Zoom zero tile substitution #31
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We don't generate metatiles with negative zoom levels, so when we get a request which would cause us to request a negative zoom metatile, we adjust the offset to keep the metatile zoom at zero. Previously, we were just clamping the metatile and offset to `0/0/0`, but as we make larger metatiles this no longer works for low zooms.
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iandees
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rmarianski
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Use "lower zoom" tiles within the zoom 0 metatile as substitutes for lower zoom tiles.
For example, if a we have a global build with metatile size 8x8, this means that the metatile zoom offset is 3, so the 256px tiles in the
0/0/0metatile are actually tiles at zoom level 3. Changes requested intilequeuewould mean generating the tiles at lower zoom levels too, allowing us to use the+0/0/0offset in the0/0/0metatile to respond to the0/0/0request.This approach means we'll always be able to answer a request for at least one tile size at coordinate zoom 0 (i.e: a tile covering the whole world). The nominal zoom will depend on the tile size which is configured. If we configure 512px tiles, then
0/0/0will be a nominal zoom 1 tile.