing device (i.e. a camera index)
bool VideoCapture::isOpened() // Returns true if video capturing has been initialized already
void VideoCapture::release() // Closes video file or capturing device
bool VideoCapture::grab() // Grabs the next frame from video file or capturing device
bool VideoCapture::retrieve(Mat& image, int channel=0) // Decodes and returns the grabbed video frame

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.

VideoCapture& VideoCapture::(operator)>>(Mat& image) // Grabs, decodes and returns the next video frame
double VideoCapture::get(int propId) // Returns the specified VideoCapture property
Parameters: 
       propId – Property identifier. It can be one of the following:

• CV_CAP_PROP_POS_MSEC Current position of the video file in milliseconds or video capture timestamp.
• CV_CAP_PROP_POS_FRAMES 0-based index of the frame to be decoded/captured next.
• CV_CAP_PROP_POS_AVI_RATIO Relative position of the video file: 0 - start of the film, 1 - end of the film.
• CV_CAP_PROP_FRAME_WIDTH Width of the frames in the video stream.
• CV_CAP_PROP_FRAME_HEIGHT Height of the frames in the video stream.
• CV_CAP_PROP_FPS Frame rate.
• CV_CAP_PROP_FOURCC 4-character code of codec.
• CV_CAP_PROP_FRAME_COUNT Number of frames in the video file.
• CV_CAP_PROP_FORMAT Format of the Mat objects returned by retrieve() .
• CV_CAP_PROP_MODE Backend-specific value indicating the current capture mode.
• CV_CAP_PROP_BRIGHTNESS Brightness of the image (only for cameras).
• CV_CAP_PROP_CONTRAST Contrast of the image (only for cameras).
• CV_CAP_PROP_SATURATION Saturation of the image (only for cameras).
• CV_CAP_PROP_HUE Hue of the image (only for cameras).
• CV_CAP_PROP_GAIN Gain of the image (only for cameras).
• CV_CAP_PROP_EXPOSURE Exposure (only for cameras).
• CV_CAP_PROP_CONVERT_RGB Boolean flags indicating whether images should be converted to RGB.
• CV_CAP_PROP_WHITE_BALANCE Currently not supported
• CV_CAP_PROP_RECTIFICATION Rectification flag for stereo cameras (note: only supported by DC1394 v 2.x backend currently)

bool VideoCapture::set(int propId, double value) // Sets a property in the VideoCapture

Parameters:

• propId –
  • CV_CAP_PROP_POS_MSEC Current position of the video file in milliseconds.
  • CV_CAP_PROP_POS_FRAMES 0-based index of the frame to be decoded/captured next.
  • CV_CAP_PROP_POS_AVI_RATIO Relative position of the video file: 0 - start of the film, 1 - end of the film.
  • CV_CAP_PROP_FRAME_WIDTH Width of the frames in the video stream.
  • CV_CAP_PROP_FRAME_HEIGHT Height of the frames in the video stream.
  • CV_CAP_PROP_FPS Frame rate.
  • CV_CAP_PROP_FOURCC 4-character code of codec.
  • CV_CAP_PROP_FRAME_COUNT Number of frames in the video file.
  • CV_CAP_PROP_FORMAT Format of the Mat objects returned by retrieve() .
  • CV_CAP_PROP_MODE Backend-specific value indicating the current capture mode.
  • CV_CAP_PROP_BRIGHTNESS Brightness of the image (only for cameras).
  • CV_CAP_PROP_CONTRAST Contrast of the image (only for cameras).
  • CV_CAP_PROP_SATURATION Saturation of the image (only for cameras).
  • CV_CAP_PROP_HUE Hue of the image (only for cameras).
  • CV_CAP_PROP_GAIN Gain of the image (only for cameras).
  • CV_CAP_PROP_EXPOSURE Exposure (only for cameras).
  • CV_CAP_PROP_CONVERT_RGB Boolean flags indicating whether images should be converted to RGB.
  • CV_CAP_PROP_WHITE_BALANCE Currently unsupported
  • CV_CAP_PROP_RECTIFICATION Rectification flag for stereo cameras (note: only supported by DC1394 v 2.x backend currently)
• value – Value of the property.

Example 1:

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#include "opencv2/opencv.hpp"
using namespace cv;

int main(int, char**)
{
    VideoCapture cap(0); // open the default camera
    if(!cap.isOpened())  // check if we succeeded
        return -1;

    namedWindow("Video",1);
    while(1)
    {
        Mat frame;
        cap >> frame;         // get a new frame from camera
        imshow("Video", frame);

        // Press 'c' to escape
        if(waitKey(30) == 'c') break;
    }
    return 0;
}

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Example 2:

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#include "opencv2/objdetect/objdetect.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
using namespace cv;

int main( int argc, const char** argv )
{
    CvCapture* capture;
    Mat frame;
    
    // Read the video stream
    capture = cvCaptureFromCAM( -1 );
    namedWindow("Video",CV_WINDOW_AUTOSIZE);
    if( capture )
    {
        while( true )
        {
            frame = cvQueryFrame( capture );
            imshow("Video",frame);

            // Press 'c' to escape
            int c = waitKey(10);
            if( (char)c == 'c' ) { break; }
        }
    }
    return 0;
}

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