Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!ames!pasteur!dent.Berkeley.EDU!davidh
From: davidh@dent.Berkeley.EDU (David S. Harrison)
Newsgroups: comp.windows.x
Subject: Using Multiple Graphics Contexts
Keywords: graphics contexts, GC
Message-ID: <8404@pasteur.Berkeley.EDU>
Date: 17 Dec 88 23:14:47 GMT
Sender: news@pasteur.Berkeley.EDU
Reply-To: davidh@ic.Berkeley.EDU
Followup-To: comp.windows.x
Organization: Electronics Research Lab, UC Berkeley
Lines: 58

[X11]

A co-worker and I recently had an argument about appropriate use of
graphics contexts in X11.  I dimly recall a similar discussion in this
news group about this subject but I cannot recall the final
recommendation.  

Should an application maintain many graphics contexts and swap between
them when appropriate or maintain a few graphics contexts and change
fields when appropriate?  My co-worker advocates the first approach.

1.  The application will run well on hardware that supports multiple
    graphics contexts.

2.  Even if the hardware does not support multiple graphics contexts,
    the server could optimize the swap between contexts by comparing
    the currently loaded context with the new context.

3.  Traffic between client and server is reduced.  The (rather large)
    amount of information in the graphics context structure is shipped
    only once for each context.  From that point on,  the contexts are
    maintained on the server and no further modifications are required.

4.  In some sense,  the complexity of the application is reduced.  No
    code has to be written to change existing graphics contexts.

However,  I vaguely remember that Robert Scheifler posted a message
encouraging applications to maintain a small number of graphics
contexts and change them when necessary.  The reasons I came up with
are:

1.  Currently,  very few hardware platforms truly support multiple
    graphics contexts.  Those that do will often have very few.  This
    means updates to hardware contexts will occur anyway.

2.  In a graphics intensive application (like the one we are porting
    to X11),  changes in graphics contexts are not frequent.  Thus,
    the increased client-server communication caused by the
    occasional request to change a graphics context is insignificant
    compared to the number of graphics requests issued by the
    application.

3.  It is possible for the server to optimize changes to the current
    graphics context by noting the fields that change in the request
    from the application.  No comparison of all graphics context
    fields is required.

4.  Using many graphics contexts consumes resources in the server.
    Many servers may have very strict limits on resources.  The best
    example is X terminals.  These devices may have very little extra
    memory for storing many graphics contexts.

Can those with greater knowledge of X11 design shed some light on this
issue?

				David Harrison
				UC Berkeley Electronics Research Lab
				(davidh@ic.Berkely.EDU, ...!ucbvax!davidh)