[comp.windows.x] what is backing store?

petree@ece.ece.utexas.edu (07/04/88)

Another beginner's question: what is backing store?

What are its advantages/disadvantages?

When is it appropriate to use it and where is it available?

I'm sure other beginners would appreciate a simple explanation from 
some knowledgeable X soul  :-)

	- Mitch Petree (petree@bell.ece.utexas.edu)

RWS@ZERMATT.LCS.MIT.EDU (Robert Scheifler) (07/06/88)

Backing store simply means having the server preserve the contents of a
window, even when all or part of the window is obscured by other windows
(and potentially even when the window is unmapped).  There are various
ways of implementing this, but basically it means the server will copy
(all or parts of) the window's contents off-screen before the window
becomes obscured, and will continue to perform graphics output to the
off-screen contents.  As regions of the window become unobscured, the
server copies the saved contents back onto the screen.  The advantage of
backing store is that the client doesn't have to repaint regions of the
window when they become unobscured.  This is a win if the contents of
the window are relatively complex and expensive to recompute/redraw.  As
to when it is appropriate to use it, I'm a mechanist, not a policist,
but one answer is "let the user decide", by providing settable
widget/program resources or command line options.

rob@daisy.UUCP (Rob Posadas) (07/08/88)

In article <19880706125708.8.RWS@KILLINGTON.LCS.MIT.EDU> RWS@ZERMATT.LCS.MIT.EDU (Robert Scheifler) writes:
>server copies the saved contents back onto the screen.  The advantage of
>backing store is that the client doesn't have to repaint regions of the
>window when they become unobscured.  This is a win if the contents of
>the window are relatively complex and expensive to recompute/redraw.  As


The two disadvantages of backing store that I know of are 
    1. Memory must be allocated by the server to save the 
       contents and 
    2. The performance may degrade if the server is performing 
       each output primitive twice, once to the window and once 
       to the backing store pixmap.

-- 
            Rob Posadas -- Daisy Systems Corp
            700 E. Middlefield Rd.
            Mt. View, CA 94039-7006
            ucbvax!imagen!atari!daisy!rob