[GRASSLIST:2189] Re: How to display satellite data correctly?

[Ian Macmillan]

  I am not sure if there is an automated way to do this, perhaps a simple script
could be written with r.mapcalc.  Anybody know how to do this?  

The manual way to do this is to use d.histogram to obtain the values at each end
of the histogram.  After that, use r.rescale to change each raster.
For example,

d.erase blue
#need the color to see the curve correctly
d.histogram map=rast.b1
r.rescale input=rast output=rast.b1x from=40,140 to=0,255
#repeat this for the 3 bands, then
d.rgb red=rast.b3x green=rast.b2x blue=rast.b1x

Good luck.
-ian

Quoting Wolfgang von Hansen <wvhansen at web.de>:

> Hi,
> 
> currently I am doing some first steps with GRASS to see how it all works.
> I've got some sample data (Landsat TM) that I would like to analyze with some
> of the available functions. However it seems that GRASS is not very good at
> displaying color images -- at least not if the histograms of the individual
> channels aren't properly equalized.
> 
> There is r.colors that should do the trick (I've tried grey.eq) but the
> results weren't too good. To my opinion the histogram is transformed to be a
> little like black-and-white but the dynamics aren't well. The display of
> d.rgb is extremely bright colored but it is difficult to see the real
> structure -- there are quite a few colored noise pixels that also degrade the
> quality.
> 
> Are there better ways to display and visually analyze satellite data? What I
> am especially missing is a simple linear stretch of the histogram with a 1%
> cutoff at each end.
> 
> Wolfgang
>
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Ian MacMillan
Geological Sciences-UCSB