[GRASS-user] Triangle plot with correspoding map

[Glynn Clements]

Hamish wrote:

> > a colleague wanted to display 3 different attributes (e.g. bare soil, 
> > vegetation, shadow), so far he did a map with a triangle plot manually (see 
> > attachment by M. Schramm). 
> > 
> > And he asked (and I wonder) if it would be possible to generate something 
> > along these lines using GRASS. Using 3 input raster and displaying a 3 
> > gradient colour map with a corresponding triangle legend. 
> 
> scale all three maps to 0-255 (if r.rescale doesn't work for maps with floating
> point data (it probably does, but may not), it can be done with r.mapcalc).
> 
> then use d.rgb to display it. (or perhaps d.his?)

d.rgb

> no ideas about how to generate the legend outside of a paint program.

#!/bin/sh
eval `g.region -g`
r.mapcalc <<-EOF
	tri.r = eval(\
		s = float(col() - 1) / $cols,\
		t = 1 - float(row() - 1) / $rows,\
		u = s - 0.5 * t,\
		v = t,\
		out = u < 0 || v < 0 || u+v > 1,\
		if(out,null(),u))
	tri.b = if(out,null(),v)
	tri.g = if(out,null(),1-u-v)
EOF
for ch in r g b ; do
    r.colors tri.$ch color=grey1.0
done

If you want an equilateral triangle, the region needs a height/width
ratio of cos(30) ~= 0.866.

-- 
Glynn Clements <glynn at gclements.plus.com>