code: protected long getNextRelativePts(long absPts, int trackIndex) {
          if (mFirstPts == 0) {
              mFirstPts = absPts;
              return 0;
          }
          return getSafePts(absPts - mFirstPts, trackIndex);
      }
docs: Return a relative pts given an absolute pts and trackIndex.
  
  This method advances the state of the Muxer, and must only
  be called once per call to {@link #writeSampleData(android.media.MediaCodec, int, int, java.nio.ByteBuffer, android.media.MediaCodec.BufferInfo)}.