bug#10397: [PATCH] Under Remote Desktop, NUMCOLORS is unreliable; workaround

[Eli Zaretskii <eliz@gnu.org>]

> Date: Thu, 29 Dec 2011 19:18:13 -0800
> From: Daniel Colascione <dancol@dancol.org>
> Cc: 10397@debbugs.gnu.org
> 
> > Hmm. Shouldn't in fact GetDeviceCaps (hdc, NUMCOLORS) always be <= 256?
> > 
> > According to http://msdn.microsoft.com/en-us/library/dd144877(v=vs.85).aspx
> > 
> >   NUMCOLORS
> >   Number of entries in the device's color table, if the device has a
> > color depth of no more than 8 bits per pixel. For devices with greater
> > color depths, 1 is returned.
> > 
> > (It says "1", but it's a typo for "-1".)
> 
> Good catch. What about this (untested) code?
> 
>   hdc = GetDC (dpyinfo->root_window);
>   if (dpyinfo->has_palette)
>     cap = GetDeviceCaps (hdc, SIZEPALETTE);
>   else if (dpyinfo->n_cbits <= 8)
>     /* According to the MSDN, GetDeviceCaps (NUMCOLORS) is valid only
>        for devices with at most eight bits per pixel.  It's supposed
>        to return -1 for other displays, but because it actually
>        returns other, incorrect values under some conditions (e.g.,
>        remote desktop), only use it when we know it's valid.  */
>     cap = GetDeviceCaps (hdc, NUMCOLORS);
>   else
>     cap = -1;

There's a comment near the end of the GetDeviceCaps documentation
saying this:

  NUMCOLORS doesn't always return one with more than 256 colors

  The documentation for NUMCOLORS is wrong, devices that support more
  than 256 colors often return -1 and some virtual machines can return
  the number of Windows reserved colors (i.e. 20), even in high color
  mode.  Combine the NUMCOLORS return value with BITSPIXEL and PLANES
  to reliably detect indexed color.

So how about using `1 << (n_planes * n_cbits)' (which we compute when
NUMCOLORS call returns -1) unconditionally?  IOW, why do we need to
call GetDeviceCaps(..., NUMCOLORS) here in the first place?