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• ppc : reduce overreads when loading 8 pixels in altivec dsp functions

  13 février 2014, par Janne Grunau

  ppc : reduce overreads when loading 8 pixels in altivec dsp functions
  Altivec can only load naturally aligned vectors. To handle possibly
  
  unaligned data a second vector is loaded from an offset of the original
  
  location and the data is recovered through a vector permutation.
  
  Overreads are minimal if the offset for second load points to the last
  
  element of data. This is 7 for loading eight 8-bit pixels and overreads
  
  are reduced from 16 bytes to 8 bytes if the pixels are 64-bit aligned.
  
  For unaligned pixels the overread is reduced from 23 bytes to 15 bytes
  
  in the worst case.
  

  • [DBH] libavcodec/ppc/dsputil_altivec.c

• How to open a remote radio stream with ffmpeg's `avformat_open_input` without segfault ?

  19 août 2020, par Keyboard embossed forhead

  I'm at the beginning stage of writing a small app to stream internet radio stations. For the moment I'm just trying to get the detected info of the input stream. Whilst I am successful in getting all the stream's details via the the command line tool (ffmpeg -i ${URL}), calling the library's avformat_open_input(...) method call results in a SEGFAULT (a stack overflow to be precise when checked in valgrind).

  


  Passing a local file url works fine though in both the command line utility and the library call.

  


  Here's a minimal example :

  


  int test() {
    const char      * station_url = "http://stream.srg-ssr.ch/m/rsc_de/aacp_96";
    const char      * test_file   = "test.mp3"; //works
    AVFormatContext * av_ctx      = avformat_alloc_context();
    int               ret         = 0;

    avformat_network_init();

    if( ( ret = avformat_open_input( &av_ctx, station_url, NULL, NULL ) ) < 0 ) { //SEGFAULT   
        printf( "Could not open file '%s': %i", station_url, ret );
        return -1;
    }

    printf( "Format %s, duration %ld us", av_ctx->iformat->long_name, av_ctx->duration );

    avformat_network_deinit();
    return 0;
}


  


  If anyone with experience in dealing with acquiring remote streams using ffmpeg libraries in C has some insights I'd be grateful. Thanks in advance.

  


  I'm using ffmpeg v4.3.1 on Linux.

  


• Cut a video in between key frames without re-encoding the full video using ffpeg ?

  1er septembre 2020, par bguiz

  I would like to cut a video at the beginning at any particular timestamp, and it need to be precise, so the nearest key frame is not good enough.

  


  Also, these videos are rather long - an hour or longer - so I would like to avoid re-encoding this altogether if possible, or otherwise only re-encode a minimal fraction of the total duration. Thus, would like to maximise the use of -vcodec copy.

  


  How can I accomplish this using ffmpeg ?

  


  NOTE : See scenario, and my own rough idea for a possible solution below.

  


  ---

  


  Scenario :

  


  

  • Original video

    

    • Length of 1:00:00

    


    • Has a key frame every 10s

    


    


  


  • Desired cut :

    

    • From 0:01:35 through till the end

    


    


  


  • Attempt #1 :

    

    • Using -ss 0:01:35 -i blah.mp4 -vcodec copy, what results is a file where :

    


    • audio starts at 0:01:30

    


    • video also starts at 0:01:30

    


    • this starts both the audio and the video too early

    


    


  


  • using -i blah.mp4 -ss 0:01:35 -vcodec copy, what results is a file where :

    

    • audio starts at 0:01:35,

    


    • but the video is blank/ black for the first 5 seconds,

      

      • until 0:01:40, when the video starts

      


      


    


    • this starts the audio on time,
but the video starts too late

    


    


  


  


  ---

  


  Rough idea

  


  

  • (1) cut 0:01:30 to 0:01:40

    

    • re-encode this to have new key frames,
including one at the target time of 0:01:35

    


    • then cut this to get the 5 seconds from 0:01:35 through 0:01:40

    


    


  


  • (2) cut 0:01:40 through till the end

    

    • without re-encoding, using -vcodec copy

    


    


  


  • (3) ffmpeg concat the first short clip (the 5 second one)
with the second long clip

  


  


  I know/ can work out the commands for (2) and (3), but am unsure about what commands are needed for (1).