Document transcripts. (#2105)

* Document transcripts.

* Update verbiage.

* Put feature trace at bottom.

* Document that sandbox/install profile hides the field.

* Remove mentions of Defaults and add screeenshots.

docs/user-documentation/transcripts.md

@@ -0,0 +1,77 @@
+# Transcripts
+
+WEBVTT transcripts, captions, or subtitles may be displayed along with audio and video media.
+Transcripts must be added to the media entity that will be playing, using a
+special field type ("Media track"). When the media is configured to use the
+"Audio with Captions" and "Video with Captions" field formatters, tracks in
+"Media track" fields will be available.
+
+## Media track field type
+
+The Islandora module provides a field type, Media Track, that accepts .vtt files.
+Once a file has been uploaded, additional options relevant to media tracks become
+available to configure.
+
+![Screenshot of the Track field with a file uploaded and configured](../assets/transcripts-media-track-field.png)
+
+
+## Using Transcripts with the Islandora Starter Site
+
+In the Islandora Starter Site, Audio and Video media are pre-configured to work with tracks.
+
+!!! note "Sandbox"
+    On the public sandbox, or other sites using the Islandora Install Profile Demo,
+    you will first need to make the "Track" field visible in the media form, at
+    Structure > Media > Audio|Video > Manage Display.
+
+### Video
+
+- Navigate to the Service File media (or whichever media is configured to play)
+for a Video item.
+- Click "Edit" to get to the Media's edit form.
+- In the "Track" field, add one or more WebVTT files.
+- Enter the details about each audio track.
+    - Under "Kind", select "Captions" or "Subtitles". (Per the HTML standard, the others are valid track types but won't be played in the viewer)
+    - The "Label" will be visible by the user when they select a track to play.
+- Save the media and refresh it.
+
+![Screenshot of captions on a video, with the menu open showing available caption tracks](../assets/transcripts-video.png)
+
+
+### Audio
+
+- Follow the same instructions as for Video.
+- For audio, only upload one Track file (others will not be displayed).
+- For audio, ensure that the Track file is marked "Default track" (otherwise it will not be displayed).
+- For audio, the selected "Kind" of media track does not have any effect.
+
+![Screenshot of captions on an audio showing a single line of text above the player](../assets/transcripts-audio.png)
+
+## Caveats
+
+!!! info "Selecting the correct media"
+    Note that if you add the transcript file to the Original File, but you have configured
+    the site to play the Service File, then you will not see the transcript.
+
+!!! info "Languages"
+    While you can add subtitles in different languages, you may only choose from
+    the site's installed languages.
+
+!!! tip "Types"
+    The five options: **captions**; **subtitles**; **descriptions**; **chapters**; and **metadata** come from
+    the [HTML standard's `<track>` element](https://html.spec.whatwg.org/multipage/media.html#the-track-element).
+    As per their definitions, captions and subtitles will be displayed as optional text over the video,
+    available through the usual [cc] icon in the viewer controls. Descriptions, chapters, and metadata
+    will not be displayed as they are intended for programmatic use.
+
+## Tracing the components of this feature
+
+| Feature Component | Source |
+|---|---|
+| Define Field type "Media track" | Islandora module |
+| Add "Media track" fields to audio and video media types | Islandora Starter Site |
+| Define IslandoraFileMediaFormatterBase, which finds Media Track fields on media, and adds them as playable tracks | Islandora module |
+| Define "Audio with Captions" field formatter, extending IslandoraFileMediaFormatterBase | Islandora Audio |
+| Define "Video with Captions" field formatter, extending IslandoraFileMediaFormatterBase | Islandora Video |
+| Select these field formatters for the Default and Source display modes for audio and video media | Islandora Starter Site |
+

mkdocs.yml

@@ -166,6 +166,8 @@ nav:
           - 'Flysystem': 'user-documentation/flysystem.md'
           # moved from Developer documentation
           - 'Large files': 'user-documentation/uploading-large-files.md'
+          - 'Transcripts': 'user-documentation/transcripts.md'
+              # Conceptual and pragmatic, for repository managers.
       - Administering Islandora:
           - 'Access control': 'user-documentation/access-control.md'
           - 'JWT authentication': 'user-documentation/jwt-authentication.md'