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specification revision to v2 [#27]

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Brandon Liu
2022-02-17 17:37:46 +08:00
parent 3a9679b952
commit 7092bd93a4
1 changed files with 5 additions and 3 deletions
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README.md
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@@ -55,14 +55,14 @@ A PMTiles archive is composed of:
### Header
* The header begins with a 2-byte magic number, "PM"
* Followed by 2 bytes, the PMTiles specification version (currently 1)
* Followed by 2 bytes, the PMTiles specification version (currently 2).
* Followed by 4 bytes, the length of metadata (M bytes)
* 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
* Followed by M bytes of metadata, which **must be a JSON string with bounds, minzoom and maxzoom properties (new in v2)**
* Followed by N * 17 bytes, the root directory.
### 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 contiguous sequence of 17 byte entries. A directory can have at most 21,845 entries. **A directory must be sorted by Z, X and then Y order (new in v2).**
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 offset/length points to a leaf directory and not a tile.
@@ -71,6 +71,8 @@ An entry consists of:
* 6 bytes: the offset of where the tile begins in the archive.
* 4 bytes: the length of the tile, in bytes.
**All leaf directory entries follow non-leaf entries. All leaf directories in a single directory must have the same Z value. (new in v2).**
### 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.