[mapserver-users] Mapserver 5.2.0 and 5.4.1,QUANTIZE_FORCE or PALETTE_FORCE do not produce8bit colour PNG file - only reduce the number of colours used.

[Steve Lime]

Hi Nikolai: I'm using the exact same output format and it works fine
for me (PALETTE_FORCE, 5.4.1). With that format my output images are
1/4 the size of their RGB counterparts. Have you tried a version without
any of the experimental PNG options enabled? 

Steve

>>> "Nikolai Nikolov" <nnikolov at gypsii.com> 07/07/09 6:48 AM >>>
Hi everybody,

 

I have tried unsuccessfully to reduce the size of my PNG image tiles,
generated by the Mapserver using the QUANTIZE_FORCE or PALETTE_FORCE
options.  The resulting file is marginally smaller than the one not using
those options.  

 

Using "IrfanView" I could get some information about the colours used and
the number of bits used to code the colours.  The repost shows a decrease of
the number of colours used from over 1600 to either 241 (Mapserver defined)
or 193 (using palette file containing 71 predefined colours).

However, tool reports 24 bits used for colour mapping.

 

If I put the PNG image through tools like "pngquant", "OptiPng" or
"pngcrush", the resulting image is much smaller and "IrfanView" reports that
only 8 bits are used for colour mapping!

 

The relevant map file part is:

 

    IMAGETYPE AGG_PNG24

    OUTPUTFORMAT

        NAME AGG_PNG24

        DRIVER AGG/PNG

        MIMETYPE  "image/png"

        IMAGEMODE RGB

        EXTENSION  "png"

        FORMATOPTION "PALETTE_FORCE=TRUE"

        FORMATOPTION "PALETTE=/var/www/html/maps_gypsii/palette.txt"

        FORMATOPTION "INTERLACE=OFF"

    END

 

The Mapserver, both 5.2.0 and 5.4.1, has been configured with
--with-experimental-png or --enable-experimental-png (I have not noticed any
difference in using either option).

 

Please, advise what I am missing here.

 

Many thanks,

Nick