[mapserver-dev] AGG/PNG && QUANTIZE_FORCE && --with-experimental-png => bad IE6 rendering

[David Fuhry]

I generated the attached tile.png with AGG/PNG and QUANTIZE_FORCE=ON, 
using a --with-experimental-png build of mapserver (to get PNG-8 
transparency).  It's an 8-bit PNG image with a transparent background. 
The image looks great in every browser except IE6, which only renders 
certain pieces of lines (see other attached image).  Is this a known 
problem?

I've tried INTERLACE=OFF with no success.  Changing the driver to GD/PNG 
works, but of course I don't get the AGG smoothness.  There is no 
AGG/GIF driver yet, so that's not an option.  And finally, a 24-bit PNG 
is not an option since IE<7 doesn't support alpha transparency.

I'm now beginning to wonder if the misrendering is due to alpha 
transparency (not supported by IE6-) on the 8-bit image.  I didn't think 
that 8-bit PNGs supported alpha transparency (GIFs don't) but according 
to this article: 
http://www.sitepoint.com/blogs/2007/09/18/png8-the-clear-winner/ indexed 
PNGs can store alpha channel values for one *or more* palette entries. 
If this is the case, then a welcome FORMATOPTION might be 
QUANTIZE_ROUND_TRANSPARENCY which would round palette entries' 
transparency up to 100 or down to 0.  Hmmm... but then (since there's no 
background color) AGG smoothness would be lost, and one might as well 
use GD/PNG, so maybe not.

I'll look into the IE htc (css behavior: ...) hack as a possible 
solution.  Better suggestions appreciated.

Thanks,

Dave Fuhry

OUTPUTFORMAT
   NAME "AGG_PNG8"
   DRIVER AGG/PNG
   IMAGEMODE RGB
   FORMATOPTION "QUANTIZE_FORCE=ON"
   FORMATOPTION "QUANTIZE_COLORS=256"
   FORMATOPTION "INTERLACE=OFF"
   TRANSPARENT ON
END
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