On Apr 10, 2010, at 12:59 PM, Thomas Worth wrote:
On Sat, Apr 10, 2010 at 10:37 AM, Bruce Wheaton
<[email protected]> wrote:
On Apr 10, 2010, at 4:07 AM, Thomas Worth wrote:
I have a loop that decodes frames using avcodec_decode_video2(), and
am spawning threads to convert the data put into the AVFrame struct
and write it to disk. However, even though I allocated multiple
structs to hold the picture data, frames are overwriting each other:
Are you serializing access to the file? Possibly the frames are
fine but all
the threads are doing overlapping writes to the file.
Bruce
Yep. Each new thread gets its own AVFrame and unique file number. Its
job is to write one frame to disk, using the passed AVFrame struct as
its picture source. It uses the passed file number (from the main for
loop) to serialize the output image sequence. So, each thread will
only ever get one frame worth of data (the result from
avcodec_decode_video2), making this problem seem impossible. However,
I'm not sure how libavcodec handles this stuff internally so I'm not
sure how to fix it.
I noticed that allocating multiple AVFrame structures in an array
doesn't make any difference versus just using the same struct over and
over.
Do I need to set up the program in such a way that the libav*
libraries know I'm doing simultaneous (threaded) reads?
Oh, I see. So you're writing each frame to disk? But you want each
thread to actually do decoding? Hmm. I'm not sure that's supported.
There's an ffmpeg-mt fork that I've been using, and it will jump off
and do decoding of simultaneous frames or slices of frames as the
codec allows, but even that comes back to a single thread to 'deliver'
the final frames.
The more I think about it, the more certain I am that you can't run
decode_video on the same movie from different threads - there's a fair
amount of 'global' state to the codec. I suggest you allow ffmpeg-mt
to multi-thread your decoding, then pick up the completed frames in
one thread, then pass them off to multiple threads to do the disk
writing.
Regards,
Bruce Wheaton
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