reparse overscaled tiles

[ansis]

Fixes #1004

@jfirebaugh this was easier to handle by adding a couple special cases in Source and TileCoord than by loading multiple copies and clipping.

• It uses the maxzoom to generate a cover, but creates tiles with the current zoom's z.
• When generating the url, it uses maxzoom if z > maxzoom.
• TileCoord's .parent() and .children() have a special case when z > maxzoom.

---

When the current zoom is past the vectortile source's maxzoom, load overscaled tiles and parse them specifically for the current integer zoom level. This is necessary to:

• make layout property functions work
• let us place labels only 1 zoom level deep (for performance)

Overscaled tile id's have the same x/y/w as their corresponding tiles from the source's maxzoom.

For example:

currentzoom: 18
maxzoom: 16

The overscaled tile { z: 18, x: 123, y: 456 }
loads the real tile { x: 16, x: 123, y: 456 }

[mourner]

• fix GeoJSON disappearing after z15
• fix tests

[mourner]

Also, considering underlying data is the same for overscaled tile and max zoom tile, could we change interpolation so that it doesn't jump when the overscaled tiles are loaded?

[ansis]

fix GeoJSON disappearing after z15

Should we just remove maxzoom for geojson sources? or would it be better to also overscale geojson tiles?

Also, considering underlying data is the same for overscaled tile and max zoom tile, could we change interpolation so that it doesn't jump when the overscaled tiles are loaded?

Yes, we should look at this more carefully when all the other labeling stuff is done.

[mourner]

Should we just remove maxzoom for geojson sources? or would it be better to also overscale geojson tiles?

We could do either way. The advantage of overscaling vs generating new clipped tiles is potentially no jumpy tiles. cc @jfirebaugh

[ansis]

Both tests and rendering tests pass (with mapbox/mapbox-gl-test-suite#17).
Geojson is also fixed.

[jfirebaugh]

👍

[kkaefer]

Can we get this ported to native?

[ansis]

@kkaefer I'll start on it soon. issue so that I don't forget: mapbox/mapbox-gl-native#912

[kkaefer]

Also, is this covered by tests?

[ansis]

mapbox/mapbox-gl-test-suite@28b6a25 adds rendering tests. Is there anything that should be covered by regular tests?

[kkaefer]

Can we use a few sample tiles and compare the output of the clipped tile algorithm with the expected result?

[ansis]

@kkaefer we're not actually clipping the data. It draws an overscaled tile the same way it used to draw it. The only change is that instead of reusing the same tile object, it creates a new tile object. It creates a new tile object so that layout property functions work and so that labelling gets reprocessed for that zoom level.