Re: MEDM -cmap Switch

[Andrew Johnson <anj@aps.anl.gov>]

Hi Mark,

On 09/27/2013 10:31 AM, Poff, Mark A wrote:
> I’m having difficulty understanding the utility of the MEDM “-cmap”
> switch.  The MEDM Reference Manual tells me how to change the default
> MEDM palette in siteSpecific.h, and how to edit the “display” block of a
> display to make it use the palette defined in another .adl file - those
> things I understand.  But the manual also says to start MEDM with the
> -cmap option to “.../use a private X colormap to circumvent the problem
> of not having enough colors available in the colormap/”.  What does that
> mean?  I placed a full-spectrum colorwheel gif image on a MEDM display
> and to me, it appears the same regardless of whether the cmap switch is
> used or not.  Can someone explain, in simple terms, what cmap provides
> and when it might be useful?

I think it is only useful if you have a very old video card that has a 
limited colormap. Early colour displays only had enough RAM to store 8 
bits per displayed pixel, thus you could only have 256 colours on the 
screen at a time and the display driver programmed the colormap to tell 
hardware what RGB value to emit for each of the 256 possible pixel 
values it read from the frame-buffer.

The X server managed its own colormap settings and allocated colours to 
the running programs on request, but it also allowed programs to define 
their own private colormap which it would switch to when the focus 
belonged to that program. This resulted in other windows looking really 
weird, but the foreground program's colours always looked right.

More details at http://tronche.com/gui/x/xlib/color/

TL;DR version: The -cmap option is not useful on modern video displays.

HTH,

- Andrew
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