[GRASS5] d.his r.his question

Glynn Clements glynn.clements at virgin.net
Wed Dec 12 20:52:38 EST 2001

Bob Covill wrote:

> I have been trying the new r.his to generate RGB files, followed by
> r.composite to assemble the files into a single combined image. I have
> compared the results to the results from my old d.his method.
> What I found is that the new r,his is written only for CELL type data.
> If the data is floating point only the integer values are used and
> consequently the integer color is used. The end rsult of this is a final
> image with distinct color breaks as opposed to a smooth color range from
> the original hue map.  In my case the original hue map is a floating
> point DEM with a custom color table. The intensity map is an integer
> shaded relief map generated from the DEM. 
> I guess the question is, should r.his be only used for imagery (CELL)
> data or should it be expanded to include FCELL type data? Would it be
> better to come up with a new program to combine a hue map with a
> intensity map? Starting to sound like the same questions asked over.

Note: r.his was closely based upon d.his, which has only ever
supported CELL data (at least, as long as it's been in CVS).

If your original composite layers look smoother, I would guess that
it's because d.his used to perform dithering. I originally planned to
add this to r.composite as an option, but had forgotten about it until

I'll add some dithering options to r.composite (at least the basic
dithering previously implemented in d.his, maybe Floyd-Steinberg as

Having said that, I'm not personally aware of any situation where
composite layers are a good idea.

AFAICT, it's always preferable to leave the data as separate R/G/B
bands. These can be exported as a 24-bit PPM image with r.out.ppm3
(PPM can be converted to ~20 different formats with the pbmplus/netpbm
tools), displayed with d.rgb, and printed using ps.map's "rgb"

However, I would appreciate any examples where there is a good reason
for merging separate R/G/B layers into a composite layer.

Glynn Clements <glynn.clements at virgin.net>

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