Media Encoding

Shaka Packager does not do transcoding internally. The contents need to be pre-encoded before passing to Shaka Packager.

General guidelines of how contents should be encoded

• Encode multiple bitrates or resolutions of the same content. Shaka Packager can then package the content into DASH / HLS formats, allowing different bitrates of the content to be served for different network conditions, achieving adaptive bitrate streaming.
• Not a must, but the multibirate content is recommended to have aligned GOPs across the different bitrate streams. This makes bitrate switching easier and smoother.
• We recommend setting GOP size to 5s or less. The streams are usually switchable only at GOP boundaries. A smaller GOP size results in faster switching when network condition changes.
• In the same stream, the bitrate should be more or less the same in the inter-GOP level.

Sample commands to generate multi-bitrate content

Let us say we have a 1080p original content original.mp4 containing an audio track in AAC and a video track in H264. The frame rate is 24. We want to encode the contents into four resolutions: 360p, 480p, 720p and 1080p with GOP size 72, i.e. 3 seconds.

We use ffmpeg here, which is a common tool used for transcoding.

H264 encoding

• 360p:

  $ ffmpeg -i original.mp4 -c:a copy \
    -vf "scale=-2:360" \
    -c:v libx264 -profile:v baseline -level:v 3.0 \
    -x264opts scenecut=0:open_gop=0:min-keyint=72:keyint=72 \
    -minrate 600k -maxrate 600k -bufsize 600k -b:v 600k \
    -y h264_baseline_360p_600.mp4
  

• 480p:

  $ ffmpeg -i original.mp4 -c:a copy \
    -vf "scale=-2:480" \
    -c:v libx264 -profile:v main -level:v 3.1 \
    -x264opts scenecut=0:open_gop=0:min-keyint=72:keyint=72 \
    -minrate 1000k -maxrate 1000k -bufsize 1000k -b:v 1000k \
    -y h264_main_480p_1000.mp4
  

• 720p:

  $ ffmpeg -i original.mp4 -c:a copy \
    -vf "scale=-2:720" \
    -c:v libx264 -profile:v main -level:v 4.0 \
    -x264opts scenecut=0:open_gop=0:min-keyint=72:keyint=72 \
    -minrate 3000k -maxrate 3000k -bufsize 3000k -b:v 3000k \
    -y h264_main_720p_3000.mp4
  

• 1080p:

  $ ffmpeg -i original.mp4 -c:a copy \
    -vf "scale=-2:1080" \
    -c:v libx264 -profile:v main -level:v 4.2 \
    -x264opts scenecut=0:open_gop=0:min-keyint=72:keyint=72 \
    -minrate 6000k -maxrate 6000k -bufsize 6000k -b:v 6000k \
    -y h264_main_1080p_6000.mp4
  

VP9 encoding

The audio is encoded into opus.

• 360p:

  $ ffmpeg -i original.mp4 \
    -strict -2 -c:a opus \
    -vf "scale=-2:360" \
    -c:v libvpx-vp9 -profile:v 0 \
    -keyint_min 72 -g 72 \
    -tile-columns 4 -frame-parallel 1 -speed 1 \
    -auto-alt-ref 1 -lag-in-frames 25 \
    -b:v 300k \
    -y vp9_360p_300.webm
  

• 480p:

  $ ffmpeg -i original.mp4 \
    -strict -2 -c:a opus \
    -vf "scale=-2:480" \
    -c:v libvpx-vp9 -profile:v 0 \
    -keyint_min 72 -g 72 \
    -tile-columns 4 -frame-parallel 1 -speed 1 \
    -auto-alt-ref 1 -lag-in-frames 25 \
    -b:v 500k \
    -y vp9_480p_500.webm
  

• 720p:

  $ ffmpeg -i original.mp4 \
    -strict -2 -c:a opus \
    -vf "scale=-2:720" \
    -c:v libvpx-vp9 -profile:v 0 \
    -keyint_min 72 -g 72 \
    -tile-columns 4 -frame-parallel 1 -speed 1 \
    -auto-alt-ref 1 -lag-in-frames 25 \
    -b:v 1500k \
    -y vp9_720p_1500.webm
  

• 1080p:

  $ ffmpeg -i original.mp4 \
    -strict -2 -c:a opus \
    -vf "scale=-2:1080" \
    -c:v libvpx-vp9 -profile:v 0 \
    -keyint_min 72 -g 72 \
    -tile-columns 4 -frame-parallel 1 -speed 1 \
    -auto-alt-ref 1 -lag-in-frames 25 \
    -b:v 3000k \
    -y vp9_1080p_3000.webm