Smaller zoom levels "leaking" in at larger zoom levels

[LukasBommes]

I noted visual artefacts at in our PMTiles as shown in the screenshots attached. To me it looks like that lower resolved overviews are "leaking in" at closer zoom levels. I observed this for mutiple differen PMTiles in PMTiles viewer and also in Openlayers (as in this example).

If it helps, I can anonymize my PMTile and provide it for debugging.

PMTiles were created from VRTs like this

gdal_translate in.vrt out.mbtiles -of MBTILES
gdaladdo -r bilinear out.mbtiles -minsize 256
pmtiles convert out.mbtiles out.pmtiles

[bdon]

have you experimented with generating a higher zoom level of mbtiles?

[LukasBommes]

You mean higher than the "native" zoom level chosen by gdal_translate with default settings (BLOCKSIZE=256, TILE_FORMAT=PNG, ZLEVEL=6, ZOOM_LEVEL_STRATEGY=AUTO)?

I did try generating MBTiles with ZOOM_LEVEL_STRATEGY=UPPER but haven't analyzed the impact on the artefact yet. I can do so for debugging purposes. But adding an additional zoom level seems wasteful to me as it significantly increases size of the resulting PMTiles.

A (maybe) important detail to add: The artefact occurs at the tile grid boundary. This becomes evident when rendering the tile grid in OpenLayers as shown in this example. To me it looks like the renderer does not properly delete the lower-zoomlevel tile when rendering the higher zoom levels.

[bdon]

It would be helpful if you could share your file, but I suspect this is related to the generation of the tiles and outside the scope of the PMTiles format itself (this repository).

[bdon]

I took a look at the openlayers example and I see artifacts due to how the DEM was created but can't reproduce what you have a screenshot of above.

Possible explanations for the above screenshots:

• if this is a partial tile pyramid of just one region, it is possibly loading in more detailed tiles in some areas (you can enable the debug tile boundaries in MapLibre to see them)
• the "leaking" could be happening because of the resampling when generating overview tiles

please re-open this issue when you are able to upload the sample to inspect.