TIFF/GeoTIFF images on MapServer 3.2

[kenboss at dilbert.dnr.state.mn.us]

> Imagine can kick out collormapped GeoTIFF, although I believe you
> need to do the color reduction (i.e. 24bit -> 8bit) as a separate
> process and then go to GeoTIFF.

> (I know there are other list users more adept at the color reduction
> step than I. Perhaps they'll post a how-to.)

I'll give it a shot, though this is veering a bit off-topic so I won't
dwell on it at length.

You can do the color reduction in Imagine using the
Interpreter->Utilities->Adv. RGB Clustering menu option.  (You can try
to RGB Clustering option above it, but in my experience the results
are generally unacceptable).  Select your input image, choose the
three bands you want to work with, and hit the "Load Image Data"
button.  Then go to the "Partition Data" tab and tweak the "(Color)
Sections" values, hitting the "Calculate Number of Classes" button
with each change.  Ideally, you'll want a number of classes
approaching, but not exceeding, 256 (and a data type of Unsigned 8
bit).  Usually some combination of 7s and 8s in the Color Sections
will get you there.  I've never tried the threshold options, you may
want to experiment with those as well.

Once you've got the parameters set where you want them, click the
Build LUT button.  Then go back to the Image I/O tab, specify your
output image, and hit the "Produce Output" button.  You'll get a new,
single-band .img file that you can then convert to GeoTIFF.

The results are generally pretty good, but the process is a bit
unwieldy and doesn't lend itself to automated processing of large
numbers of images.  We use a product called "Image Alchemy"
(http://www.handmadesw.com/) for such purposes - it reads (but doesn't
write) .img files and will output to a huge variety of image formats,
including TIFF.  I don't think it does GeoTIFF, though as Steve points
out you can just concoct an ESRI world file in that case.  Of course
there are plenty of other packages that could be used as well, but I
don't know of any others off-hand that can read .img files, if that's
an issue.

============================================================================
Ken Boss                                
Digital Image Analyst / Web Stuff       Forestry Resource Assessment
ken.boss at dnr.state.mn.us                Minnesota Dept. of Natural Resources
Voice: 218 327 4449 ext. 237            413 SE 13th Street                    
Fax:   218 327 4517                     Grand Rapids, MN  55744         USA
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