This documentation is heavily based on the man pages of Mark J. Kilgard's GLUT library.

OpenGL is a trademark of Silicon Graphics, Inc. X Window System is a trademark of X Consortium, Inc. Spaceball is a registered trademark of Spatial Systems, Inc.

The author has taken care in preparation of this documentation but makes no expressed or implied warranty of any kind and assumes no responsibility for errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising from the use of information or programs contained herein.

The OpenGL Utility Toolkit (GLUT) is a programming interface for writing window system independent OpenGL programs. Currently there are implementations for the X Window System, the Windows family, OS/2, and Mac. The toolkit supports the following functionality:

• Multiple windows for OpenGL rendering.
• Callback driven event processing.
• Sophisticated input devices.
• An idle routine and timers.
• A simple, cascading pop-up menu facility.
• Utility routines to generate various solid and wire frame objects.
• Support for bitmap and stroke fonts.
• Miscellaneous window management functions, including managing overlays.

This documentation serves as both a specification and a programming guide. If you are interested in a brief introduction to programming with GLUT, have a look at the relevant parts of http://www.opengl.org/ and the vast amount of books on OpenGL, most of them use GLUT.

The remainder of this section describes GLUT's design philosophy and usage model. The following sections specify the GLUT routines, grouped by functionality. The final sections discuss usage advice and the logical programmer visible state maintained by GLUT.

One of the major accomplishments in the specification of OpenGL was the isolation of window system dependencies from OpenGL's rendering model. The result is that OpenGL is window system independent.

Window system operations such as the creation of a rendering window and the handling of window system events are left to the native window system to define. Necessary interactions between OpenGL and the window system such as creating and binding an OpenGL context to a window are described separately from the OpenGL specification in a window system dependent specification. For example, the GLX specification describes the standard by which OpenGL interacts with the X Window System.

The predecessor to OpenGL is IRIS GL. Unlike OpenGL, IRIS GL does specify how rendering windows are created and manipulated. IRIS GL's windowing interface is reasonably popular largely because it is simple to use. IRIS GL programmers can worry about graphics programming without needing to be an expert in programming the native window system. Experience also demonstrated that IRIS GL's windowing interface was high-level enough that it could be retargeted to different window systems. Silicon Graphics migrated from NeWS to the X Window System without any major changes to IRIS GL's basic windowing interface.

Removing window system operations from OpenGL is a sound decision because it allows the OpenGL graphics system to be retargeted to various systems including powerful but expensive graphics workstations as well as mass-production graphics systems like video games, set-top boxes for interactive television, and PCs.

Unfortunately, the lack of a window system interface for OpenGL is a gap in OpenGL's utility. Learning native window system APIs such as the X Window System's Xlib or Motif can be daunting. Even those familiar with native window system APIs need to understand the interface that binds OpenGL to the native window system. And when an OpenGL program is written using the native window system interface, despite the portability of the program's OpenGL rendering code, the program itself will be window system dependent.

Testing and documenting OpenGL's functionality lead to the development of the tk and aux toolkits. The aux toolkit is used in the examples found in the OpenGL Programming Guide. Unfortunately, aux has numerous limitations and its utility is largely limited to toy programs. The tk library has more functionality than aux but was developed in an ad hoc fashion and still lacks much important functionality that IRIS GL programmers expect, like pop-up menus and overlays.

GLUT is designed to fill the need for a window system independent programming interface for OpenGL programs. The interface is designed to be simple yet still meet the needs of useful OpenGL programs. Features from the IRIS GL, aux, and tk interfaces are included to make it easy for programmers used to these interfaces to develop programs for GLUT.

GLUT simplifies the implementation of programs using OpenGL rendering. The GLUT application programming interface (API) requires very few routines to display a graphics scene rendered using OpenGL. The GLUT API (like the OpenGL API) is stateful. Most initial GLUT state is defined and the initial state is reasonable for simple programs. The GLUT routines also take relatively few parameters.

The GLUT API is (as much as reasonable) window system independent. For this reason, GLUT does not return any native window system handles, pointers, or other data structures. More subtle window system dependencies such as reliance on window system dependent fonts are avoided by GLUT; instead, GLUT supplies its own (limited) set of fonts.

For programming ease, GLUT provides a simple menu sub-API. While the menuing support is designed to be implemented as pop-up menus, GLUT gives window system leeway to support the menu functionality in another manner (pull-down menus for example).

Two of the most important pieces of GLUT state are the current window and current menu. Most window and menu routines affect the current window or menu respectively. Most callbacks implicitly set the current window and menu to the appropriate window or menu responsible for the callback. GLUT is designed so that a program with only a single window and/or menu will not need to keep track of any window or menu identifiers. This greatly simplifies very simple GLUT programs.

GLUT is designed for simple to moderately complex programs focused on OpenGL rendering. GLUT implements its own event loop. For this reason, mixing GLUT with other APIs that demand their own event handling structure may be difficult. The advantage of a builtin event dispatch loop is simplicity.

GLUT contains routines for rendering fonts and geometric objects, however GLUT makes no claims on the OpenGL display list name space. For this reason, none of the GLUT rendering routines use OpenGL display lists. It is up to the GLUT programmer to compile the output from GLUT rendering routines into display lists if this is desired.

GLUT routines are logically organized into several sub-APIs according to their functionality. The sub-APIs are:

• Initialization: Command line processing, window system initialization, and initial window creation state are controlled by these routines.
• Beginning Event Processing: This routine enters GLUT's event processing loop. This routine never returns, and it continuously calls GLUT callbacks as necessary.
• Window Management: These routines create and control windows.
• Overlay Management: These routines establish and manage overlays for windows.
• Menu Management: These routines create and control pop-up menus.
• Callback Registration: These routines register callbacks to be called by the GLUT event processing loop.
• Color Index Colormap Management: These routines allow the manipulation of color index colormaps for windows.
• State Retrieval: These routines allows programs to retrieve state from GLUT.
• Font Rendering: These routines allow rendering of stroke and bitmap fonts.
• Geometric Shape Rendering: These routines allow the rendering of 3D geometric objects including spheres, cones, icosahedrons, and teapots.
• Debugging: This routine reports any pending GL errors.
• Device Control: These routines allow setting the key repeat and polling the joystick.
• Game Mode: These routines allow programs to enter/leave a full-screen mode with specified properties.

A number of terms are used in a GLUT-specific manner throughout this document. The GLUT meaning of these terms is independent of the window system GLUT is used with. Here are GLUT-specific meanings for the following GLUT-specific terms:

• Callback: A programmer specified routine that can be registered with GLUT to be called in response to a specific type of event. Also used to refer to a specific callback routine being called.
• Colormap: A mapping of pixel values to RGB color values. Used by color index windows.
• Dials and button box: A sophisticated input device consisting of a pad of buttons and an array of rotating dials, often used by computer-aided design programs.
• Display mode: A set of OpenGL frame buffer capabilities that can be attributed to a window.
• Idle: A state when no window system events are received for processing as callbacks and the idle callback, if one is registered, is called.
• Layer in use: Either the normal plane or overlay. This per-window state determines what frame buffer layer OpenGL commands affect.
• Menu entry: A menu item that the user can select to trigger the menu callback for the menu entry's value.
• Menu item: Either a menu entry or a sub-menu trigger.
• Modifiers: The Shift, Ctrl, and Alt keys that can be held down simultaneously with a key or mouse button being pressed or released.
• Multisampling: A technique for hardware antialiasing generally available only on expensive 3D graphics hardware. Each pixel is composed of a number of samples (each containing color and depth information). The samples are averaged to determine the displayed pixel color value. Multisampling is supported as an extension to OpenGL.
• Normal plane: The default frame buffer layer where GLUT window state resides; as opposed to the overlay.
• Overlay: A frame buffer layer that can be displayed preferentially to the normal plane and supports transparency to display through to the normal plane. Overlays are useful for rubber-banding effects, text annotation, and other operations, to avoid damaging the normal plane frame buffer state. Overlays require hardware support not present on all systems.
• Pop: The act of forcing a window to the top of the stacking order for sibling windows.
• Pop-up menu: A menu that can be set to appear when a specified mouse button is pressed in a window. A pop-menu consists of multiple menu items.
• Push: The act of forcing a window to the bottom of the stacking order for sibling windows.
• Reshape: The act of changing the size or shape of the window.
• Spaceball: A sophisticated 3D input device that provides six degrees of freedom, three axes of rotation and three axes of translation. It also supports a number of buttons. The device is a hand-sized ball attached to a base. By cupping the ball with one's hand and applying torsional or directional force on the ball, rotations and translationsare generated.
• Stereo: A frame buffer capability providing left and right color buffers for creating stereoscopic renderings. Typically, the user wears LCD shuttered goggles synchronized with the alternating display on the screen of the left and right color buffers.
• Sub-menu: A menu cascaded from some sub-menu trigger.
• Sub-menu trigger: A menu item that the user can enter to cascade another pop-up menu.
• Subwindow: A type of window that is the child window of a top-level window or other subwindow. The drawing and visible region of a subwindow is limited by its parent window.
• Tablet: A precise 2D input device. Like a mouse, 2D coordinates are returned. The absolute position of the tablet "puck" on the tablet is returned. Tablets also support a number of buttons.
• Timer: A callback that can be scheduled to be called in a specified interval of time.
• Top-level window: A window that can be placed, moved, resized, etc. independently from other top-level windows by the user. Subwindows may reside within a top-level window.
• Window: A rectangular area for OpenGL rendering.
• Window display state: One of shown, hidden, or iconified. A shown window is potentially visible on the screen (it may be obscured by other windows and not actually visible). A hidden window will never be visible. An iconified window is not visible but could be made visible in response to some user action like clicking on the window's corresponding icon.
• Window system: A broad notion that refers to both the mechanism and policy of the window system. For example, in the X Window System both the window manager and the X server are integral to what GLUT considers the window system.