Specifications for kakotile - metadata tileset for backup tiles
Kakotile tileset is a metadata tileset for backup tiles generated using qdltc. This document explains the specifications for backup tiles first, and then the specifications for kakotile.
By the way, kako (過去) means the past in Japanese language.
Backup tileset contains old tiles. The backup tile of the original tile at
{t}/{z}/{x}/{y}.{ext}
shall be stored as
{t}/bak/{z}/{x}/{y}.{date}.{ext}
where {date} is the modification date (mtime) of the original tile. A backup tile would normally be generated as a 'sheltered tile' when the tile is overwritten by the newer version. qdltc behaves as such.
Kakotile tileset contains the dates of the backup tiles at each tile location. The location and the format of the kakotile tileset are as the following:
If you have
{t}/{z}/{x}/{y}.{ext}
as the original tile, and
{t}/bak/{z}/{x}/{y}.{date}.{ext}
as the backup tile, the kokotile would be located at
{t}/kakotile/{z}/{x}/{y}.csv
Each kakotile contains comma-separated list of modification dates of the original tile.
For instance, if the content of the std/kakotile/15/29101/12903.csv is
20150606,20150522,20150129
, there must be 3 old versions of the original tile at std/15/29101/12903.png. And the old tiles would be located at:
std/bak/15/29101/12903.20150606.png
std/bak/15/29101/12903.20150522.png
std/bak/15/29101/12903.20150129.png
Please note that there would be 4 (not 3) versions of the tile because there is also original tile. The time-line of the each tile would be reconstructed from the modification time (mtime) of the current (original) tile and the dates stored in the kakotile.
A reference implementation of kakotile generator is available at kakotile-generator.