Document Physical Dimensions extension

[azaroth42]

Following #1269, the Physical Dimensions service is no longer a service but an extension. We need to document it, as it is used in the wild. E.g.: https://data.ucd.ie/api/img/manifests/ucdlib:33064

[azaroth42]

Can we find someone other than an editor to do this, as an easy test run of submitting an extension to be registered?

[workergnome]

Here's a first pass at this: https://gist.github.com/workergnome/01a3c617b0f5a6ee8a2fb51fc44666bf

To go along with it, here's a first pass at the documentation for writing extensions as tested on both this and a revised version of my palette extension.

https://gist.github.com/workergnome/a161d3376a950ac7e283e6ec6630e1ec

[zimeon]

I wrote a comment on the gist but perhaps better to add here: The physdim extension looks good to me, I would be happy to import this into the prezi3 branch as /api/annex/exensions/physdim/index.md (to eventually become http://iiif.io/api/annex/exensions/physdim/) and roll any review/revision into the v3 process. We could then also use this extension as an example in the Image API v3 spec (in the full example, 5.8)

[workergnome]

Fine by me.

[azaroth42]

Some minor comments on the referenced gist ...

• Instead of type PhysicalDimension, it seems more like a Ratio or Scale. I would intuitively expect an array of PhysicalDimension objects referenced from a physicalDimensions property. PhysicalRatio ?
• Do we need to talk about Duration for v3? I don't think so as there's no real scaling here.

And meta-wise:

• Do we want the contexts to be in the extension HTML pages, or just referenced?

[zimeon]

I was assuming context should be just referenced but was included in the gist for convenience

[zimeon]

To the points above:

• I think Ratio would be wrong -- this is a dimensioned concept: pixels to something with a length unit. I could imagine Scale but I don't really see any advantage in clarity overPhysicalDimesion
• I agree that duration is not required, we already have real seconds

[workergnome]

I think a near-the-top link to the context would be useful, but I don't think there's a lot of value in explicitly writing it out--the documentation is more useful.

Don't have strong opinions abut the name--my only argument would be changing the name away from what it was called in prezi2 would add even more cognitive barriers for upgraders.

[azaroth42]

For 4.0, proposal to bring this into the main spec, as any AR use will require it for 3d