diff options
| -rw-r--r-- | maketiles/README | 28 |
1 files changed, 28 insertions, 0 deletions
diff --git a/maketiles/README b/maketiles/README new file mode 100644 index 000000000..64c6bb380 --- /dev/null +++ b/maketiles/README @@ -0,0 +1,28 @@ +File formats and structures + +Tiles are generated as PNG files. At the moment these are full-colour PNGs but +in due course we will probably want to use dithered images with a limited +palette. Generated tiles are named for their own SHA1 checksums, so that we +can eliminate duplicate tiles. The first-stage output of these scripts, +therefore, consists of a set of PNG files named by SHA1 and a text index which +maps tile locations to SHA1s. These are arranged as: + + toplevel/index text index of tiles + toplevel/tiles/0/0/0 location for images with SHA1s beginning + 000 + toplevel/tiles/0/0/0/00000000000000000000.png + first possible image file + +This format is not really acceptable for final deployment because many of the +image files won't be much larger than a filesystem block, so a large amount of +disk space would be wasted in storing them. Nor is the text index from tile +location to SHA1 acceptable. So a second program packs the image files into +catenated files with an index from SHA1 to offset and length in each file. +Separately, the index from tile location to SHA1 is turned into another index. +Both indices use Bernstein-style CDB files. + + toplevel/index.cdb binary index from tile location to SHA1 + toplevel/tiles/0/0/0/packed catenated file of images with SHA1s + beginning 000 + toplevel/tiles/0/0/0/index.cdb index of images in packed + |