How Extract Video Elementary Stream from MPEG DASH segment 

Shortly on MPEG-DASH

DASH is a complex hierarchical schema:  MPD-file (the manifest file) is divided into periods, each period is divided into adaptation sets, adaptation set is divided in representations and each representation is divided into segments:

 

 

 

 

How Generate MPEG-DASH with shaka-packager

A single DASH segment is commonly not decodable since SPS/PPS are not present in the meta-data of segments. They are present in a special ‘init’ mp4-file. Even if you extract the video stream from such mp4 segment it would not playable, you need add SPS/PPS.

To extract the video stream form a mp4 segment i suggest to perform the following operations:

1)  to concatenate ‘init’ mp4 file with the segment file (e.g. by using Linux command ‘cat’)

2) then to apply ffmpeg to extract video stream with SPS/PPS present.  For example:

       ffmpeg -i test.mp4 -vcodec copy -vbsf h264_mp4toannexb -f h264 test.h264

 

Example: How Shaka packager generates MPEG-DASH mp4 segments with the init-file (test_init.mp4):

/packager-linux ‘in=test.ts , stream=video, init_segment=test_init.mp4, segment_template=test_$Number$.mp4’ –profile vod –mpd_output test.mpd –dump_stream_info –min_buffer_time=0.1 –segment_duration=0.5 –strip_parameter_set_nalus=false

As a results test_1.mp4, test_2.mp4, … segments are generated and in addition test_init.mp4 file is also produced (this file test_init.mp4 contains SPS/PPS).

 

How Assembly All Segments

Shaka Packager produces tons of mp4-segments and it can be required to extract video excerpts from all mp4-segments and to assemble the overall elementary video stream. The following bash-script   ExtractVideoSegments.sh extracts all video excerpts and compose them in correct order (consequently the overall elementary video stream is generated):

#!/bin/bash

if (( $#<2 ))then

    echo ‘Usage: indir  init-mp4  outstream  tempfolder (default  /.)’ >&2

   exit 1

fi

 

indir=$1

initmp4=$2

outfile=$3

tempdir=${4:-./}

if [ ! -d $indir ]; then

    echo $indir  is not directory or not exist

    exit 1

fi

if [ ! -f $initmp4 ]; then

    echo $initmp4  is not exist

    exit 1

fi

if [ ! -d $tempdir ]; then

     echo $tempdir  is not directory or not exist

    exit 1

fi

if [  -d $outfile ]; then

    echo $outfile  is directory

    exit 1

fi

if [ -f $outfile ]; then   

    echo $outfile exist and to be overwritten   

   rm   $outfile

fi

# add ‘/’ to the end folder path if it’s absent

lastSym=”${indir: -1}”

if [[ $lastSym != “/” ]]; then   

     indir=”${indir}/”

fi

lastSym=”${tempdir: -1}”

if [[ $lastSym != “/” ]]; then  

    tempdir=”${tempdir}/”

fi

 nfiles=0

for mp4file in $( ls ${indir}*.mp4 );

do   

    bname=$(basename “$mp4file”)    # filename without directory path

    fname=”${bname%.*}”                  # filename without extension

   lastSym=”${fname: -1}”    # files with names not ending numerals are not segments, even if they are segments we can’t sorted 

   if [[ $lastSym == [0-9] ]]; then 

       mp4segment=”${tempdir}${fname}.mp4″

        h264segment=”${tempdir}${fname}.264″

        cat $initmp4  $mp4file > $mp4segment 

       ffmpeg  -loglevel quiet  -i $mp4segment -vcodec copy -vbsf h264_mp4toannexb -f h264 -y $h264segment 

     # get file pattern

     if (( nfiles==0 ))   

     then

            filepattern=${fname%_*}          

           echo “file pattern  $filepattern” 

     fi   

    rm $mp4segment  

   let nfiles++   

fi

    done

    echo

    echo “total number of segments $nfiles”

    echo

    cnt=1

    touch $outfile   # create empty file

    while [ $cnt -le $nfiles ]   

   do

         filename=”${tempdir}${filepattern}_${cnt}.264″    

        if [ ! -f $filename ]; then

            echo $filename   not exist

            exit 2    

        fi  

      cat $filename >> $outfile 

       let cnt++  

  done

  exit 0

 

 

Usage of the above script is simple:

indir – the directory where mp4-segments are located. Notice that each segment should have the extension ‘mp4’ and patterned as ‘xxxx_<num>.mp4’ , e.g ‘slava_13.mp4’

init-mp4   – init-mp4 file

 outstream – output h264 file

tempfolder – folder where intermediate files are stored