Hi,
I have a question about the image color within GRASS.
I tried to compose a color spot image with 20 level colors,
it finished, but I couldn't show it.
I tried to compose it with 256 level colors,
it couldn't finish in 3 hours.
Is there a limit for the color level to compose color
image within GRASS? at most 10 level?
My computer is Silicon Graphics workstation. It supports
16777216 colors( 24 bit).
Thanks a lot.
Tian Qing
On Sun, 6 Apr 1997, Tian Qing wrote:
Hi,
I have a question about the image color within GRASS.
I tried to compose a color spot image with 20 level colors,
it finished, but I couldn't show it.
^^^^^^^^^^^^^^^^^^
I'm not quite sure exactly what you mean by that. 20 color levels means
20 shades of red x 20 shades of green x 20 shades of blue, or a total of
8000 colors. As the display program has to look up a color entry for each
pixel, this tends to make for a slow display. Just as a note, color
ranges in GRASS will display much quicker than long color tables (though
I realize that this isn't much help with imagery).
I tried to compose it with 256 level colors,
it couldn't finish in 3 hours.
Let's see, that's 256^3, or 16,777,216 entries in your colortable. There
could be problems.
Is there a limit for the color level to compose color
image within GRASS? at most 10 level?
Not that I know of - I've certainly worked with 16 color levels in
r.combine before.
My computer is Silicon Graphics workstation. It supports
16777216 colors( 24 bit).
Fine, but you can't deal with a 16 million line lookup table for dynamic
display. The GRASS Xdriver will only display 256 colors (and I'm not sure
that it's any different for the IRIS driver), so it's a moot point
for most of us. Although it is theoretically possible to print 16 million
colors from GRASS using ps.map, generating the maps would take forever. I
generally use 12 to 16 colorlevels when I want a good looking map on a
24-bit dye sublimation printer.
Remember, in most commercial remote sensing packages you are displaying
three different bands at the same time to derive your 24-bit image; you
are not writing a 24-bit lookup table. By the way, have you played with
the SG3d module? It _does_ make full use of your display capabilities
(and it's really cool, as well!).
Have fun,
-Malcolm Williamson
Thanks a lot.
Tian Qing