[GRASS-dev] any way to know color assigned to a cell?

Michael Barton michael.barton at asu.edu
Wed Apr 18 12:30:04 EDT 2007

This is fine, but I'm afraid that it won't help planned implementation. This
is partly because I didn't phrase my question correctly. What I really need
to know is what color is assigned to a cat value. The idea is to do
histogramming in wxPython. It might be nice to have the histogram show the
map colors like d.histogram can. To do that, I need to know the color
assigned to a cat. However, to get colors by querying every cell in a map
with r.what would take a very long time.


On 4/18/07 7:05 AM, "Bob Covill" <bcovill at tekmap.ns.ca> wrote:

> On Thu, 2007-04-19 at 01:51 +1200, Hamish wrote:
>> Michael:
>>>>> If have a map with values from 0-255 I create a set of color rules
>>>>> such that
>>>>> 0% = green
>>>>> 100% = red
>>>>> GRASS will create a color table that will assign a nice gradient of
>>>>> colors from 0:255:0 to 255:0:0 to the values from 0-255.
>>>>> Is there any way when querying a single cell of that map to know
>>>>> what color has been assigned to it?
>> Glynn:
>>>> Set a 1x1 region around the cell, then r.out.ppm | pnmnoraw | sed.
>>>> Or r.mapcalc 'out = (r#map * 256 + g#map) * 256 + b#map', query the
>>>> composite map and decode the category number.
>>>>> Similarly, is there any way to know what color has
>>>>> been assigned to any value in the 0-255 range?
>>>> Set a 1x1 region, r.mapcalc "tmp = $value", r.out.ppm ...
>>>> Seriously; this is how d.rast.edit.tcl does it.
>>>> Yeah; we could probably do with a "r.what.colors" module.
>> Michael:
>>> I was contemplating a wxPython histograming module, like the TclTk
>>> profiling module, and liked the option in d.histogram of being able to
>>> use the color table for the histogram colors. But this seems like a
>>> pretty intensive batch of calculations to do it.
>> you could try "r.profile -c" with a profile which starts and stops at
>> the cell center (length=0). Also it should be very simple to add a flag
>> just like "r.profile -c" to r.what, if that helps.
> I have a version of r.what that I modified to optionally output RGB
> color. If interested I could commit to CVS?
> --
> Bob
>> But probably the most direct solution is a SWIG-Python interface to the
>> libgis color-lookup fns themselves.
>> Hamish
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Michael Barton, Professor of Anthropology
School of Human Evolution & Social Change
Center for Social Dynamics & Complexity
Arizona State University

phone: 480-965-6213
fax: 480-965-7671
www: http://www.public.asu.edu/~cmbarton

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