WMS PNG transparency issue

[Jesse Cleary]

Hi folks

I've got a sticky WMS transparency problem I can't resolve.  I'm serving 
a WMS layer to another Mapserver instance for further display and having 
very odd PNG transparency issues.  The issue is that my layer appears in 
their map with a colored but slightly transparent background (usually 
green, purple, or red) .  The features in my layer are not transparent, 
just the background.  The background color can vary upon map refresh.  
Has anyone run into this colored  pseudo-transparency before?

This remains the case when:
1. I serve an RGB PNG and they use an OFFSITE value.
2. I serve an RGB JPEG and they use an OFFSITE value.
3. I serve an RGBA PNG with TRANSPARENT TRUE on the my end and no 
OFFSITE on the client end.

The only way to avoid this behavior is to set the client mapfile to 
serve PC256 PNGs which causes rendering issue with the existing legends 
in their application.  However when my layer is displayed, the rendering 
issues are minimized, when my layer is off,  legend colors render 
poorly.  I've tried many different permutations of other image formats 
on both ends, but these are really shots in the dark  My version is MS 
4.8.1 the client instance is MS 4.0.1 - they are also using 
PHP/MapScript 4.0.1 to manipulate the mapfile for their UI, but I don't 
see any image format changes in that code...

Thanks for whatever pointers folks might suggest!

Jesse


My relevant mapfile bits:
MAP
  NAME ""
  STATUS ON
  EXTENT -79.069 32.643 -73.341 37.103
  SIZE 666 500
  SHAPEPATH "../data"
  SYMBOLSET "../etc/symbols.sym"
  FONTSET "../etc/fonts.txt"
  IMAGETYPE PNG
  DEBUG ON
  IMAGECOLOR 255 255 255
  OUTPUTFORMAT
    NAME png
    MIMETYPE "image/png"
    DRIVER "GD/PNG"
    EXTENSION "png"
    IMAGEMODE RGB
    FORMATOPTION "INTERLACE=OFF"
    TRANSPARENT OFF
  END
.....


Client mapfile bits:
MAP
  NAME ""
  STATUS ON
  EXTENT -91.5 22 -71.5 36.5
  SIZE 456 384
  SHAPEPATH "../data"
  SYMBOLSET "../symbols.sym"
  FONTSET "../fonts.txt"
  IMAGECOLOR 230 247 254
  UNITS DD
  IMAGETYPE PNG
  OUTPUTFORMAT
    NAME PNG
    DRIVER "GD/PNG"
    MIMETYPE "image/png"
    IMAGEMODE PC256  # this works, RGB or RGBA do not
    EXTENSION "png"
  END
....

-- 
Jesse Cleary				
Department of Marine Sciences	
UNC Chapel Hill

17-6 Venable Hall
jcleary at email.unc.edu
(919) 962-4323