Irregular time stamps from web camera

Question

Niraj 2013-6-6

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/78196-irregular-time-stamps-from-web-camera

Hi, I am trying to do some real time processing and I need to capture frames from my webcamera at 5fps. My webcamera is Logitech (QuickCam Communicate Deluxe (winvideo-1) ). I have captured 30 frames. Here is my code:

        vid = videoinput('winvideo', 1, 'RGB24_640x480');
        src = getselectedsource(vid);
        vid.FramesPerTrigger = 30;
        src.FrameRate = '5.0000';
        vid.ReturnedColorspace = 'grayscale';
        start(vid);
        wait(vid,Inf);
        numframes = get(vid,'FramesAvailable');
        [frames,time] = getdata(vid,numframes);

I would expect that since the frame rate has been set to 5, the time stamps from camera should be regularly spaced at 0.200, but this is what i get timestamps from camera:

and the time interval is :

Can anyone explain why is the time interval irregular. Although it is approximately close to 0.200, but still why is it not exact 0.200? Am i missing any change in the default settings? I need to keep the fps = 5 exactly.

Please comment and help! Thanks

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Iain 2013-6-6

This is not really a matlab problem.

You CAN operate the camera faster, and discard some frames.

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Answer 1

Walter Roberson 2013-6-6

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https://ww2.mathworks.cn/matlabcentral/answers/78196-irregular-time-stamps-from-web-camera#answer_87914

If you could get 5 frames per second "exactly" then you would be able to define the SI unit of time in terms of your frame rate, which would save rather a lot of bother with super-cooled cesium ion traps or femtosecond ultraviolet lasers.