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authorTimothy Pearson <kb9vqf@pearsoncomputing.net>2011-07-10 15:24:15 -0500
committerTimothy Pearson <kb9vqf@pearsoncomputing.net>2011-07-10 15:24:15 -0500
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+<td align="right" valign="center"><img src="logo32.png" align="right" width="64" height="32" border="0"></td></tr></table><h1 align=center>OpenGL Overlay X11 Example</h1>
+
+
+<p>
+<b>Warning:</b> From version 5.0 onwards, the Qt OpenGL Extension includes
+direct support for use of OpenGL overlays. For many uses of overlays,
+this makes the technique described below redundant. See the <a href="opengl-overlay-example.html">overlay</a> example program. The
+following is a discussion on how to use non-QGL widgets in overlay
+planes.
+<p> Overlayrubber: An example program showing how to use Qt and Qt OpenGL
+Extension with X11 overlay visuals.
+<p> See <tt>$QTDIR/examples/opengl/overlay_x11</tt> for the source code.
+<p> Background information for this example can be found in the
+information on <a href="opengl-x11-overlays.html">overlays</a>.
+<p> The example program has three main parts:
+<p> <ol type=1>
+<li> <em>GearWidget</em> - a normal, simple <a href="qglwidget.html">QGLWidget</a>. This renders the usual
+gears. It has been modified to print a debug message every time it
+redraws (renders) itself. Thus, you can easily confirm that drawing in
+the overlay plane does not cause redrawings in the main plane where
+the QGLWidget resides.
+<p> <li> <em>RubberbandWidget</em> - Very simple standard (non-GL) Qt widget that
+implements rubberband drawing. Designed for use in an overlay plane.
+It takes the plane's transparent color as a constructor argument and
+uses that for its background color. Thus, the widget itself will be
+invisible, only the rubberbands it draws will be visible.
+<p> <li> <em>main.cpp</em> Creates a GearWidget and a Rubberbandwidget and puts the
+latter on top of the former. Contains a routine that checks that the
+default visual is in an overlay plane, and returns the transparent
+color of that plane.
+</ol>
+<p> <h2> Running the Example
+</h2>
+<a name="1"></a><p> Start the <tt>overlayrubber</tt> executable. Click and drag with the left
+mouse button to see rubberband drawing. Observe that the <a href="qglwidget.html">QGLWidget</a>
+does not redraw itself (no redraw debug messages are output), and yet
+the image is not destroyed. Marvel at the coolness of X11 overlays!
+<p> <h2> Using this technique in a real application
+</h2>
+<a name="2"></a><p> For clarity, this example program has been kept very simple. Here are
+some hints for real application usage:
+<p> <ul>
+<p> <li> <em>All normal widgets are in the overlay plane.</em> This means that you
+can put all kinds of Qt widgets (your own or standard Qt widgets) on
+top of the OpenGL image (widget), e.g. pushbuttons etc., and they can
+be moved, resized, or removed without destroying the OpenGL image.
+<p> <li> <em>Using with geometry management.</em> The <a href="qlayout.html">QLayout</a> classes don't permit
+putting one widget (the overlay) on top of another (the OpenGL
+widget); that would defy the whole purpose of the automatic layout.
+The solution is to add just one of them to the QLayout object. Have it
+keep a pointer to the other (i.e. the <a href="qglwidget.html">QGLWidget</a> knows about its
+overlay widget or vice versa). Implement the resizeEvent() method of
+the widget you put in the layout, and make it call setGeometry() on
+the other widget with its own geometry as parameters, thus keeping the
+two widgets' geometries synchronized.
+<p> <li> <em>Using together with <a href="qpalette.html">QPalette</a> and <a href="qcolorgroup.html">QColorGroup</a>.</em> Instead of the
+simplistic setBackgroundColor( transparentColor ), you can
+use Qt's QPalette system to make your overlay widgets use
+transparent color for what you want. This way, the normal Qt widgets
+can be used as overlays for fancy effects. Just create a palette for
+them with the transparent color for the relevant color roles, e.g.
+Background and Base, in the Normal and/or Active modes. This way, you
+can create see-through QPushButtons etc.
+</ul>
+<p> <p>See also <a href="opengl-examples.html">OpenGL Examples</a>.
+
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+<td align=right><div align=right>Qt 3.3.8</div>
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