Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!tut.cis.ohio-state.edu!att!cbnews!jec
From: jec@cbnews.att.com (John Collier)
Newsgroups: comp.windows.x
Subject: How do I use a read/write color cell?
Keywords: read,write,alloc,SetColor
Message-ID: <1990Sep26.190951.21408@cbnews.att.com>
Date: 26 Sep 90 19:09:51 GMT
Distribution: na
Organization: AT&T Bell Laboratories
Lines: 69

I've been fiddling around with a "paint mixer" program, mostly just to learn
how things work (those of you who've already done this may smile and nod your
heads knowingly...:-).  This is the "traditional" paint mixer where I show
three sliders, one each for red, green, and blue intensities, with the idea
that when I move one of the sliders, the background color inside a window 
will change dynamically.  This would be really nice; I just can't seem to make
it work.

I made a call to XAllocColorCells to allocate one read/write color cell.  The
call looks like:

   unsigned long *plane_masks = 0 ;

   int rtn = XAllocColorCells (dsp, cmap, True, plane_masks, 0, &pixel, 1) ;

This seems to yield a valid value for "pixel" usually something "25".  I 
also check the return code and it never comes back "0", so it seems like this
part works okay.  Later I call XStoreColor to initialize the color cell to
"black":

   XColor xc ;

   xc.pixel = pixel ;
   xc.red = 0 ;
   xc.green = 0 ;
   xc.blue = 0 ;
   xc.flags = DoRed | DoGreen | DoBlue ;

   XStoreColor (dsp, cmap, &xc) ;

As far as I can tell, this works.  My Xlib Reference Manual doesn't mention a
return code, so I don't know if I can check one to see it the call worked.
Also, my test area shows black, which may or may not mean anything under the
circumstances.

Once I've allocated this, I assign the background color of an OpenLook Static
Text widget to use the "pixel" I got from the previous call.  This looks like:

   XFreeColors (dsp, cmap, old_pixel_value, 1, 0) ;   /* Free previous color */

   XColor xc ;

   xc.pixel = new_pixel ;    /* Pixel value obtained from XAllocColorCells */

   int rtn = XAllocColor (dsp, cmap, &xc) ;

   /* Build an Xt Args list with XtNbackground set to "new_pixel" */
   /* Use XtSetValues to set the widget background color to "new_pixel" */

I check the return code from "XAllocColor", but it is never zero, so I assume
this call worked as well.  Later, in the callback function for the slider, I
call XStoreColor again to set a new RGB value for the "pixel".  It is 
identical to the one above except only one field is set and "flags" is set
correspondingly.  I also left-shift the color by 8 (<< 8) before I call
XStoreColor.

Now, with all of that, when I run the program and move the sliders, the window
background never changes color.  If someone could tell me what I'm doing wrong
or, conversely, a better way to do it right, I would really appreciate it.
The explanation can deal with what I want to do on the Xlib level; I can
translate it into my "stuff" from there.  

Any ideas, help, or suggestions would be appreciated.

-- 
-------------------------------------------------------------------------------
 Working around here is like going into a swamp | John Collier, AT&T Bell Labs
 full of alligators...you'll either be eaten    | ARPA:  jec@cbnea.ATT.COM
 alive or wind up with some really nice luggage!| uucp/e-mail:  att!cbnea!jec