pmtiles with non-continuous zoom levels

[etienneJr]

Hi,
Following #321, I am considering reducing the size of my pmtiles file (MVT tiles based on OSM data using OMT scheme) by including only a few zoom levels, for example (let's say) zoom levels 6, 9, 12, 14 (instead of all levels from 6 to 14).
Thus, levels 10 and 11 (for example) would be obtained by overzooming level 9 (using the errorOnMissingTile option). After a few adjustments to ensure that zoom 9 tiles contain all the necessary information, I do not expect a significant drop in display quality. But I expect a very significant gain in file size!!
In your opinion, what are the different possible ways of creating such a file?
Thanks !

[wipfli]

It does not help much for total archive size to remove low zoom levels. Most data will be in your highest zoom level. That is because of the number of tiles. If you have n tiles at z14, you have n / 4 tiles at z13, n / 16 tiles at z12 etc

[etienneJr]

You're right, since my maxzoom is 14, I especially want to remove z13 for the objective of decreasing total file size.
But in fact I have another goal: decrease the amount of data downloaded by the user when zooming in or out.
Today (almost) all zoom levels are fetched when the user zooms, this is an issue for mobile low speed or limited connections.
I know that MapLibre overzooms the previous level until the next one has been downloaded, but it still downloads a lot of redundant data (z_x+1 is almost the same than z_x).
It seems to me that the ratio between 2 zoom levels comes from raster tiles history to avoid pixelisation, but it could be increased when working with vector tiles.

[wipfli]

If you just want to prototype it you could do something like this

maplibregl.addProtocol('my-protocol', (params, abortController) => {
  // get z x y from params.url
  // if z === 13, throw error
  // else get tile from pmtiles file or zxy endpoint
})

[etienneJr]

Oh yes, of course, I hadn't thought of that smart solution. I'll give it a try, thank you!

[etienneJr]

I managed to get the expected behavior by modifying directly my pmtiles.js file here by adding if(z>10 && z<14) {throw new Error("skipping zoom level "+z)};. Zoom levels 11, 12 and 13 are skipped, and zoom level 10 is overzoomed until tile z14 is found.

I would much rather write a custom protocol, but I did not succeed...

Today in my style file, I am using the direct and simple solution:

"sources": {
    "openmaptiles": {
      "type": "vector",
      url: `pmtiles:///tiles/planet.pmtiles`,
    }
}

I tried to replace the url by url: "custom://test/" but in this case params is an Object { url: "custom://test", type: "json" } so I can't read z/x/y.
To read them, I had to replace url by tiles: ["custom://test/{z}/{x}/{y}.mvt"] but in this case I don't know how to "simply" get the tile from the pmtiles. I would prefer to avoid rewriting the entire pmtiles protocol in my custom protocol, so I was expecting something to throw back to the pmtiles protocol. I tried this but it is not working:

maplibregl.addProtocol('custom', (params, abortController) => {
  console.log(params)
  const url = params.url.replace('custom://test/', 'pmtiles:///tiles/planet.pmtiles/');
  return {
    data: {
      tiles: [url],
    },
  };
})

[wipfli]

Here is what I do in Mapterhorn:

        const protocol = new pmtiles.Protocol({ metadata: true, errorOnMissingTile: true });
        maplibregl.addProtocol('mapterhorn', async (params, abortController) => {
            const [z, x, y] = params.url.replace('mapterhorn://', '').split('/').map(Number);
            const name = z <= 12 ? 'planet' : `6-${x >> (z - 6)}-${y >> (z - 6)}`;
            const url = `pmtiles://https://download.mapterhorn.com/${name}.pmtiles/${z}/${x}/${y}.webp`;
            const response = await protocol.tile({ ...params, url }, abortController);
            if (response['data'] === null) throw new Error(`Tile z=${z} x=${x} y=${y} not found.`);
            return response;
        });

        const map = new maplibregl.Map({
            container: 'map',
            hash: 'map',
            zoom: 10.5,
            center: [9.0788, 47.1194],
            style: {
                version: 8,
                sources: {
                    hillshadeSource: {
                        type: 'raster-dem',
                        tiles: ['mapterhorn://{z}/{x}/{y}'],
                        encoding: 'terrarium',
                        tileSize: 512,
                        attribution: '<a href="https://github.com/mapterhorn/mapterhorn">© Mapterhorn</a>'
                    },
                },
                layers: [
                    {
                        id: 'hillshade',
                        type: 'hillshade',
                        source: 'hillshadeSource',
                    },
                ],
            },
        });

[bdon]

but it could be increased when working with vector tiles.

Most optimized vector tile sets perform simplification (using Douglas-Peucker or other algorithms) and rendering z10 tiles at z11 or 12 will look over-simplified. You may be able to make a trade-off by storing less simplified tiles at 10 and overzooming, but keeping with a complete set of powers-of-two is the most general solution.