This article is in need of a technical review.
Dynamic Adaptive Streaming over HTTP (DASH) is an adaptive streaming protocol. This means that it allows for a video stream to switch between bit rates on the basis of network performance, in order to keep a video playing.
Firefox 21 includes an implementation of DASH for HTML5 WebM video which is turned off by default. It can be enabled via "about:config" and the "media.dash.enabled" preference.
Using DASH - Server Side
First you'll need to convert your WebM video to a DASH manifest with the accompanying video files in various bit rates. To start with you'll need:
- ffpmeg - with libvpx and libvoribis support for WebM video and audio (ffmpeg.org).
- libwebm - specifically for the samplemuxer tool (git clone https://gerrit.chromium.org/gerrit/p/webm/libwebm.git).
- webm-tools - specifically for the manifest creation tool, webm_dash_manifest (git clone https://gerrit.chromium.org/gerrit/p/webm/webm-tools.git).
1. Use your existing WebM file to create one audio file and multiple video files.
Create the audio using:
ffmpeg -i my_master_file.webm -vn -acodec libvorbis -ab 128k my_audio.webm
And create the video files using:
ffmpeg -i my_master_file.webm -vcodec libvpx -vb 250k -keyint_min 150 -g 150 -an my_video-250kbps.webm ffmpeg -i my_master_file.webm -vcodec libvpx -vb 100k -keyint_min 150 -g 150 -an my_video-100kbps.webm ffmpeg -i my_master_file.webm -vcodec libvpx -vb 50k -keyint_min 150 -g 150 -an my_video-50kbps.webm
2. Align the clusters to enable switching at cluster boundaries.
samplemuxer -i my_video-250kbps.webm -o my_video-250kbps-final.webm
Although we don't switch audio streams, it's still necessary to run it through samplemuxer to ensure a cues element is added. Note: to be compatible with playing on Chrome, it is suggested to change the track number to something other than the one in the video files, most likely 0.
samplemuxer -i my_audio.webm -o my_audio-final.webm -output_cues 1 -cues_on_audio_track 1 -max_cluster_duration 2 -audio_track_number
3. Create the manifest file:
webm_dash_manifest -o my_video_manifest.mpd \ -as id=0,lang=eng \ -r id=0,file=my_video-250kbps-final.webm \ -r id=1,file=my_video-100kbps-final.webm \ -r id=2,file=my_video-50kbps-final.webm \ -as id=1,lang=eng \ -r id=4,file=my_audio-final.webm
Put the manifest and the associated video files on your web server or CDN. DASH works via HTTP, so as long as your HTTP server supports byte range requests, and it's set up to serve .mpd files with mimetype="application/dash+xml", then you're all set.
Using DASH - Client Side
You'll want to modify your web page to point to the DASH manifest first, instead of directly to a particular video file:
<video> <source src="movie.mpd"> <source src="movie.webm"> Your browser does not support the video tag. </video>
That's it! If DASH is supported by the browser, your video will now stream adaptively.