Provisional patch, since 'git review' doesn't work on kartotherian repo:

maps.wikimedia.org is really a debug interface for the maps -- we could of course add the DPI detection there, but the same work will still have to be done on the client that uses it.

We could add the allowed scaling values into the https://maps.wikimedia.org/osm-intl/info.json

In T112952#1650998, @Yurik wrote:

maps.wikimedia.org is really a debug interface for the maps -- we could of course add the DPI detection there, but the same work will still have to be done on the client that uses it.
We could add the allowed scaling values into the https://maps.wikimedia.org/osm-intl/info.json

Hmm, I wonder if it would be crazy to allow sending a scaling factor (from window.devicePixelRatio or native or whatever) as a query string parameter to that info.json and have it return a pre-bracketed value that's known to work?

Also, is it possible to have different available scaling levels depending on the tile source being used? That might make it trickier to expose the correct values.

Per @Yurik this project is using GitHub as primary, so resubmitted via a pull req: https://github.com/kartotherian/kartotherian/pull/13

i made some changed to @brion's patch, and deployed to the server, so i suspect we can close it.

Scaling factors are hardcoded for each source, but I simply picked the most common ones to reduce cache fragmentation. In theory, there could be a source that's pointless to scale, e.g. a raster source which presents the entire planet as a TIFF at zoom 5. When user asks for zoom 2, we give them a scaled down portion of that image. If they ask for scaling, we scale less for the same area (higher dpi). But if user asks for higher scale at zoom 5 (the max zoom), the raster there exists at 1:1, so we will waste bandwidth - we will scale the image without adding any data, something the client could have done itself.
All this requires advanced logic on the client, something to analyse the info.json and come up with the best downloading strategy.
I think we should write a simple js lib for that and host it as part of the maps service.

@brion, do you think it also makes sense to update the official tutorial at https://github.com/MaxSem/maps-demo ? We're running it at http://maps.beta.wmflabs.org for example.

Yes that'd seem ideal. :D

https://github.com/MaxSem/maps-demo/pull/1

Max, I have updated Brion's last patch, take a look in Kartotherian repo

So can I close this out as resolved, or should that wait for it to be working on http://maps.beta.wmflabs.org too?

I'm fine with closing it.