trouble displaying ortho

[Frank Warmerdam]

Dylan Keon wrote:
> I'm having trouble displaying a TIFF image (merged orthophoto).  It
> looks pretty noisy in my mapserver app.
>
> Bad image: http://gis.nacse.org/mstemp/109224912061641.png
> What it should look like: http://gis.nacse.org/mstemp/ortho_small.png
>
> I tried displaying it as both 8-bit and 24-bit with no difference, but
> it should display ok as 8-bit anyway.  The funny thing is it looks
> correct in an existing ArcIMS app, and in the eog and kview image
> viewers.  But then it looks noisy in the mapserver app, in gimp, and
> also when I view it in Irfanview on Windows.  Not sure what's up.

Dylan,

MapServer uses nearest neighbour sampling when downsampling image data.
This is much faster than averaging or other techniques but can produce
"noisy" effects under some circumstances.  It isn't wrong, but it can be
less appealing.

One thing you can do is to pre-build overviews on the file in question
with averaging.  This can be done with the "gdaladdo" utility from the
GDAL distribution.

eg.

   gdaladdo -r average ortho.tif 2 4 8 16 32 64

There are various "issues" that can arise but hopefully this will work out
of the box for you.

Best regards,
--
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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and watch the world go round - Rush    | Geospatial Programmer for Rent