Silicon Graphics systems allow you to write OpenGL applications that are portable and run well across the Silicon Graphics workstation product line. This chapter introduces the basic issues you need to know about if you want to write an OpenGL application for Silicon Graphics systems. The chapter contains the following topics, which are all discussed in more detail elsewhere in this guide:
The OpenGL graphics library is not limited to a particular window system. The platform's window system determines where and how the OpenGL application is displayed and how events (user input or other interruptions) are handled. Currently, OpenGL is available for the X Window System, Microsoft Windows, Mac OS X, and other major window systems. If you want your application to run under several window systems, the application's OpenGL calls can remain unchanged, but window system calls are different for each window system.
 | Note: If you plan to run an application under different window systems, isolate the windowing code to minimize the number of files that must be special for each system. |
All Silicon Graphics systems use the X Window System. Applications on a Silicon Graphics system rely on Xlib calls to manipulate windows and obtain input. An X-based window manager (usually 4Dwm) handles iconification, window borders, and overlapping windows. The IRIX Interactive Desktop environment is based on X, as is the Silicon Graphics widget set, IRIS IM. IRIS IM is the Silicon Graphics port of OSF/Motif.
A full introduction to X is beyond the scope of this guide; for detailed information about X, see the sources listed in “Background Reading”.
The OpenGL extension to the X Window System (GLX) provides a means of creating an OpenGL context and associating it with a drawable window on a computer that uses the X Window System. GLX is provided by Silicon Graphics and other vendors as an adjunct to OpenGL.
For additional information on using GLX, see “GLX Extension to X” in Chapter 2. More detailed information is in Appendix D, “OpenGL Extensions to the X Window System” of the OpenGL Programming Guide. The glxintro man page also provides a good introduction to the topic.
When you prepare a program to run with the X Window System, you can choose the level of complexity and control that suits you best, depending on how much time you have and how much control you need.
This section describes different tools and libraries for working with OpenGL in an X Window System environment. It starts with easy-to-use toolkits and libraries with less control and then describes the Xlib library, which is more primitive but offers more control. Most application developers usually write at a higher level than Xlib, but you may find it helpful to understand the basic facts about the lower levels of the X Window System that are discussed in this guide.
Note that the different tools are not mutually exclusive: You may design most of the interface with one of the higher-level tools, then use Xlib to fine-tune a specific aspect or add something that is otherwise unavailable. Figure 1-1 illustrates the layering:
IRIS ViewKit (only supported on IRIX systems) and Open Inventor are layered on top of IRIS IM, which is on top of Xlib.
GLX links Xlib and OpenGL.
Open Inventor uses GLX and OpenGL.
Note: If you write an application on IRIX using IRIS Viewkit or Open Inventor, the graphical user interface will be visually consistent with the IRIX Interactive Desktop.
The Open Inventor library uses an object-oriented approach to make the creation of interactive 3D graphics applications as easy as possible by letting you use its high-level rendering primitives in a scene graph. It is a useful tool for bypassing the complexity of X and widget sets, as well as many of the complex details of OpenGL.
Open Inventor provides prepackaged tools for viewing, manipulating, and animating 3D objects. It also provides widgets for easy interaction with X and Xt, and a full event-handling system.
In most cases, you use Open Inventor, not the lower-level OpenGL library, for rendering from Open Inventor. However, the Open Inventor library provides several widgets for use with X and OpenGL (in subclasses of the SoXtGLWidget class) that you can use if OpenGL rendering is desired. For instance, the SoXtRenderArea widget and its viewer subclasses can all perform OpenGL rendering. SoXtGLWidget is, in turn, a subclass of SoXtComponent, the general Open Inventor class for widgets that perform 3D editing.
Components provide functions to show and hide the associated widgets, set various parameters (such as title and size of the windows), and use callbacks to send data to the calling application. The viewer components based on SoXtRenderArea handle many subsidiary tasks related to viewing 3D objects. Other components handle anything from editing materials and lights in a 3D scene, to copying and pasting 3D objects.
Note that if you are using libInventorXt, you need only link with libInventorXt (it automatically “exports” all of the routines in libInventor, so you never need to use -lInventorXt -lInventor, you need only -lInventorXt).
For detailed information on Open Inventor, see The Inventor Mentor: Programming Object-Oriented 3D Graphics with Open Inventor, Release 2, published by Addison-Wesley and available online through IRIS InSight.
The IRIS ViewKit library is a C++ application framework designed to simplify the task of developing applications based on the IRIS IM widget set. The ViewKit framework promotes consistency by providing a common architecture for applications and improves programmer productivity by providing high-level, and in many cases automatic, support for commonly needed operations.
When you use Viewkit in conjunction with OpenGL, it provides drawing areas that OpenGL can render to.
For more information, see the IRIS ViewKit Programmer's Guide, available online through IRIS InSight.
The IRIS IM widget set is an implementation of OSF/Motif provided by Silicon Graphics. You are strongly encouraged to use IRIS IM when writing software for Silicon Graphics systems. IRIS IM integrates your application with the desktop's interface. If you use it, your application conforms to a consistent look and feel for Silicon Graphics applications. See the sources listed in “Background Reading” for further details.
The X library, Xlib, provides function calls at a lower level than most application developers want to use. Note that while Xlib offers the greatest amount of control, it also requires that you attend to many details you could otherwise ignore. If you do decide to use Xlib, you are responsible for maintaining the Silicon Graphics user interface standards.
Not all of the toolkits just described are available on all Silicon Graphics platforms and if you are targeting both IRIX and Linux, you should be aware of the differences. IRIS ViewKit is only supported on IRIX systems, but Integrated Computer Solutions Incorporated (ICS) makes a commercial version of ViewKit for Linux and other platforms. The IRIS IM widget set includes widgets specific to SGI and supported only on IRIX. However, the OSF/Motif implementation on Linux supports most of the same functionality.
In addition to the toolkits and widget sets described earlier, similar GUI functionality is available from open source packages such as the Gnome Toolkit (GTK), Qt from Trolltech, and many others. SGI provides industry-standard versions of some of these packages with SGI Linux systems, and some are also available prebuilt for IRIX through the IRIX Freeware site, http://freeware.sgi.com/. Although SGI does not recommend any specific alternative, you may find these toolkits useful.
The OpenGL standard is designed to be as portable as possible and also to be expandable with extensions. Extensions may provide new functionality, such as several video extensions, or extend existing functionality, such as blending extensions.
An extension's functions and tokens use a suffix that indicates the availability of that extension. For example, the suffix ARB is used for extensions reviewed and approved by the OpenGL Architecture Review Board. ARB extensions are likely to be more widely supported on different vendor platforms than are any other type of extension, as they represent a consensus of the graphics industry. For a complete listing of suffixes, see Chapter 5, “Introduction to OpenGL Extensions”.
The glintro man page provides a useful introduction to extensions; many extensions are also discussed in detail in the following chapters in this guide:
Note that both the X Window System and OpenGL support extensions. GLX is an X extension to support OpenGL. Keep in mind that OpenGL (and GLX) extensions are different from X extensions.
If you want a fast application, think about performance from the start. While making sure the program runs reliably and bug-free is important, it is also essential that you think about performance early on. Applications designed and written without performance considerations can rarely be suitably tuned.
If you want high performance, read the following performance chapters in this guide before you start writing the application:
Silicon Graphics provides a variety of debugging tools for use with OpenGL programs:
The ogldebug tool helps you find OpenGL programming errors and discover OpenGL programming style that may slow down your application. You can set breakpoints, step through your program, and collect a variety of information.
For general-purpose debugging, you can use standard UNIX debugging tools such as dbx or gdb.
The CASE tools are only available on IRIX for general-purpose debugging. For more information on the CASE tools, see ProDev WorkShop and MegaDev Overview and CASEVision/Workshop User's Guide.
The OpenGL Performer application development environment from Silicon Graphics automatically optimizes graphical applications on the full range of Silicon Graphics systems without changes or recompilation. Performance features supported by OpenGL Performer include data structures to use the CPU, cache, and memory system architecture efficiently; tuned rendering loops to convert the system CPU into an optimized data management engine; and state management control to minimize overhead.
For OpenGL Performer documentation, see the SGI Technical Publications Library, http://docs.sgi.com.