[openstreetmap/openstreetmap-website] Add retina support (#104)

[Andy Allan]

> My first interaction on the entire repo and I get the issue closed. sweat_smile

No worries! It wasn't your comment in particular - when I looked back at the previous comments I realised it was worth stepping in.

> Along the lines of: _This website uses provider X for the default tiles, and they cannot provide High-DPI tiles because of reason Y, which is discussed in issue Z, where they need help with V?_

This website uses tiles provided by the OpenStreetMap Foundation (specifically, a service run by the OSMF Operations Working Group) to provide tiles for the default layer (the "Standard" layer). They cannot provide High-DPI tiles because their current infrastructure does not easily support doing so. Specifically, the mod_tile-based infrastructure works on a query-then-render-then-store-then-serve basis (note store, not cache, which is a subtle but important difference), which has big implications for disk space used on the tile servers, in addition to the overheads of fully rendering everything twice. 

They need help in developing alternative infrastructure, which will likely involve either rendering on-the-fly from vector tiles (i.e. still delivering rasters, but only storing the underlying vector tiles, which saves storing lots of rasters) or perhaps client-side rendering directly from vector tiles. The decisions involved in choosing the solution are complex, since they have implications for cost, cartography, server infrastructure (e.g. changing the needs between powerful database servers, adding lightweight rendering servers, using object stores for vector storage, etc), software choices and further software development, and then the operational overhead of doing so at scale, which can rule out options that are otherwise used in smaller scale setups. 

The old adage of "if it was easy, it would have been done already" clearly applies.

In the time I was writing this @tomhughes has also replied, so I hope between these two comments you get enough of the background!

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