Image Processing

[Cristina Isabel Seabra [Ambiente]]

Hi Grass users!

I'm a little confused with all the commands that change the colors!!

When running r.support, there is an option to update the statistics (histogram
and range). I've looked at those. The histogram gives the number of pixels for
each category (that is each intensity level?), is this right?

I tried to do a contrast stretching; i.grey.scale and grey.eq (from r.colors)
do the same. What I expect from a contrast stretch is that the intensity values
are "stretch" to use the entire available range (that is 0-255). When I display the image
it looks like it has been stretched, but the number of categories doesn't change.
This works fine if it is a single band (I'm using Landsat TM images), but if I try it in a
color composite the image becomes black and white. So maybe what happens is that when
running i.composite the result is already "stretched". Is it?

If that color composite image is already stretched, that range of categories it has is the
maximum that it handles. Right?
What type of stretching is it applying? Linear or equalized?
Is there any way to control (or change) the stretching? Let's say the range is from 1 to
215, but I want it 10-250. Is it possible?

Hope there's someone that can explain me these things!!

Thanks,

Cristina Seabra
Lisbon, Portugal


cis at fct.unl.pt