[GRASSLIST:6018] Re: r.his and i.his.rgb confusion

[Glynn Clements]

Dylan Beaudette wrote:

> > AFAIK, r.his exists solely to provide a specific visual effect. If you
> > use an existing coloured map as the hue map, create an intensity map
> > which simulates solar illumination (e.g. by obtaining the slope from a
> > DEM using r.slope.aspect) and a saturation map by scaling a DEM, the
> > end result looks (slightly) like an illuminated relief map, tinted
> > according to the hue map and with "fog" in the valleys.
> 
> I have used r.his with a hue and intensity map, for a semi-transparent raster 
> overlay type image.... but i don't quite understand what you mean by using a 
> rescaled DEM for the saturation component... could you please elaborate a bit 
> on this, as it sounds rather interesting!

The saturation component controls an interpolation between the
original colour (sat=255) and grey (sat=0).

If you rescale a DEM to the 0-255 range and use the result as the
saturation map, higher altitudes (peaks) will have their colour
determined by the hue and intensity maps, while the colour will "fade"
to grey at lower altitudes (valleys).

The effect is similar to looking down through fog; the peaks can be
seen clearly while the valleys blend into the fog.

Essentially, the idea is to represent altitude in intuitive manner.

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