[imsc] forcedDisplay and visibility="hidden" (#484)

nigelmegitt has just created a new issue for https://github.com/w3c/imsc:

== forcedDisplay and visibility="hidden" ==
Currently the specification for [`itts:forcedDisplay`](https://www.w3.org/TR/ttml-imsc1.1/#itts-forcedDisplay) in both IMSC 1.0.1 and IMSC 1.1 suggests in the introduction that it only applies to content for which the value of `tts:visibility="visible"` but the normative text does not actually make that distinction.

We have a use case for using `itts:forcedDisplay="true"` _and_ `tts:visibility="hidden"` for the same content: the idea is that when text is burned into video and has no equivalent sound an "invisible" caption is forced on to trigger reading by a screen reader, i.e. producing an audible presentation without producing "any visible rendering".

We want to do this in the short to medium term without having to advance to full adoption of `condition`, and this (direction of text to a screen reader) is not a candidate for the use of `tta:speak` which is itself not conditional on there being an active screen reader.

I don't think this alone is adequate to trigger the release of a new version of IMSC, however if one is being prepared for any reason, I would propose changing the opening sentence from:

> `itts:forcedDisplay` can be used to hide content whose computed value of `tts:visibility` is `"visible"` when the processor has been configured to do so via the application parameter `displayForcedOnlyMode`.

to 

> `itts:forcedDisplay` can be used to prevent visible rendering of content when the processor has been configured to do so via the application parameter `displayForcedOnlyMode`, regardless of the computed value of `tts:visibility`.

and add a note:

> Note: Applications that send presented text to a screen reader can be configured to do so even when normal display of captions is disabled by the user, without making any visible mark, by using `itts:forcedDisplay="true"` in conjunction with `tts:visibility="hidden"`. This can be used to cause on-screen text with no audio equivalent to be presented in an alternative medium for users who cannot read that text.

Please view or discuss this issue at https://github.com/w3c/imsc/issues/484 using your GitHub account

Received on Monday, 5 August 2019 14:34:42 UTC