Path: utzoo!attcan!uunet!cs.utexas.edu!tut.cis.ohio-state.edu!ucbvax!bloom-beacon!HLERUL5I.BITNET!laffra
From: laffra@HLERUL5I.BITNET
Newsgroups: comp.windows.x
Subject: Re:  rubberbanding with Xlib
Message-ID: <8908290738.AA08852@rulcs.uucp>
Date: 29 Aug 89 07:38:08 GMT
Sender: daemon@bloom-beacon.MIT.EDU
Organization: The Internet
Lines: 38

In response to:

     Does anyone have a good way of doing rubberbanding with Xlib, by good
     I mean where rubberband box keeps up with the cursor. The normal way of
     doing rubberbanding

     erasebox(flush buffer)-redrawbox(flush buffer)-erasebox..

     requires a roundtrip for each request (batching requests makes it even
     worse) and the cursor is always ahead of the box being drawn. Has anyone
     worked a solution where rubberbanding box always tracks thr cursor very
     closely.

There is a way:
You have to make sure that you do not redraw your box with every motion event.
What I generally do (and it works very nicely) is the following:

   XNextEvent(&event);
   switch( type_of(&event) ) {
   case MotionNotify :
             if (event_queue_is_empty_now()) {
                        erase_box(...); draw(box(...);
             else
                        ignore();
   }

The 'event_queue_is_empty_now()' function calls XPending() (XLib function).
In this way you only redraw the box when the mouse pointer is not moving
anymore. This reduce the delay of redrawing and the box will follow the
cursor better.

If your picture is very complicated you might want to test the queue for
new motion event even while you are drawing the picture. The you do not
finish the picture but erase the sofar drawn parts.

Chris Laffra
laffra@HLERUL5I.BITNET
h