r.out.ppm converts a GRASS raster map into a PPM image
at the pixel resolution of the CURRENTLY DEFINED REGION.
To get the resolution and region settings of the raster map, run:
g.region -p rast=[mapname]
before running r.out.ppm.
By default the PPM file created is 24-bit color, rawbits storage.
You can use the -G flag to force r.out.ppm to
output an 8-bit greyscale instead.
The greyscale conversion uses the NTSC conversion:
Y = .30*Red + .59*Green + .11*Blue
One pixel is written for each cell value, so if ew_res and
ns_res differ, the aspect ratio of the resulting image will be off.
A few ppm file comments are written: the name of the GRASS
raster map, resolution, etc. Although these are perfectly legal,
I've found one PD image utility that chokes on them, so if you need
a commentless PPM file, use 'out=- > outfile.ppm'. (When sending
output to stdout, no comments are written.)
You can create a PNG image with NULL values represented by a transparent
background by using the PNG driver with
GRASS_TRANSPARENT set to TRUE.
Alternatively, you can use the pnmtopng program from
netpbm to do this:
pnmtopng -transparent white raster.ppm > raster.png
Bill Brown, UIUC
Last changed: $Date$