(spec version 2) enforce sorted order for directories

protomaps / PMTiles

Cloud-optimized + compressed single-file tile archives for vector and raster maps

https://protomaps.com/docs/pmtiles/

BSD 3-Clause "New" or "Revised" License

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(spec version 2) enforce sorted order for directories #27

Closed bdon closed 2 years ago

bdon commented 2 years ago

This should allow O(log n) lookup of tile membership in a directory without creating a copy.

bdon commented 2 years ago

This is now the default behavior of the python writer and https://github.com/protomaps/go-pmtiles , though is not officially part of the spec yet. Spec version 2 clients should be able to depend on this sorted assumption.

bdon commented 2 years ago

Other things to consider for v2:

Mandatory GZIP compression for tiles?
Fetching mode that requires less than 512kB header? (make multiple requests)
Anything special needed around ETags?

bdon commented 2 years ago

possible v2:

Enforce presence of certain fields like bounds, center, minzoom, maxzoom

bdon commented 2 years ago

possible v3:

either gzip compression for directories, or a very simple, data-specific Huffman-style encoding for directories.

bdon commented 2 years ago

possible v3:

use extra bits in entry Z to indicate a tile is canonical for all children; do bottom-up computation of identical parent-child relationships at directory write time, compacting leaves upward

bdon commented 2 years ago

Spec v2 is now described in the readme, changes in bold: https://github.com/protomaps/PMTiles#specification

Mandatory sorted directory order
Mandatory json in metadata with minzoom, maxzoom, bounds keys (center was omitted as the zoom level is somewhat arbitrary, you can calculate this yourself)

Closing this for now until we revisit for v3.