Constant DPI regardless of resolution

[Nikos Chantziaras]

After RTFM and Google, I can't seem to find a way to force xorg to use a 
constant DPI value regardless of the current resolution.  It calculates 
the DPI with the known formula (screen dimensions and resolution).  But 
in my case, when changing resolution, the DFP (digital flat panel) is 
set-up to simply keep the picture centered, with black borders around it 
if the resolution is smaller than the DFP's native resolution.  The DPI 
does not change (only the resolution changes.)

Starting X with "-dpi 86" also won't help; the DPI still changes after 
switching resolution (starting in 1024x768 and then going 1280x1024 for 
example results in huge fonts).  The desktop's environment (KDE in this 
case) "Force 96DPI" is sub-optimal; my DFP is not 96DPI.

This is also an issue when running xorg in a virtual machine (the VM 
window for example is 1024x768 while the monitor is running at 
1280x1024).  Resizing the window changes the resolution but 
unfortunately xorg also changes DPI even though it shouldn't.

If there is a way, how can I force a constant DPI regardless of resolution?