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  1. I want to step through a .mov file frame-by-frame looking at geometry of an aircraft in the video to establish the rotational speed of the propeller. This should be fairly straightforward by looking at how much the propeller moves between each frame and using the exposure time of each frame.

    However the .mov file is from an iPhone 4S and looking at the details, has a variable frame rate. What I need to know is the duration that each frame is displayed for in order to do my analysis. Does anyone know how to find out this information?
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  2. Maybe a DirectShow media player such as MPC-HC in combination with ffdshow and it's on screen display.

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    Stepping forward through a video frame by frame should be fine but I wouldn't trust DirectShow to navigate back and forth particularly frame accurately. You can navigate to a specific frame via MPC-HC's Navigate/GoTo menu.
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  3. That works just great, many thanks
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  4. Member racer-x's Avatar
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    Mar 2003
    3rd Rock from the Sun
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    I don't see how you can accurately calculate that, given that even at a standard idle speed would be about 1,000 RPM. That's well above any framerate an iphone can record.

    I guess if one blade was painted an obvious color, you might have a chance....
    Got my retirement plans all set. Looks like I only have to work another 5 years after I die........
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  5. And you're going to have problems with rolling shutter on the iphone. The geometry will be distorted, but maybe that's not important for what you're trying to do
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  6. Member Cornucopia's Avatar
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    Oct 2001
    Deep in the Heart of Texas
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    Even assuming a global shutter, sampling theory explains that it is impossible to know for sure what the true speed of something is once it has gone up to and/or above the Nyquist frequency (as it could be one of a number of alternate frequency harmonics).

    Example: non-moving camera with fan blade that is rotating at something equal to or faster than 30Hz. If you even luck out in having a sharp image by doing a strobe or a narrow shutter angle, and you see the "special color" blade appearing at the EXACT SAME spot every second (and only every second), that means the fan could be rotating at 30Hz, 60Hz, 90Hz, 120Hz, 150Hz, 180Hz, ...and on and on (within reason). Which one is it?
    No real way to tell unless you can be assured that it should fall within a very narrow range of speeds (so that you can discount the alternates). If the motor was rated to run between 80-100Hz, you could then assume the 90Hz to be the true speed.

    Last edited by Cornucopia; 24th Oct 2014 at 20:10.
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