24bpp strange

Jeremy Chatfield xig "at" dircon.co.uk
Mon, 23 Mar 1998 20:39:36 +0000


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Tristan Richardson wrote:
> 
> > I cannot run vncviewer in 24 bpp mode. vncviewer works fine in
> > 8bpp,16bpp,32bpp.
> >
> > the vncviewer  window is created  and imediatly killed.
> 
> None of the servers or viewers support 24-bit packed format at the moment
> [stupid idea if you ask me :-)].  I suppose the viewer really should inform
> you rather than waiting for the server to refuse the connection.

There's nothing fundamentally wrong with 24bpp packed support.  It's
just that it shouldn't be exposed to the client.  VNC does work with
some Servers that support 24bpp packed, in any case.

24bpp packed offers higher resolutions, more non-display memory (meaning
more room for performance improving caches) and better bus bandwidth
use.  The bus bandwidth decrease alone accounts for about 25% speed
improvement.  However, if you then expose 24bpp packed to the
application, you lose about 12-17% performance on Intel systems because
three in four memory accesses are not word aligned.  Net performance
gain is only 8-12% (less than half the maximum).  Even worse, few
applications are prepared to handle it, because it Just Isn't Done,
except by one recent X Server.

A sensible X Server design would present a 24bpp packed Server as a
32bpp image interface to the application.  The application will be
totally unaware of repacking for handling the bus and graphics board. 
You get the full 25% speed increment and all existing apps work
unmodified.  We've been doing it that way for years ;-)

It's all a question of the representation in system memory and display
memory.  How it is exposed to the application, and what gets passed to
the graphics board, need not be the same memory layout, so long as the
24 bits of color data are made available on the display screen. 
Conversion between the two forms can be done with very low (negligble)
system impact.

Yes, if you are not going to support 24bpp packed exposed to the client,
I believe that it would be better to make it clear, rather than
appearing to work and giving up!

Cheers, JeremyC.
-- 
Jeremy Chatfield, Xi Graphics  mailto:jdc "at" xig.com  tel:+44(0)1234.710030 
 Commercial X Products: Servers, CDE, contracts and custom development
    http://www.xig.com ftp://ftp.xig.com/ mailto:majordomo "at" xig.com
     tel:+1.303.298.7478  fax:+1.303.298.1406  mailto:info "at" xig.com
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