[libav-api] Avoiding deinterlacing for photo finish system

Wayne Kelly w.kelly at qut.edu.au
Mon Nov 28 13:24:49 CET 2011


I'm new to this area. I'm trying to create a (Microsoft Windows based) home made photo finish system (for athletics).
The races will be filmed using a video camera which produces HD M2TS video files - 50 fps (PAL) interlaced.
I need to be able to fast forward/rewind and step forward and back one frame at a time to identify the
exact frame when each athlete crosses the finish line (and when the flash goes off for the race start).
I realize that this is not how professional photo finish systems work - they work at hundreds of frames per second
and so are able to produce times accurate to 0.01 seconds. I only require times accurate to about 0.1 seconds.
Do you have any feel for whether the frames per second generated by a home video camera will be
even/consistent enough to provide a reliable basis for timing? Or, how I would even test that?

I've followed the basic libav tutorial at http://dranger.com/ffmpeg/ and am able to create a simple app that saves each frame image to file.
The de-interlacing seems to be done implicitly somewhere in that tutorial - I'm not sure where.
Is it possible to possible to get access to the un-deinterlaced frames? At present I get 25 full progressive frames per second.
I'd rather 50 half frames per second (to provide better temporal resolution). I assume that such half frames could still be displayed
at half vertical resolution? I'd appreciate any examples that you could point me to that might do anything like this.

At present, my best working prototype is a Windows Media Player control embedded in a C# windows application. It decodes the m2ts file
and provides excellent deinterlacing and effective frame rate. I'm able to step forward one (half) frame at a time and determine which frame I'm at,
but I can't get it to rewind or fast forward m2ts video files. So, I've turned to libav in hope of being able to avoid such restrictions.

I'm also happy to consider DirectShow solutions (which I understand are based on ffdshow which is based on libav???).

Whatever is the simplest solution is ...

Cheers, Wayne.
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