Garbled icons in png256 tiles

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I'm creating transparent tiles with a bunch of markers (PointSymbolizer) which I plan to overlay onto google maps. I'm using the generate_tiles.py script from osm, which by default creates 'png256' tiles. The icons are garbled. This doesn't happen with 'png' format.

I tried this on windows using latest mapnik 0.6.1 and imagemagick 6.5.6.8-Q8 binaries. I also tried on Debian following the DebianInstall instructions from the wiki to build from svn sources. I got the same results on both machines.

I'm attaching the files needed to reproduce this.

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[altaurog] The icons are garbled even when the background is not transparent.

However I did successfully generate png256 tiles with the osm map including these icons, and they were not garbled.

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[altaurog] I tried compressing the icons with pngnq to 8-bit palette mode before running through mapnik and got the same results.

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[springmeyer] see also #447.

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[springmeyer] images inlined:

[[BR]]
||[[Image(test-png256.png)]] || [[Image(test-png.png)]]||

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[springmeyer] see also #202

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[mar_rud] Cause of problem is reduce_4 function in png_io.hpp. When number of colors in computed palette is less then 16, more packed format is selected, that saves two pixels per byte bits are swapped up. Same goes for #447 where adding outline just increased number of colors, so reduce_8 was used instead.

Solution (against trunk):
{{{
--- include.old/mapnik/png_io.hpp 2009-12-30 21:31:50.589649710 +0100
+++ include/mapnik/png_io.hpp 2009-12-30 21:32:15.637646168 +0100
@@ -134,12 +134,12 @@
#ifdef MAPNIK_BIG_ENDIAN
mapnik::rgb c((val>>24)&0xff, (val>>16)&0xff, (val>>8) & 0xff);
byte index = tree.quantize(c);

•        if (x%2 >  0) index = index<<4;
  

•        if (x%2 == 0) index = index<<4;
       if (!(val&0x80)) index = 0;//alfa
  

  #else
  mapnik::rgb c((val)&0xff, (val>>8)&0xff, (val>>16) & 0xff);
  byte index = tree.quantize(c);

•        if (x%2 >  0) index = index<<4;
  

•        if (x%2 == 0) index = index<<4;
       if (!((val>>24)&0x80)) index = 0;//alfa
  

  #endif
  row_out[x>>1] |= index;
  }}}

Discovered it while trying to implement solution for semitransparent png256 support (#202).

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[springmeyer] great, thanks Marcin!, applied to the 0.7 branch in r1503 and trunk in r1504