HOW TO USE X11 OVERLAYS WITH THE QT OPENGL EXTENSION

X11 overlays is a powerful mechanism that allows one to draw
annotations etc. on top of an image without destroying it, thus saving
significant image rendering time. For more information, consult the
highly recommended book "OpenGL Programming for the X Window System"
(Mark Kilgard, Addison Wesley Developers Press 1996).

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UPDATE: 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 'overlay'
example program. The following is a discussion on how to use non-QGL
widgets in overlay planes.
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In the typical case, X11 overlays can easily be used together with the
current version of Qt and the Qt OpenGL Extension. The following
retquirements apply:

1) Your X server and graphics card/hardware must support overlays (of
   course). For many X servers, overlay support can be turned on with
   a configuration option; consult your X server installation
   documentation.

2) Your X server must (be configured to) use an overlay visual as the
   default visual. Most modern X servers do this, since this has the
   added advantage that pop-up menus, overlapping windows etc. will
   not destroy underlying images in the main plane, saving expensive
   redraws.

3) The best (deepest) visual for OpenGL rendering is in the main
   plane. This is the normal case. Typically, X servers that support
   overlays provide a 24 bit deep TrueColor visuals in the main plane,
   and an 8 bit PseudoColor (default) visual in the overlay plane.

The provided example program "overlayrubber" will check for all this
and tell you what is wrong, if anything. See "About X11 Visuals" below
for more information.


How it works:
-------------

Given the above, a QGLWidget will by default use the main plane
visual, while all other widgets will use the overlay visual. Thus, one
can place a normal widget on top of the QGLWidget, and do drawing in
it, without destroying the image in the OpenGL window. In other words,
one can use all the drawing capabilities of QPainter to draw the
annotations, rubberbands, whatever. For the typical use of overlays,
this is much easier than using OpenGL for rendering the annotations.

An overlay plane has a specific color called the transparent
color. Pixels drawn in this color will not be visible, instead the
underlying OpenGL image will show through. In the example program
"overlayrubber", the file main.cpp contains a routine that returns a
QColor containing the transparent color. For the overlay widget, one
will typically want to set the background color to the transparent
color, so that the OpenGL image shows through except where explicitly
overpainted.

Note: To use this technique, you must not use the "ManyColor" or
"TrueColor" ColorSpec for the QApplication, because this will force
the normal Qt widgets to use a TrueColor visual, which will typically
be in the main plane, not in the overlay plane as desired.



About X11 visuals:
------------------

The utilities directory contains two small programs that can help you
determine the capabilities of your X server. These programs are from
the OpenGL book mentioned above, see utilities/NOTICE for copyright
information. The full set of example programs from this book is
available at ftp://ftp.sgi.com/pub/opengl/opengl_for_x/

"glxvisuals" will list all the GL-capable visuals the X server provides,
together with the depth and other GL-specific information for
each. Note especially the column "lvl"; a number in this column means
the visual is in an overlay plane.

"sovinfo" will list all available visuals, and provides special
transparency information for overlay visuals.

The overlayrubber example program will output what visual is used for
the normal Qt widgets, and what visual is used by the QGLWidget.



Comments are welcome at info@trolltech.com.