Re: X11 backing store

[Harold L Hunt II <huntharo at msu dot edu>]

Hans Werner Strube wrote:
> If X11 graphics programs request backing store, for instance, with
>     drawattr.backing_store = Always;
>     XChangeWindowAttributes(display, drawable, CWBackingStore, &drawattr);
> this seems to have no effect. The same program works correctly in Solaris
> (SPARC) and IRIX (MIPS).
> The memory of my graphics card (8 MB, Rage Mobility) appears sufficient,
> since the whole screen would only use 2.3 MB with 24 bits/pixel. It would
> be desirable to have this possibility to avoid explicit redrawing after
> a portion of the window has been covered or the window has been iconized
> and reopened.
Hans,

The amount of memory on your video card is not relevant.  All 
Cygwin/XFree86 graphics operations are computed in system memory and 
only the resulting pixels are transferred to the screen.

Cygwin/XFree86 has support for backing store, but it is not enabled by 
default.  You have to pass the ``+bs'' parameter to XWin.exe to enable 
backing store support:

XWin +bs


We have not confirmed whether the backing store actually performs to 
specification.  I just ran XWin with the backing store enabled and xterm 
running under the mwm window manager failed to redraw its client area 
correctly when I moved one xterm over another xterm.  I cannot tell you 
if this is a bug with general XFree86 code, with Cygwin-specific XFree86 
code, with mwm, or with xterm.

I have seen various posting that seem to indicate that backing store 
support is not trusted anymore... perhaps it is no longer being 
maintained.  It is also possible that I have simply misread or 
misremembered things.  Maybe backing store support is still fully 
maintained and perhaps we are simply not using it correctly.

Harold