HLS Project Overview
This project has three primary duties:
- Download and parse playlist files
- Implement the HTMLVideoElement interface
- Feed content bits to a SourceBuffer by downloading and transmuxing video segments
Playlist Management
The playlist loader handles all of the details of requesting, parsing, updating, and switching playlists at runtime. It's operation is described by this state diagram:
During VOD playback, the loader will move quickly to the HAVE_METADATA state and then stay there unless a quality switch request sends it to SWITCHING_MEDIA while it fetches an alternate playlist. The loader enters the HAVE_CURRENT_METADATA when a live stream is detected and it's time to refresh the current media playlist to find out about new video segments.
HLS Tech
Currently, the HLS project integrates with video.js as a tech. That means it's responsible for providing an interface that closely mirrors the <video> element. You can see that implementation in videojs-http-streaming.js, the primary entry point of the project.
Transmuxing
Most browsers don't have support for the file type that HLS video segments are stored in. To get HLS playing back on those browsers, contrib-hls strings together a number of technologies:
- The Netstream in video.js SWF has a special mode of operation that allows binary video data packaged as an FLV to be provided directly
- videojs-contrib-media-sources provides an abstraction layer over the SWF that operates like a Media Source
- A pure javascript transmuxer that repackages HLS segments as FLVs
Transmuxing is the process of transforming media stored in one container format into another container without modifying the underlying media data. If that last sentence doesn't make any sense to you, check out the Introduction to Media for more details.
Buffer Management
Buffering in contrib-hls is driven by two functions in videojs-hls.js: fillBuffer() and drainBuffer(). During its operation, contrib-hls periodically calls fillBuffer() which determines when more video data is required and begins a segment download if so. Meanwhile, drainBuffer() is invoked periodically during playback to process incoming segments and append them onto the SourceBuffer. In conjunction with a goal buffer length, this producer-consumer relationship drives the buffering behavior of contrib-hls.