[mapserver-users] MS 6.0 beta6 and faulty image/png; mode=8bit output if source image has overviews

[Rahkonen Jukka]

Hi,
 
I am sometimes getting images with totally odd colours with MS 6.0 beta
6 (and the previous beta versions) with format image/png; mode=8bit.  I
have been doing some testing and I believe I can now reproduce the
error.
 
The error happens only if the raster image has overviews. If I delete or
rename the external .ovr file the 8bit png output is correct at any
scales. To be sure that there is nothing odd with the overview file I
recreated it with FWTools v. 2.4.2 by using a basic command
gdaladdo -r average -ro suomi1m.tif 2 4 8
 
I prepared also my own outputformat so I could compare the outputs from
MS 6.0 and older MS 5.6.5 version. This is my outputformat which I have
understood to mean just the same as the inbuild MS 6.0 outputformat
"image/png; mode=8bit".
OUTPUTFORMAT
      NAME 'AGG_Q'
      MIMETYPE "image/png"
      DRIVER "AGG/PNG"
      EXTENSION "png"
      IMAGEMODE RGB
      FORMATOPTION "QUANTIZE_FORCE=ON"
      FORMATOPTION "QUANTIZE_DITHER=OFF"
      FORMATOPTION "QUANTIZE_COLORS=256"
  END
 
With this outputformat the resulting map is correct with MS 5.6.5 at any
scales even if the .ovr file is in use.
 
My conclusion is that exectly MS 6.0 version is creating at some scales
through ADD driver an 8 bit png output with messed up colours, but it
happens only if the source image has overviews.
 
If someone is interested in having a look at this I can prepare and send
my mapfile and some imagery to play with.
 
-Jukka Rahkonen-
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