[mapserver-users] Re: NoDATA value, transparency for paletted IMG file

[ChrisFern]

Dear Frank, dear all,

thanks for your reply. 
I think I have narrowed down the problem and it isn't really a mapserver
issue - the 8bit paletted img gets rendered to a 4bit-per-pixel png whereas
the correctly transparent RGB img gets rendered to an 8bit-per-pixel png.
Seems that the Google Earth API can't detect/deal with the NoData value in
this 4bpp png and renders this value sort of "shaded black" (which I would
like to get ridd of). GIMP for example can deal with the transparency in
this 4bpp png.

If I switch to RGBA in the output format, then I have the transparency
however the image is much bigger:

OUTPUTFORMAT
   NAME png
   DRIVER "GD/PNG"
   MIMETYPE "image/png"
#   IMAGEMODE RGBA
   IMAGEMODE PC256
   EXTENSION "png"
   TRANSPARENT ON
   FORMATOPTION "transparent=on"
END

Is there a way I can force the mapserver to produce 8bpp pngs?


THANKS for your help - highly appreciated - and greetings,



Chris

PS here's the gdalinfo output for the 4bpp png:
Driver: PNG/Portable Network Graphics
Files: lb_usa.png
Size is 1024, 768
Coordinate System is `'
Corner Coordinates:
Upper Left  (    0.0,    0.0)
Lower Left  (    0.0,  768.0)
Upper Right ( 1024.0,    0.0)
Lower Right ( 1024.0,  768.0)
Center      (  512.0,  384.0)
Band 1 Block=1024x1 Type=Byte, ColorInterp=Palette
  NoData Value=0
  Image Structure Metadata:
    NBITS=4
  Color Table (RGB with 9 entries)
    0: 1,1,1,0
    1: 0,0,0,255
    2: 12,16,120,255
    3: 31,62,140,255
    4: 33,110,158,255
    5: 42,156,154,255
    6: 61,184,104,255
    7: 59,214,45,255
    8: 113,235,47,255

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