Hi Steve,
there is some info by Frank in the following bug report (assuming it is
not outdated):
http://mapserver.gis.umn.edu/bugs/show_bug.cgi?id=684
Are you able to preprocess your tiles to 8-bit using a script that GDAL
provides?
My experience with PNG24 is that they become huge (aerial photos of
about 500 by 500 pixels approach 1 Mb) and that ofcourse IE 6 (and
lower) has problems with alpha transparency.
Best regards,
Bart
Stephen Woodbridge wrote:
Hi all,
Sorry for the cross-post, but this seems valid for both groups.
I have mapserver configured to work with ka-map and things are working
fine except we have noticed color shifts in tiles. We are using:
OUTPUTFORMAT
NAME PNG8
DRIVER "GD/PNG"
EXTENSION "png"
MIMETYPE "image/png"
IMAGEMODE RGBA
TRANSPARENT OFF
FORMATOPTION "QUANTIZE_FORCE=ON"
FORMATOPTION "QUANTIZE_DITHER=OFF"
FORMATOPTION "QUANTIZE_COLORS=256"
END
in our mapfile and my hypotheses is that the GD QUANTIZE process is
selecting different colors based on the colors available in the various
meta-tiles. The mapserver source code calls the following function for
this process and from the GD manual:
void gdImageTrueColorToPalette(gdImagePtr im, int ditherFlag, int
colorsWanted) gdImagePtr gdImageCreatePaletteFromTrueColor(gdImagePtr
im, int ditherFlag, int colorsWanted) (FUNCTION)
gdImageCreatePaletteFromTrueColor returns a new image.
gdImageTrueColorToPalette permanently converts the existing image.
The two functions are otherwise identical.
The function converts a truecolor image to a palette-based image,
using a high-quality two-pass quantization routine. If ditherFlag is
set, the image will be dithered to approximate colors better, at the
expense of some obvious "speckling." colorsWanted can be anything up
to 256. If the original source image includes photographic
information or anything that came out of a JPEG, 256 is strongly
recommended. 100% transparency of a single transparent color in the
original truecolor image will be preserved. There is no other support
for preservation of alpha channel or transparency in the destination
image.
For best results, don't use this function -- write real truecolor
PNGs and JPEGs. The disk space gain of conversion to palette is not
great (for small images it can be negative) and the quality loss is
ugly. However, the version of this function included in version
2.0.12 and later does do a better job than the version included prior
to 2.0.12.
So, a few questions:
1) Has anyone looked into the impact of using PNG24 images on
a) image sizes
b) ka-map tile-cache sizing
c) compatibility with browsers
d) impact on browser memory usage or performance
2) I thought Paul Spencer said to use DITHERED=OFF, is this correct
3) Has anyone looked at the GD routine? Would it be possible to add a
palette of say 32 colors that would be "sticky" and priority over
other colors?
-SteveW
--
Bart van den Eijnden
OSGIS, Open Source GIS
http://www.osgis.nl
