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Brandon Liu
2021-04-15 10:48:51 +08:00
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@@ -31,34 +31,43 @@ Example of a raster PMTiles archive decoded and displayed in Leaflet:
## Specification ## Specification
![layout](layout.png) PMTiles is a binary serialization format designed for two main access patterns: over the network, via HTTP 1.1 Byte Serving (`Range:` requests), or via memory-mapped files on disk. All integer values are little-endian
A detailed specification is forthcoming. PMTiles is a binary serialization format designed for two main access patterns: over the network, via HTTP 1.1 Byte Serving (`Range:` requests), or via memory-mapped files on disk. A PMTiles archive is composed of:
* a fixed-size 512,000 byte header section
* Followed by any number of tiles in arbitrary format
* Optionally followed by any number of *leaf directories*
### Design considerations ### Header
* Directories are recursive, with a maximum of 21,845 entries per directory. * The header begins with a 2-byte magic number, "PM"
* *21845 is the total tiles of a pyramid with 8 levels, or 1+4+16+64+256+1024+4096+16384* * Followed by 2 bytes, the PMTiles specification version (currently 1)
* Deduplication of tile data is handled by multiple entries pointing to the same offset in the archive. * Followed by 4 bytes, the length of metadata (M bytes)
* The order of tile data in the archive is unspecified; an optimized implementation should arrange tiles on a 2D space-filling curve. * Followed by 2 bytes, the number of entries in the *root directory* (N entries)
* Followed by M bytes of metadata, by convention a JSON object
### Details * Followed by N * 17 bytes, the root directory.
* The first 512,000 bytes of a PMTiles archive are reserved, and contain the headers as well as a root directory.
* All integer values are little-endian.
* The headers begin with a 2-byte magic number, "PM"
* 2 bytes: the PMTiles specification version, right now always 1.
* 4 bytes: the length of metadata (M bytes)
* 2 bytes: the number of entries in the root directory (N)
* M bytes: the metadata, by convention a JSON object.
* N * 17 bytes: the root directory.
### Directory structure ### Directory structure
A directory is a contiguous sequence of 17 byte entries. A directory can have at most 21,845 entries.
A directory is a sequence of 17 byte entries. An entry consists of: An entry consists of:
* 1 byte: the zoom level (Z) of the entry, with the top bit set to 1 instead of 0 to indicate the data is a child directory, not tile content. * 1 byte: the zoom level (Z) of the entry, with the top bit set to 1 instead of 0 to indicate the offset/length points to a leaf directory and not a tile.
* 3 bytes: the X (column) of the entry. * 3 bytes: the X (column) of the entry.
* 3 bytes: the Y (row) of the entry. * 3 bytes: the Y (row) of the entry.
* 6 bytes: the offset of where the data begins in the archive. * 6 bytes: the offset of where the tile begins in the archive.
* 4 bytes: the length of the data. * 4 bytes: the length of the tile, in bytes.
![layout](layout.png)
### Notes
* A full directory of 21,845 entries holds exactly a complete pyramid with 8 levels, or 1+4+16+64+256+1024+4096+16384.
* A PMTiles archive with less than 21,845 tiles should have a root directory and no leaf directories.
* Multiple tile entries can point to the same offset; this is useful for de-duplicating certain tiles, such as an empty "ocean" tile.
* Analogously, multiple leaf directory entries can point to the same offset; this can avoid inefficiently-packed small leaf directories.
### Implementation suggestions
* PMTiles is designed to make implementing a writer simple. Reserve 512KB, then write all tiles, recording their entry information; then write all leaf directories; finally, rewind to 0 and write the header.
* The order of tile data in the archive is unspecified; an optimized implementation should arrange tiles on a 2D space-filling curve.
* PMTiles readers should cache directory entries by byte offset, not by Z/X/Y. This means that deduplicated leaf directories result in cache hits.
## License ## License