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| 32 | <td align="right" valign="center"><img src="logo32.png" align="right" width="64" height="32" border="0"></td></tr></table><h1 align=center>How to use X11 overlays with the Qt OpenGL extension</h1>
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| 33 |
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| 34 |
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| 35 |
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| 36 | <p> X11 overlays are a powerful mechanism for drawing
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| 37 | annotations etc., on top of an image without destroying it, thus saving
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| 38 | a great deal of image rendering time. For more information, consult the highly
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| 39 | recommended book <em>OpenGL Programming for the X Window System</em> (Mark
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| 40 | Kilgard, Addison Wesley Developers Press 1996).
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| 41 | <p> <b>Warning:</b> From version 5.0 onwards, the Qt OpenGL Extension includes
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| 42 | direct support for the use of OpenGL overlays. For many uses of
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| 43 | overlays, this makes the technique described below redundant. See the
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| 44 | <a href="opengl-overlay-example.html">overlay</a> example program. The
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| 45 | following is a discussion on how to use non-QGL widgets in overlay
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| 46 | planes.
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| 47 | <p> In the typical case, X11 overlays can easily be used together with the
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| 48 | current version of Qt and the Qt OpenGL Extension. The following
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| 49 | requirements apply:
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| 50 | <p> <ol type=1>
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| 51 | <li> Your X server and graphics card/hardware must support overlays.
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| 52 | For many X servers, overlay support can be turned on with
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| 53 | a configuration option; consult your X server installation
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| 54 | documentation.
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| 55 | <p> <li> Your X server must (be configured to) use an overlay visual as the
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| 56 | default visual. Most modern X servers do this, since this has the
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| 57 | added advantage that pop-up menus, overlapping windows etc., will
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| 58 | <em>not</em> destroy underlying images in the main plane, thereby
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| 59 | avoiding expensive redraws.
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| 60 | <p> <li> The best (deepest) visual for OpenGL rendering is in the main
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| 61 | plane. This is the normal case. Typically, X servers that support
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| 62 | overlays provide a 24 bit deep TrueColor visual in the main plane,
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| 63 | and an 8 bit PseudoColor (default) visual in the overlay plane.
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| 64 | </ol>
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| 65 | <p> The provided example program <a href="opengl-overlay-example.html">X11
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| 66 | overlay</a> will check for these and report if anything is wrong.
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| 67 | See <a href="#x11visuals">About X11 Visuals</a>, below for more
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| 68 | information.
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| 69 | <p> <h2> How it works
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| 70 | </h2>
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| 71 | <a name="1"></a><p> Given the above, a <a href="qglwidget.html">QGLWidget</a> will by default use the main plane
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| 72 | visual, while all other widgets will use the overlay visual. Thus, we
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| 73 | can place a normal widget on top of the QGLWidget, and do drawing on
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| 74 | it, without destroying the image in the OpenGL window. In other words,
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| 75 | we can use all the drawing capabilities of <a href="qpainter.html">QPainter</a> to draw the
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| 76 | annotations, rubberbands, etc. For the typical use of overlays,
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| 77 | this is much easier than using OpenGL for rendering the annotations.
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| 78 | <p> An overlay plane has a specific color called the transparent
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| 79 | color. Pixels drawn in this color will not be visible; instead the
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| 80 | underlying OpenGL image will show through. In the example program
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| 81 | <a href="opengl-overlay-example.html">X11 overlay</a>, the file
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| 82 | <tt>main.cpp</tt> contains a routine that returns a <a href="qcolor.html">QColor</a> containing the
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| 83 | transparent color. For the overlay widget, you will typically want to
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| 84 | set the background color to the transparent color, so that the OpenGL
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| 85 | image shows through except where explicitly overpainted.
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| 86 | <p> Note: to use this technique, you must not use the "ManyColor" or
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| 87 | "TrueColor" ColorSpec for <a href="qapplication.html">QApplication</a>, because this will force
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| 88 | the normal Qt widgets to use a TrueColor visual, which will typically
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| 89 | be in the main plane, not in the overlay plane as desired.
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| 90 | <p> <a name="x11visuals"></a>
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| 91 | <h2> About X11 visuals
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| 92 | </h2>
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| 93 | <a name="2"></a><p> The utilities directory contains two small programs that can help you
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| 94 | determine the capabilities of your X server. These programs are from
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| 95 | the OpenGL book mentioned above, see utilities/NOTICE for copyright
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| 96 | information. The full set of example programs from this book is
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| 97 | available at <a href="ftp://ftp.sgi.com/pub/opengl/opengl_for_x/">ftp://ftp.sgi.com/pub/opengl/opengl_for_x/</a>.
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| 98 | <p> <tt>glxvisuals</tt> will list all the GL-capable visuals the X server
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| 99 | provides, together with the depth and other GL-specific information
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| 100 | for each. Note especially the column "lvl"; a number in this column
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| 101 | means the visual is in an overlay plane.
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| 102 | <p> <tt>sovinfo</tt> will list all available visuals, and provides special
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| 103 | transparency information for overlay visuals.
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| 104 | <p> The <a href="opengl-overlay-example.html">X11 overlay</a> example
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| 105 | program will output what visual is used for the normal Qt widgets, and
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| 106 | what visual is used by the <a href="qglwidget.html">QGLWidget</a>.
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| 107 | <p>
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