[core] Filter out child tiles that overlap with parent tile

[alexshalamov]

Whenever map is panned or zoomed and some of the tiles at the current zoom level are not loaded, cached parent tile may be used until missing tile is loaded. The problem is that whenever circle layer is non-opaque or has blur, parent tile will be drawn together with children (overdraw) causing flickering, as the non-opaque pixels are blended together.

This PR adds filter for circle layer in order to avoid rendering parent and children tiles covering same area.

Checklist

• briefly describe the changes in this PR
• add unit tests

[asheemmamoowala]

Related GL-JS issue: mapbox/mapbox-gl-js#6768

[pozdnyakov]

LGTM % nit

[pozdnyakov]

nit: RenderLayer::filterRenderTiles(std::move(tiles_));

[ansis]

Could this approach introduce flickering in existing tiles? I'm trying to understand what would happen in this case.

• you load and display the z0/x0/y0 tile centered on the northwest corner of the world
• you zoom in and the z1/x0/y0 tile is loaded
• now you pan east and eventually you get to an area that needs both z1/x0/y0 (already loaded) and z1/x1/y0 (needs to be loaded)
• as you are loading z1/x1/y0 the cached z0/x0/y0 is rendered

Does the z1/x0/y0 disappear for a bit while z1/x1/y0 loads? Does this introduce flickering if the z1 tile had data that the z0 tile didn't? (due to simplification and data thinning)

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If it does introduce flickering it could still be much better than the current flickering!

I'm not sure how else this could be fixed. Maybe a uniform could be passed to the vertex shader that specifies which parts of the tile are covered by child tiles? It could then use that to drop features from the parent tile that are in areas covered by child tiles.

[alexshalamov]

@ansis

Does the z1/x0/y0 disappear for a bit while z1/x1/y0 loads? Does this introduce flickering if the z1 tile had data that the z0 tile didn't? (due to simplification and data thinning)

Yes, it does. Also, I think there is a problem related to cached tiles. One missing child tile should not make rest of loaded / cached tiles non-renderable.

I need to rewrite PR and approach the issue differently, options:

• Drop vertices in vertex shader (proposed by @ansis)
• Drop features. Something like 'cross-tile-geometry-index' that can be used by circle and heatmap layers

[alexshalamov]

Need to rewrite PR.