XWindows - how capture window ?

[Valery Reznic]

OK, I found something interesting.
It's turn out that for some reason on-screen rendering is a problem too.

Greatly oversimplified description of my application:
There is a mai windows with 2 buttons ('A', and 'B')
When buttons 'A' pressed Windows 'W' (with a lot of child windows) created and shown on the screen. It's done with function 'callback_A'

When I press button 'B' I want following to happened:
1. Window 'W" created, like it was created, when button 'A' pressed
2. Window 'W" content is captured.

So I have callback_B like this:
void callback_B()
{
    callback_A(); // create Window 'W'
    capture_window();
}

So far so good. The only trouble is that requests sent to XServer in the function callback_A have no chance to be processed before call to capture_window() function, because application does not retirned to it's main application loop, which process events.
To get XServer chance to process events callback_B was modified as following:

void callback_B()
{
    callback_A(); // create Window 'W'
    handle_events();
    capture_window();
}

Where handle_events looks like:
void handle_events()
{
   XFlush(display);
   XSync(display, False);

     while (XtAppPending(appContext))
     {
       XtAppProcessEvent(appContext,Mask );
   }
}

This functions used in this application in other cases when events should be handled and it's works OK.
But in this case, Window 'W' was only partially drawn.

I found workaround modifie callback_B
void callback_B()
{
    callback_A(); // create Window 'W'
    handle_events();

    usleep(100000);
    handle_events();

    capture_window();
}

After some sleep and addition handle_events call Window 'W' rendered on screen as expected.

But I don't why first approach didn't work.
From the XSync man page:
-------------------------------------------------------------------------
       The XSync function flushes the output buffer and then waits until all
       requests have been received and processed by the X server.  Any errors
       generated must be handled by the error handler.  For each protocol
       error received by Xlib, XSync calls the client application’s error han-
       dling routine (see section 11.8.2).  Any events generated by the server
       are enqueued into the library’s event queue.
-------------------------------------------------------------------
So it's looks like XSync alone should do the job.
Obviously it was not - and it was a reason, that event_handle function was written.

What I did with sleep and two calls to even_handle is work, but it's ugly.
Anyone has idea why XSync alone is not enough and how I can wait to all requests to be processed by XServer ?

Regards,
Valery.


--- On Sun, 3/7/10, Nadav Har'El <nyh at math.technion.ac.il> wrote:

> From: Nadav Har'El <nyh at math.technion.ac.il>
> Subject: Re: XWindows - how capture window ?
> To: "Erez D" <erez0001 at gmail.com>
> Cc: "Valery Reznic" <valery_reznic at yahoo.com>, "linux-il." <linux-il at cs.huji.ac.il>
> Date: Sunday, March 7, 2010, 10:38 AM
> On Thu, Mar 04, 2010, Erez D wrote
> about "Re: XWindows - how capture window ?":
> > composite window managers (i.e. compiz, baryl) work by
> drawing the original
> > window off screen, then read it as a 2D picture, and
> render it back to the
> > screen with certain effects.
> > So i know it is possible to grab an off screen window.
> I do not know however
> > how to make it off screen.
> 
> This is done using the "Composite" extension. See
> http://www.freedesktop.org/wiki/Software/CompositeExt
> 
> But please note that all these "extensions", as their title
> implies, are
> not available in every installation of X. The "right way"
> to use them is
> to use them when they're available, but fall back to some
> slower or uglier
> alternative when they aren't.
> 
> > There is an X Extension called XDamage, which reports
> changes on the window
> > so you do not have to poll it for changes. These
> window managers use it.
> 
> Right. http://www.freedesktop.org/wiki/Software/XDamage
> 
> 
> -- 
> Nadav Har'El           
>             |   
>     Sunday, Mar  7 2010, 21 Adar 5770
> nyh at math.technion.ac.il 
>        
>    |-----------------------------------------
> Phone +972-523-790466, ICQ 13349191 |Business jargon is the
> art of saying
> http://nadav.harel.org.il       
>    |nothing while appearing to say a lot.
>