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This is a point that can be argued about but one opinion is there is
no way for an application to know the appropriate visual -- it has to
be specified by the user.  If you disagree with this then your
application probably falls into the category of always using the
default visual or it is hardware specific and expects some particular
visual such as 24bit TrueColor with an OverlayPlane extension (or some
such).

Why?  No application runs in isolation.  Depending on the way a server
allocates resources I may not always want your application to run in
TrueColor mode if it is going to mess up my other applications.  I may
be very upset if it chooses to run in GreyScale instead of PsuedoColor
or just monochrome.

As an example, on a low end color Sun server there are many different
possible visuals: monochrome, 256 entry colormap, static gray, static
color, and a 3/3/2 TrueColor.  The SGI Iris's offer all the above 
plus 12 bit TrueColor, 24 bit TrueColor, an Overlay Plane.

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