TIFF input and GIF quality

Pericles S. Nacionales pnaciona at gis.umn.edu
Tue Nov 21 17:17:08 EST 2000


On Tue, 21 Nov 2000, Marc Jacquin wrote:

> Hi folks,
> I am facing a quite serious problem. In our market we usually use SPOT or
> Landsat images with 3 bands (RGB). Unfortunately the only images supported
> by MapServer are 8 bits.
> Did somebody worked around that ? It requires to apply dithering (24->8
> bits) which is not so obvious but feasible.

This can be done with image manipulation software such as Photoshop or
Gimp.  In those two programs, it's as simple as opening the image and
converting into indexed color.  The software typically asks you whether it
should dither or not.  I never do as my images usually don't have that
many colors.

One thing to be aware of is that the two programs I mentioned above don't
support geotiff--you'll need to have a world file with your tiff image or
you'll lose the georeference info.

> Also we encounter quality problems with nearest neighbour resampling,
> bi-cubic would be useful.

I'm not quite sure what you mean here.  If it's something to do with
perceptible color quality, then all you can do is play with dithering.

If you're using Erdas Imagine or some other Image processing software, you
can probably play with red, green, and blue color combinations.


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