Your application's 3D viewing area can be encapsulated by an X Toolkit widget. There are two approaches to rendering OpenGL into a widget. You can render OpenGL into a standard Motif drawing area widget, or you can use a special OpenGL drawing area widget.
The Motif drawing area widget would seem a natural widget for OpenGL rendering. Unfortunately, the X Toolkit's design (upon which Motif relies) allows programmers to specify a widget's visual only if its class is derived from the shell widget class. Shell widgets are often called ``top level'' widgets because they are designed to communicate with the window manager and act as containers for other widgets. Non-shell widgets inherit the depth and visual of their parent widget. The Motif drawing area widget class (like most widget classes) is not derived from the shell widget class. It is impossible (without resorting to programming widget internals) to set the visual of a standard non-shell Motif widget differently than its ancestor shell widget.
But in OpenGL, the X notion of a visual has expanded importance for determining the OpenGL frame buffer capabilities of an X window. In many cases, an application's 3D viewing area is likely to demand a deeper, more capable visual than the default visual which Motif normally uses.
There are two options:
The paperplane example in the appendix is written to support either scheme depending on how the code is compiled. By default, the code compiles to use the OpenGL-specific widget. If the noGLwidget C preprocessor symbol is defined, the standard Motif drawing area widget will be used, forcing the use of a single visual throughout the example's widget hierarchy. The code differences between the two schemes in the paperplane example constitute seven changed lines of code.
The preferable approach is to use the OpenGL-specific widget, since
you can run most of the application's user interface in the default visual
and use a deeper, more capable visual only for 3D viewing areas.
Limiting the use of deeper visuals can save memory and increase
rendering speed for the user interface windows.
If you use a 24-bit visual for your 3D viewing
area and use the same visual for your entire application,
that means that the image memory for pixmaps used by non-OpenGL windows
is four times what it would be for an 8-bit visual.
Some X rendering operations
might also be slower for 24-bit windows compared with 8-bit windows.
There can be advantages to running your entire application in a single visual. Some workstations with limited colormap resources might not be capable of showing multiple visuals without colormap flashing. Such machines which support OpenGL should be rare. Even if running in a single visual is appropriate, nothing precludes doing so using an OpenGL-specific widget.