support multiple URLs/sources for vector tiles

[3nids]

This is a draft/POC to allow a discussion.

Goal: rendering Mapbox Vector Tile layer having multiple sources

for instance: https://vectortiles.geo.admin.ch/styles/ch.swisstopo.basemap.vt/style.json
(see online: https://cms.geo.admin.ch/fmc/bm.html)

{
  "version": 8,
  "id": "2fa0964e-8ce3-4fe9-91b3-2a0d18663c9d",
  "name": "basemap_v1.15.0",
  "sources": {
    "terrain_v1.0.0": {
      "url": "https://vectortiles.geo.admin.ch/tiles/ch.swisstopo.relief.vt/v1.0.0/tiles.json",
      "type": "vector"
    },
    "base_v1.0.0": {
      "url": "https://vectortiles.geo.admin.ch/tiles/ch.swisstopo.base.vt/v1.0.0/tiles.json",
      "type": "vector"
    }
  },
  "layers": [
…

This cannot be solved by using several QGIS layers since the symbols ordering doesn't necessarily match with the layers/sources.

The idea is to allow listing several URLs in the layer. The rendering of each tile will be done at once when every source are fetched.

I have adapted QgsVectorTileRawData so that it contains a list of QByteData instead of a single one. The MVT decoder has been adapted accordingly.

For the download algorithm I suppose we need to recombine the data into a single array.

The layer names are considered unique across sources. It might be necessary to map layer names to the proper source.

I have successfuly tested this for asynchronous tile fetching. Sync is implemented but not tested.

There is no UX yet to play with this, it will be done if the approach is accepted.
But here is a project to play with: mapbox-multi-sources.qgs.zip

Screenshot with the log of the 2 sources fetched:

Looking forward to read your thoughts :)

[nirvn]

@3nids , really happy about this one, it was literally less than two months ago that I had stumbled on this very issue.

[github-actions]

🪟 Windows builds ready!

Windows builds of this PR are available for testing here. Debug symbols for this build are available here.

(Built from commit 4f79b5b)

[nirvn]

Should we just remove this block altogether? Or Re-enable it the tiles size == 1 (i.e. when we only have one source)?

[m-kuhn]

@3nids any feedback on this one?

[3nids]

I did not added this ifdef. I dropped it.

[3nids]

@wonder-sk @troopa81 @nyalldawson @rouault do you have an opinion on this approach?

[m-kuhn]

Thank you for this, I am excited to see this land !

@nirvn are the latest updates ok for you?
If yes, should we merge it this week to have enough cooldown time before release?

[nirvn]

@m-kuhn , yep, I'm good with the latest updates.

[qgis-bot]

@3nids

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[DelazJ]

@3nids what is the status of this feature in regards of the docs? I've seen a series of mvt PRS but can't connect the points enough to tell if and what we could mention in docs...

[3nids]

This is the one #58663

[DelazJ]

Thanks @3nids