[OSM-dev] speeding up tile rendering using a custom "convert -color 255" program.

[Jon Burgess]

While experimenting with the new tile renderer I noticed that the
"convert -color 255" step of the tile generation was taking more CPU
than anything else. To attempt to optimise this I hacked together a
quick C program which uses the same ImageMagick library but with
hardcoded options. For me this is several times faster than the stock
convert command. 

I don't know the complete story behind why this custom version is so
much faster. Perhaps it is because I've not provided any colour space
correction or dithering parameters. Maybe i've lost some quality in the
output, but the tiles I've generated with it so far look OK to me.

Let me know if you want this added to SVN (i'm thinking maybe a subdir
of utils/mapnik). Or perhaps you can come up with some options on
convert which will make it just as quick. 

	Jon

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