[Live-devel] FramedFilter Performance & Questions

[Julian Lamberty]

Hi!

I've implemented a FramedFilter subclass that transcoders video from 
MPEG-2 to MPEG-4 using the libavcodec library.

Therefore I use a live-"chain" that looks like:

MPEG1or2VideoRTPSource -> Transcoder (my class) -> 
MPEG4VideoStreamDiscreteFramer -> MPEG4ESVideoRTPSink

The Transcoder's structure is:

//Begin Code
doGetNextFrame()
{
    gettimeofday(begin_frame, NULL);
    fInputSource->getNextFrame(..., afterGettingFrame, ...);
}

afterGettingFrame(void* clientData, unsigned numBytesRead, ...)
{
    Transcoder* transcoder = (Transcoder*)clientData;
    transcoder->afterGettingFrame1(numBytesRead, ...);
}

afterGettingFrame1(unsigned numBytesRead, ...)
{
    size = numBytesRead;
    while(size > 0)
    {
        gettimeofday(&begin_decoding_slice, NULL);
        dec_bytes = decode();
        size -= dec_bytes;
        gettimeofday(&end_decoding_slice, NULL);
       
        decoding_frame_time += decoding_slice_time;
       
        if(got_frame)
        {
            decoding_frame_time = 0;
            gettimeofday(&begin_encoding_frame, NULL);
            enc_bytes = encode();
            gettimeofday(&end_encoding_frame, NULL);
                     
            memcpy(fTo, outbuf, ...);
            frame_ready = true;
        }
    }
    if(!frame_ready)
    {
        fInputSource->getNextFrame(..., afterGettingFrame, ...);
    }
    else
    {
        frame_ready = false;
       afterGetting(this);
       gettimeofday(&end_frame, NULL);
    }
}
//End Code

 From the timevals inserted I calculate decoding, encoding and total 
frame processing time.
While decoding takes between 3ms and 10ms per frame and encoding takes 
between 8ms and 15ms per frame, the total processing time is sometimes 
greater than 50ms and exceeds the sum of decoding and encoding time by 
more than 15ms or even more.

This seems to be quite much just for fetching new data from the 
source... where could this come from? With more than 40ms average 
processing time the Transcoder loses the ability to do his work in 
real-time.

Another question: Can I use a MPEG1or2VideoStreamFramer after the 
MPEG1or2VideoRTPSource, so that I can pass complete frames into the 
decoder instead of chunks?

Thanks
Julian



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