Path: utzoo!attcan!uunet!lll-winken!lll-tis!ames!mailrus!tut.cis.ohio-state.edu!bloom-beacon!EXPO.LCS.MIT.EDU!jim
From: jim@EXPO.LCS.MIT.EDU (Jim Fulton)
Newsgroups: comp.windows.x
Subject: Re: color map question
Message-ID: <8806101338.AA25708@EXPO.LCS.MIT.EDU>
Date: 10 Jun 88 13:38:01 GMT
References: <8806091630.AA02763@velveeta>
Sender: daemon@bloom-beacon.MIT.EDU
Organization: X Consortium, MIT Laboratory for Computer Science
Lines: 46


There are a couple of problems with your example.  One is relatively minor and
the other is the reason why it failed. 

>         nplanes = XDefaultDepth (dpy, scr);
>         ncells = XCellsOfScreen (scrn);
>
>         XAllocColorCells(dpy, XDefaultColormap(dpy, scr), 0, plane_masks,
>                         nplanes, cmap_cells, ncells);

First off, the definitions of nplanes and ncells should probably be:

          nplanes = DisplayPlanes (dpy, scr);
	  ncells = DisplayCells (dpy, scr);

And, you'll have to make sure that plane_masks and cmap_cells are arrays
containing nplanes and ncells entries each.

But, in any case, it won't work.

You are trying to allocate ncells*2^nplanes pixels (which on a display with
256 color cells and 8 planes will be 65536) out of a colormap that has at most
254 entries remaining (since BlackPixel and WhitePixel have been allocated
by the server)!  The reason why you are getting zeros is because the request
is failing and you're not checking the return status.

What you'll probably want to do is to try to allocate 256 pixels (through some
combination of cells and planes), and, if that fails, create a new colormap
using XCreateColormap with something along the lines of:

	visual = /* select the proper visual if DefaultVisual isn't right */
	cmap = XCreateColormap (dpy, RootWindow (dpy, scr), visual, All);

The visual determines what sort of colormap should be allocated.  This is 
necessary for running on displays that provide more than one type of window
(for example: 24 bit DirectColor and 16 bit PseudoColor, or 8 bit PseudoColor
and 1 bit StaticGray).

Once you have a colormap with all of the entries allocated, you can set them
to be whatever you want.  Then, whenever you are ready, you can install the
colormap.  Note that this will cause the rest of the screen to go "technicolor"
if there is only one colormap.  The Inter-Client Communications Conventions
Manual describes installing colormaps in more detail (since it is something
that affects the entire screen).

Jim Fulton
MIT X Consortium