Server/Viewer pixel depth questions

[Grant McDorman]

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On December 17, 2002 04:09 am, Shing-Fat Fred Ma wrote:
> Hello,
>
> I thought I overcame the problem of 8-bit
> desktops crashing as soon as a viewer connects
> to it from a 24-bit desktop.  I might have been
> wrong.  I just tried again and the 8-bit desktop
> died.  The only difference between
>
> When I launch the viewer, I get the message
>
>     Connected to VNC server, using protocol version 3.3
>     VNC server default format:
>       8 bits per pixel.
>       True colour: max red 7 green 7 blue 3, shift red 0 green 3 blue 6

This is the server, which is 8 bit TrueColour. Some applications may not like 
this <grin>.

>     Using default colormap which is TrueColor.  Pixel format:
>       32 bits per pixel.
>       Most significant byte first in each pixel.
>       True colour: max red 255 green 255 blue 255, shift red 16 green 8
>     blue 0

This is the local (viewer) machine, which has a TrueColour display.  The pixel 
information refers to the viewer's internal colour handling.

For comparison, the output from the latest RealVNC viewer on my Sun Sparc 
connecting to a TrueColour windows machine is:
 VNC server default format:
   32 bits per pixel.
   Least significant byte first in each pixel.
   True colour: max red 255 green 255 blue 255, shift red 16 green 8 blue 0
 Using default colormap and visual, TrueColor, depth 24.
 Using viewer's native pixel format:
   32 bits per pixel.
   Most significant byte first in each pixel.
   True colour: max red 255 green 255 blue 255, shift red 0 green 8 blue 16
The output in this version is a little clearer.

> Obviously, the 8bpp is for the server while the 32bpp is
> for the desktop from which the viewer was launched.
> This is strange, since the latter has 24bpp.  The message
> remains the same even if the viewer is invoked with
> "-depth 8".  And the log file for the 8bpp server shows
> that client (i.e. the viewer) has 32bpp format, regardless
> of whether the viewer was launched with "-depth 8".

The -depth option for the viewer could be better described as a 'hint'; i.e. 
in your case, you're asking the viewer to use depth 8 *if possible*. However, 
most hardware does not support multiple simutaneous depths (the only kind I 
know of personally that does is some Sun Sparc display boards).

On top of that, the '-depth' option is pretty useless without the '-owncmap' 
option. Lacking the latter, the viewer will still use the default colourmap 
even if an 8 bit visual is available. So to get an 8-bit visual and 
colourmap, you have to use '-depth 8 -owncmap'.

In my case, the output from 'vncviewer' with these options is:
VNC server default format:
  32 bits per pixel.
  Least significant byte first in each pixel.
  True colour: max red 255 green 255 blue 255, shift red 16 green 8 blue 0
PseudoColor visual with private colormap, depth 8.
Using viewer's native pixel format:
  8 bits per pixel.
  Colour map (not true colour).

Incidentally, in testing this I found a problem with the viewer: if there are 
resources (xrdb -query) that specify colours for the controls in the viewer, 
the viewer will crash.

- -- 

Grant McDorman <grant.mcdorman@cedara.com>, Sr. Software Design Consultant
Cedara Software Corp.  <URL:http://www.cedara.com>
  (formerly I.S.G. Technologies Inc.)
Mississauga, Ontario, Canada

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