[Tilecache] png256 transparency and nodata with raster, mapnik

[Matt Bartolome]

Hi, this is a tilecache + mapnik question.

I'm wondering if it is possible to set a nodata value to transparent
when rendering raster data with png256 in tilecache. I wanted to use
the png256 extension in my tilecache.cfg but when the bounding box
exceeds the raster I get a black boundary for the missing data. With
the default extension (regular png) in tilecache it is transparent so
the black doesn't show up however each tile can be upwards of 200K,
which I am currently using pngnq to compress. It would be nice if it
could be accomplished in one swoop.

-Matt