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Text tracks

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Text tracks

Overview

The rx-player allows to display text tracks - such as subtitles or closed captions - directly over your content:

Example of a textrack on acontent

Adding text tracks to contents can be done by two means:

  • by using a Manifest declaring those text tracks
  • by manually adding text track(s) when you call the loadVideo API

You can then choose the right track through the different text track-related API, all documented in the general API documentation.

Supported text track formats

The rx-player supports the following formats:

  • TTML (TTML1, EBU-TT and IMSC1)
  • WebVTT
  • SAMI
  • SRT
  • TTML embedded in an MP4 file
  • WebVTT embedded in an MP4 file

Text tracks indicated in a manifest

Each streaming technology supported by the Manifest defines a way to add text track directly in their Manifests files.

This chapter explains what is supported by the RxPlayer.

In DASH

In DASH, text tracks are defined by AdaptationSet elements, which have a contentType attribute equal to text.

Those AdaptationSet can also define a lang, codecs and mimeType, which are then exploited by the RxPlayer.

The lang attribute is used to know the language the track is in.

The RxPlayer understands the following standards:

  • ISO 639-1 (2 letters)
  • ISO 639-2 (3 letters)
  • ISO 639-3 (3 letters)

More complex combined formats are also understood, as long as it begins by one of the understood standards, followed by a dash ("-").

For example “en-US” is translated into just “en”, and then inferred to be english.

The mimeType attribute is used to know in which format the track is in.

The RxPlayer understands the following ones:

  • application/ttml+xml: TTML in plain text
  • text/vtt: WebVTT in plain text
  • application/x-sami: SAMI in plain text
  • application/mp4: Text track embedded in an MP4 container.
  • text/plain: Generic plain text mimeType

For the last two, the codecs attribute of the AdaptationSet will be exploited to know the exact format.

The rx-player uses the codecs attribute for text tracks in only two cases:

  • the mimeType is equal to application/mp4
  • the mimeType is equal to text/plain

For the first case, both WebVTT and TTML can be embedded in an MP4 file. To know which one we’re dealing with, the codecs attribute should be equal to:

  • stpp for TTML
  • wvtt for WebVTT

For the second case ("text/plain"), this is specifically to support plain SubRip (SRT) subtitles. To use them you need to set codecs simply to srt.

To know if we’re dealing with a closed caption text track, the RxPlayer uses the DVB-DASH specification.

That is, an AdaptationSet is inferred to be a closed caption for the hard of hearing, if it contains an Accessibility descriptor with the following attributes:

  • SchemeIdUri set to urn:tva:metadata:cs:AudioPurposeCS:2007
  • value set to 2

In Microsoft Smooth Streaming

In Smooth Manifests, a StreamIndex is inferred to be for a text track if its Type attribute is equal to text.

The FourCC attribute is used to infer the format of the text track. Only TTML is understood, and is translated to be a TTML track embedded in an MP4 container.

Adding support for other formats is very simple, open an issue if you want us to add a standardized FourCC code for another supported format.

The Language is used to know the language the track is in. The rules are the same than for DASH:

The rx-player understand the following standards:

  • ISO 639-1 (2 letters)
  • ISO 639-2 (3 letters)
  • ISO 639-3 (3 letters) More complex combined formats are also understood, as long as it begins by one of the understood standards, followed by a dash ("-").

For example “en-US” is translated into just “en”, and then inferred to be english.

The Subtype attribute is used to know if the language is a closed caption or not.

At the moment, the RxPlayer infers the track to be a closed caption only if its value is DESC.

Text tracks added manually

It is also possible to add a supplementary text track dynamically, by using the TextTrackRenderer tool. You can read its documentation here.

Text track display modes

There is two ways the text track can be displayed:

  • "native": The text track is displayed in <track> elements, which are directly in the linked videoElement.

  • "html": The text track is displayed in a separate <div> element.

The second ones allows for a better track stylisation. The distinction between those two is pretty simple and is explained here, in the loadVideo options documentation.