Allow alt text for external content resources beyond label

[azaroth42]

Content resources might not be accessible to all users, such as images for users with visual impairments or audio for those with auditory impairment. The resource can have a label, but this is not the same thing -- a textual representation of an audio file is not the name of the audio file.

The property should allow internationalized text, following the languageMap (#755) decision.

[azaroth42]

Needs a proposal, or could defer until 3.1 if there's insufficient interest?

[tomcrane]

Is it just description?

[azaroth42]

Now summary according to #1242. I think so? Just a case of documenting this in the definition?

[azaroth42]

Discussion on 11/22 call:

Question: Where does this actually live? Doesn't go on the image's info.json, not per tile, nor on the canvas which could have many images or time based media.
Need to recognize that we're building an application, not a replacement for and need different solutions to just an alt text property.

No consensus as to the correct way forwards.

[anarchivist]

Per comments by me and @workergnome on 22 Nov call:

• Alt text is usually very short (~30 words or less)
• Image description for accessibility can be longer. See, for example Art Beyond Sight and MCA Chicago's Coyote documentation
• Image description for a11y is not necessarily the same as summary. Doing so places probably undesirable semantic constraints on summary.
• @workergnome had some conversations with Sina Bahram (Prime Access Consulting), who worked on MCA's Coyote project; recommendation is that we treat IIIF viewers more as ARIA applications.

[jpstroop]

Use case here: #1777 (comment)

Finally, a text version should be provided for screen readers so that all users are aware of the content and purpose of the image.

[jpstroop]

Some text that is more descriptive than label seems like it could go with a logo, or any other image or a/v content for that matter

[azaroth42]

Editors agree it's important, especially with 3d that adds even more accessibility issues. Would be good to have an informed and holistic solution, perhaps via a new TSG?

[tomcrane]

Dropped into Slack #a11y

Editors have been discussing #1288
But needs a wider discussion. There are a set of aria-* tags that a IIIF client might expose, drawing them from IIIF properties, but what are those properties and what are the mappings?