given: mapnik2 old trunk, SRTM geotiff, stylesheets with rastersymbolizer/colorizer, gdal datasource.
if projections aren't exactly the same, (even if their descriptions don't match which is the case when using nik2img standalone or tirex which resets the projection by itself), raster gets a grid of (probably) bad rounding.
Would it be possible to post a reduced test case?
Well, sorry for not chopping it down too much, here is the stylesheet on screenshot above:
to reproduce, you can just drop entries for CleanTopo and planet_osm_line. To generate srtmm.xml, just take a tiff from http://gis-lab.info/data/srtm-tif/,
gdalwarp -s_srs "+proj=longlat" -t_srs "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +no_defs +over" -overwrite -r cubicspline sourcetiff.tif resulttiff.tif
after that, do:
gdalbuildvrt srtmm.xml *.tif
to add those to gdal virtual raster.
seems to me you can also get the same result without reprojecting tiffs to mercator but taking them as-is in epsg4326.
closing, will address over at #1049