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• Decompressing time-To-Sample table (STTS) in an MP4 file

  30 mai 2016, par man-r

  i have an mp4 video byte array and i need to generate a thumbnail for it using its middle frame (e.g. if the video length is 10 seconds then i need to get the picture from 5th second).

  i managed to parse through the file and extract its boxes (atom). i have also managed to get the video length from the mvhd box. also i managed to extract
  1. the time-To-Sample table from stts box,
  2. the sample-To-Chunk table from stcs box,
  3. the chunk-Offset table from stco box,
  4. the sample Size table from stsz box,
  5. the Sync Sample table from stss box

  i know that all the actual media are available in the mdat box and that i need to correlate the above table to find the exact frame offset in the file but my question is how ? the tables data seems to be compressed (specially the time-To-Sample table) but i don’t know how decompress them.

  any help is appreciated.

  below are code samples

  code to convert byte to hex

  public static char[] bytesToHex(byte[] bytes) {
  
      char[] hexChars = new char[bytes.length * 2];
  
      for ( int j = 0; j < bytes.length; j++ ) {
  
          int v = bytes[j] & 0xFF;
  
  
  
          hexChars[j * 2] = hexArray[v >>> 4];
  
          hexChars[j * 2 + 1] = hexArray[v & 0x0F];            
  
      }
  
      return hexChars;
  
  }

  code for getting the box offset

  final static String MOOV                          = "6D6F6F76";
  
  final static String MOOV_MVHD                     = "6D766864";
  
  final static String MOOV_TRAK                     = "7472616B";
  
  final static String MOOV_TRAK_MDIA                = "6D646961";
  
  final static String MOOV_TRAK_MDIA_MINF           = "6D696E66";
  
  final static String MOOV_TRAK_MDIA_MINF_STBL      = "7374626C";
  
  final static String MOOV_TRAK_MDIA_MINF_STBL_STSD = "73747364";
  
  final static String MOOV_TRAK_MDIA_MINF_STBL_STTS = "73747473";
  
  final static String MOOV_TRAK_MDIA_MINF_STBL_STSS = "73747373";
  
  final static String MOOV_TRAK_MDIA_MINF_STBL_STSC = "73747363";
  
  final static String MOOV_TRAK_MDIA_MINF_STBL_STCO = "7374636F";
  
  final static String MOOV_TRAK_MDIA_MINF_STBL_STSZ = "7374737A";
  
  
  
  static int getBox(char[] s, int offset, String type) {
  
      int typeOffset = -1;
  
      for (int i = offset*2; i-1) {
  
                  break;
  
              } 
  
          }
  
          i+=(size*2);
  
      }
  
  
  
      return typeOffset;
  
  }

  code for getting the duration and timescale

  static int[] getDuration(char[] s) {
  
      int mvhdOffset = getBox(s, 0, MOOV_MVHD);
  
      int timeScaleStart = (mvhdOffset*2) + (4 + 4 + 1 + 3 + 4 + 4)*2;
  
      int timeScaleEnd   = (mvhdOffset*2) + (4 + 4 + 1 + 3 + 4 + 4 + 4)*2;
  
  
  
      int durationStart  = (mvhdOffset*2) + (4 + 4 + 1 + 3 + 4 + 4 + 4)*2;
  
      int durationEnd    = (mvhdOffset*2) + (4 + 4 + 1 + 3 + 4 + 4 + 4 + 4)*2;
  
  
  
      String timeScaleHex = new String(Arrays.copyOfRange(s, timeScaleStart, timeScaleEnd));
  
      String durationHex = new String(Arrays.copyOfRange(s, durationStart, durationEnd));
  
  
  
      int timeScale = Integer.parseInt(timeScaleHex, 16);
  
      int duration = Integer.parseInt(durationHex, 16);
  
  
  
      int[] result = {duration, timeScale};
  
      return result;
  
  }

  code to get the time-To-Sample table

  static int[][] getTimeToSampleTable(char[] s, int trakOffset) {
  
      int offset = getBox(s, trakOffset, MOOV_TRAK_MDIA_MINF_STBL_STTS);
  
      int sizeStart = offset*2;
  
      int sizeEnd   = offset*2 + (4)*2;
  
  
  
      int typeStart = offset*2 + (4)*2;
  
      int typeEnd   = offset*2 + (4 + 4)*2;
  
  
  
      int noOfEntriesStart = offset*2 + (4 + 4 + 1 + 3)*2;
  
      int noOfEntriesEnd   = offset*2 + (4 + 4 + 1 + 3 + 4)*2;
  
  
  
      String sizeHex = new String(Arrays.copyOfRange(s, sizeStart, sizeEnd));
  
      String typeHex = new String(Arrays.copyOfRange(s, typeStart, typeEnd));
  
      String noOfEntriesHex = new String(Arrays.copyOfRange(s, noOfEntriesStart, noOfEntriesEnd));
  
  
  
      int size = Integer.parseInt(sizeHex, 16);
  
      int noOfEntries = Integer.parseInt(noOfEntriesHex, 16);
  
  
  
      int[][] timeToSampleTable = new int[noOfEntries][2];
  
  
  
      for (int i = 0; icode>

• avformat/yuv4mpegdec : Use table instead of repeated "else if"

  21 juillet 2021, par Andreas Rheinhardt

  avformat/yuv4mpegdec : Use table instead of repeated "else if"
  The code savings more than offset the size of the table
  
  (1936B vs 768B with GCC 10.3 at -O3).
  Reviewed-by : Paul B Mahol <onemda@gmail.com>
  
  Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
  

  • [DH] libavformat/yuv4mpegdec.c

• Révision 17606 : reparer un peu l’insertion en table avec une valeur sur la cle primaire,

  1er avril 2011, par cedric -

  mais reste le probleme fondamental de synchronisation de l’autoincrement