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115 lines
5.7 KiB
115 lines
5.7 KiB
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<title>How to use X11 overlays with the TQt OpenGL extension</title>
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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>How to use X11 overlays with the TQt OpenGL extension</h1>
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<p> X11 overlays are a powerful mechanism for drawing
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annotations etc., on top of an image without destroying it, thus saving
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a great deal of image rendering time. For more information, consult the highly
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recommended book <em>OpenGL Programming for the X Window System</em> (Mark
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Kilgard, Addison Wesley Developers Press 1996).
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<p> <b>Warning:</b> From version 5.0 onwards, the TQt OpenGL Extension includes
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direct support for the use of OpenGL overlays. For many uses of
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overlays, this makes the technique described below redundant. See the
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<a href="opengl-overlay-example.html">overlay</a> example program. The
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following is a discussion on how to use non-TQGL widgets in overlay
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planes.
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<p> In the typical case, X11 overlays can easily be used together with the
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current version of TQt and the TQt OpenGL Extension. The following
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requirements apply:
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<p> <ol type=1>
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<li> Your X server and graphics card/hardware must support overlays.
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For many X servers, overlay support can be turned on with
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a configuration option; consult your X server installation
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documentation.
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<p> <li> Your X server must (be configured to) use an overlay visual as the
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default visual. Most modern X servers do this, since this has the
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added advantage that pop-up menus, overlapping windows etc., will
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<em>not</em> destroy underlying images in the main plane, thereby
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avoiding expensive redraws.
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<p> <li> The best (deepest) visual for OpenGL rendering is in the main
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plane. This is the normal case. Typically, X servers that support
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overlays provide a 24 bit deep TrueColor visual in the main plane,
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and an 8 bit PseudoColor (default) visual in the overlay plane.
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</ol>
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<p> The provided example program <a href="opengl-overlay-example.html">X11
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overlay</a> will check for these and report if anything is wrong.
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See <a href="#x11visuals">About X11 Visuals</a>, below for more
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information.
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<p> <h2> How it works
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</h2>
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<a name="1"></a><p> Given the above, a <a href="tqglwidget.html">TQGLWidget</a> will by default use the main plane
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visual, while all other widgets will use the overlay visual. Thus, we
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can place a normal widget on top of the TQGLWidget, and do drawing on
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it, without destroying the image in the OpenGL window. In other words,
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we can use all the drawing capabilities of <a href="tqpainter.html">TQPainter</a> to draw the
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annotations, rubberbands, etc. For the typical use of overlays,
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this is much easier than using OpenGL for rendering the annotations.
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<p> An overlay plane has a specific color called the transparent
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color. Pixels drawn in this color will not be visible; instead the
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underlying OpenGL image will show through. In the example program
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<a href="opengl-overlay-example.html">X11 overlay</a>, the file
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<tt>main.cpp</tt> contains a routine that returns a <a href="tqcolor.html">TQColor</a> containing the
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transparent color. For the overlay widget, you will typically want to
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set the background color to the transparent color, so that the OpenGL
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image shows through except where explicitly overpainted.
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<p> Note: to use this technique, you must not use the "ManyColor" or
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"TrueColor" ColorSpec for <a href="tqapplication.html">TQApplication</a>, because this will force
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the normal TQt widgets to use a TrueColor visual, which will typically
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be in the main plane, not in the overlay plane as desired.
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<p> <a name="x11visuals"></a>
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<h2> About X11 visuals
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</h2>
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<a name="2"></a><p> The utilities directory contains two small programs that can help you
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determine the capabilities of your X server. These programs are from
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the OpenGL book mentioned above, see utilities/NOTICE for copyright
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information. The full set of example programs from this book is
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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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<p> <tt>glxvisuals</tt> will list all the GL-capable visuals the X server
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provides, together with the depth and other GL-specific information
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for each. Note especially the column "lvl"; a number in this column
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means the visual is in an overlay plane.
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<p> <tt>sovinfo</tt> will list all available visuals, and provides special
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transparency information for overlay visuals.
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<p> The <a href="opengl-overlay-example.html">X11 overlay</a> example
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program will output what visual is used for the normal TQt widgets, and
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what visual is used by the <a href="tqglwidget.html">TQGLWidget</a>.
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<p>
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<a href="troll.html">Trolltech</a><td align=center><a href="trademarks.html">Trademarks</a>
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<td align=right><div align=right>TQt 3.3.8</div>
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