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CvCapture

lastmod 2019-10-16created 2015-07-1510 edits

Notes on the CvCapture object of OpenCV.

Attributes

Property

Meaning

Notes

cv2.CAP_PROP_FRAME_WIDTH

Frame width

-

cv2.CAP_PROP_FRAME_HEIGHT

Frame height

-

cv2.CAP_PROP_FRAME_COUNT

Total number of frames

-

cv2.CAP_PROP_FPS

Frame rate

-

cv2.CAP_PROP_FOURCC

Codec code

-

cv2.CAP_PROP_BRIGHTNESS

Image brightness

Camera only

cv2.CAP_PROP_CONTRAST

Image contrast

Camera only

cv2.CAP_PROP_SATURATION

Image saturation

Camera only

cv2.CAP_PROP_HUE

Image hue

Camera only

cv2.CAP_PROP_GAIN

Image gain

Camera only

cv2.CAP_PROP_EXPOSURE

Image exposure

Camera only

cv2.CAP_PROP_POS_MSEC

Frame playback time

Returns ms

cv2.CAP_PROP_POS_FRAMES

Current frame

Less than the total number of frames

cv2.CAP_PROP_POS_AVI_RATIO

Relative position in the video file

0 = start, 1 = end

Camera example 

#include <opencv/highgui.h>

int main()
{
    char const * TITLE = "CAMERA";

    cvNamedWindow(TITLE, 0);
    cvResizeWindow(TITLE, 320, 240);

    CvCapture * capture = cvCreateCameraCapture(0);

    IplImage * frame = nullptr;
    char keycode = 0;

    while (true) {
        // Get the camera frame.
        frame = cvQueryFrame(capture);

        if (!frame) {
            break;
        }

        cvShowImage(TITLE, frame);

        keycode = cvWaitKey(33);

        // Check the ESC keycode.
        if (keycode == 27) {
            break;
        }
    }

    cvReleaseCapture(&capture);
    cvReleaseImage(&frame);

    cvDestroyWindow(TITLE);

    return 0;
}

Caveats!

The IplImage returned by the cvQueryFrame function above must never be released. The comment in the corresponding source code is as follows. 

/* Just a combination of cvGrabFrame and cvRetrieveFrame
   !!!DO NOT RELEASE or MODIFY the retrieved frame!!!      */
CVAPI(IplImage*) cvQueryFrame( CvCapture* capture );

Also, after cvShowImage you must WAIT using cvWaitKey(); for the image to be displayed.

End of file 

capVideo.open("CarsDrivingUnderBridge.mp4");

if (!capVideo.isOpened()) {
    std::cout << "error reading video file\n";
    return 0;
}
if (capVideo.get(CV_CAP_PROP_FRAME_COUNT) < 2) {
    std::cout << "error: video file must have at least two frames\n";
    return 0;
}

if ((capVideo.get(CV_CAP_PROP_POS_FRAMES) + 1) < capVideo.get(CV_CAP_PROP_FRAME_COUNT)) {
    capVideo.read(imgFrame2);
} else {
    std::cout << "end of video\n";
}

See also

Favorite site

IP CAMERA

author Zer0 sitemap