Is there a way to request a IIIF resource via the image API, but set the tileFormat via OpenSeadragon?

I’ve been wondering on the best way to use the tileFormat option from IIIFTileSource, with a tileSources array that just specifies IIIF endpoints.

Using the JSON response directly and including the tileFormat key in there seems to do the trick. However the output from my image server doesn’t include tileFormat as this appears to be non-standard. I do have a formats array, but the values in here don’t seem to be used (i.e. if this only contains ‘png’ it will still choose ‘jpg’).

I could request each resource outside of OpenSeadragon and append the tileFormat myself (and I suspect this will be the simplest approach), but I wondered if there’s a way I’m missing that would signal to OpenSeadragon to use a different tileFormat for all requests?

Edit: In our particular use case, we have transparency and upon reflection we’d probably like other applications which consume these resources to use png and leverage that. For that reason we’re also considering if updating the response from the image server to include tileFormat might serve our needs better.

[tomcrane]

preferredFormats is already supported, from the v3 work that @lutzhelm did:

            if(data.preferredFormats) {
                for (var f = 0; f < data.preferredFormats.length; f++ ) {
                    if ( OpenSeadragon.imageFormatSupported(data.preferredFormats[f]) ) {
                        data.tileFormat = data.preferredFormats[f];
                        break;
                    }
                }
            }

https://github.com/openseadragon/openseadragon/blob/master/src/iiiftilesource.js#L245

This should make OSD request tiles in the format hinted in the info.json, rather than the jpg default (unless jpg is the preferredFormat!)

This is ideal for when:

• the publisher knows that the default jpeg is not the best format for this image
• the publisher can intervene in the info.json generation to say so

In practice, in large scale digitisation workflows, neither of these are often true. Instead, it’s a page-generation-time decision, e.g.,

• you know that a particular collection is line-art therefore better as pngs
• there’s some descriptive metadata, not available to the image server, that would lead you to prefer a different format for a particular item

We had a very similar need - the preferredFormats is really useful, but we also need to hint it from the client, optionally providing it where the info.json does not have it. This isn’t the same as forcing a tileFormat, because it’s a preference - if the image service can’t supply a webp or a heif or whatever you would like, you still want OSD to request the default, or a lower-ranked preference that it does support.

It would be nice to have an easy way to insert a client-side preferredFormats list into the tileSource (the info.json data) - this could even work for pre-version-3 image services.

If you have the info.json data to hand already as the tileSource, you could just add it, but if you are passing OSD the image service URL I don’t know how best to push it in there.

[iangilman]

@altert Would that be a change to OSD? I don’t really have a sense whether it would be a good addition… How true to IIIF would it be?

@azaroth42 @tomcrane Any thoughts on this hack?