Re: 24-bit color with writable color maps

[davidf(at)dfanning.com (David Fanning)]

Kenneth P. Bowman (bowman@null.edu) writes:

> There have been a number of queries about 24-bit color lately, including
> the use of writable color maps on systems that support it (24-bit
> DirectColor visual).  I attach for the interested reader a sample program
> that plots a 24-bit color graph and manipulates the color tables.
> 
> The SGI O2 device looks like this:
> 
> Current graphics device: X
>     Server: X11.0, Silicon Graphics, Release 6001
>     Display Depth, Size: 24 bits, (1280,1024)
>     Visual Class: DirectColor (5)
>     Bits Per RGB: 8
>     Physical Color Map Entries (Used / Total): 256 / 256
>     Colormap: Private, 16777216 colors.  Translation table: Enabled
>     Graphics pixels: Decomposed,        Dither Method: Ordered
>     Write Mask: 16777215 (decimal) ffffff (hex)
>     Graphics Function: 3 (copy)
>     Current Font: <default>,    Current TrueType Font: <default>
>     Default Backing Store: Req from Server.

This is an interesting article, because I just taught an IDL
class this week on 8 SGI 24-bit color machines. The default
visual class for these machines was DirectColor, 24-bits just
like Ken's machine. But, on about half of these machines
the private color table caused severe color flashing and
the colors were only correct when the cursor was in the
IDL graphics window. On the other half, the colors worked
exactly as Ken describes. 

I tried everything I could think of to get the lousy 
machines working like the good machines, but I'm pretty 
convinced there is nothing you can do within IDL to make
it happen. It has to be a machine configuration thing.
I'd be extremely grateful to anyone who could shed more
light on this for me. :-(

We ended up putting the machines in a 24-bit TrueColor
visual class and I am happy to report everything behaved
*exactly* as I expected, with one very pleasant surprise:
true-color images, even with color decomposition OFF,
were *never* routed though the color tables like they
are on PCs. 

So turning color decomposition ON before you display a
24-bit image is only necessary on PCs (or Macs?), 
running 24-bit color, without the gray-scale
color table loaded, on Fridays or Mondays, when ...

Sigh...

Cheers,

David

-- 
David Fanning, Ph.D.
Fanning Software Consulting
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