Talk about rendering tilesets in README

README.md

@@ -65,3 +65,21 @@ an entire copy of the file into the process address space,
 which isn't going to work for large files on 32-bit machines.
 Performance, especially at low zoom levels, will be much better if the file actually fits
 in memory instead of having to be swapped in.
+
+Generating a tileset
+--------------------
+
+The <code>enumerate</code> and <code>render</code> programs work together
+to generate a tileset for whatever area there is data for. If you do,
+for example,
+
+  $ enumerate -z14 dirname | xargs -L1 -P8 ./tools/make-tile
+
+<code>enumerate</code> will output a list of all the zoom/x/y
+combinations that appear in <code>dirname</code> through zoom 18,
+and <code>make-tile</code> will invoke <code>render</code> on each
+of these to generate the tiles into <code>tiles/dirname</code>.
+
+The <code>-P8</code> makes xargs invoke 8 instances of <code>make-tile</code>
+at a time. If you have a different number of CPU cores, a different number
+may work out better.

tools/make-tile

@@ -1,5 +1,6 @@
 #!/bin/sh
 
-mkdir -p tiles/$1/$2/$3
-echo "$1 $2 $3 $4"
-./render -t 0 -c FF8800 $1 $2 $3 $4 > tiles/$1/$2/$3/$4.png
+dir=$(basename $1)
+mkdir -p tiles/$dir/$2/$3
+echo "$* -> tiles/$dir/$2/$3"
+./render -t 0 -c FF8800 $1 $2 $3 $4 > tiles/$dir/$2/$3/$4.png