FREEGLUT Application Program Interface
Author: John F. Fay

# The Open-Source

# OpenGL Utility Toolkit

# (freeglut 2.0.0)

# Application Programming Interface

# Version 4.0

## The freeglut Programming Consortium

## July, 2003

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 authors have taken care in preparation of this documentation but make 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.

# 1.0 Contents

1.0 Contents

2.0 Introduction

3.0 Background

> 3.1 Design Philosophy
> 3.2 Conventions
> 3.3 Terminology
> 3.4 Differences from GLUT 3.7

4.0 Initialization Functions

> 4.1 glutInit
> 4.2 glutInitWindowPosition, glutInitWindowSize
> 4.3 glutInitDisplayMode
> 4.4 glutInitDisplayString

5.0 Event Processing Functions

> 5.1 glutMainLoop
> 5.2 glutMainLoopEvent
> 5.3 glutLeaveMainLoop

6.0 Window Functions

> 6.1 glutCreateWindow
> 6.2 glutCreateSubwindow
> 6.3 glutDestroyWindow
> 6.4 glutSetWindow, glutGetWindow
> 6.5 glutSetWindowTitle, glutSetIconTitle
> 6.6 glutReshapeWindow
> 6.7 glutPositionWindow
> 6.8 glutShowWindow, glutHideWindow, glutIconifyWindow
> 6.9 glutPushWindow, glutPopWindow
> 6.10 glutFullScreen

7.0 Display Functions

> 7.1 glutPostRedisplay
> 7.2 glutPostWindowRedisplay
> 7.3 glutSwapBuffers

8.0 Mouse Cursor Functions

> 8.1 glutSetCursor
> 8.2 glutWarpPointer

9.0 Overlay Functions

> 9.1 glutEstablishOverlay
> 9.2 glutRemoveOverlay
> 9.3 glutUseLayer
> 9.4 glutPostOverlayRedisplay
> 9.5 glutPostWindowOverlayRedisplay
> 9.6 glutShowOverlay, glutHideOverlay

10.0 Menu Functions

> 10.1 glutCreateMenu
> 10.2 glutDestroyMenu
> 10.3 glutGetMenu, glutSetMenu
> 10.4 glutAddMenuEntry
> 10.5 glutAddSubMenu
> 10.6 glutChangeToMenuEntry
> 10.7 glutChangeToSubMenu
> 10.8 glutRemoveMenuItem
> 10.9 glutAttachMenu, glutDetachMenu

11.0 Global Callback Registration Functions

> 11.1 glutTimerFunc
> 11.2 glutIdleFunc

12.0 Window-Specific Callback Registration Functions

> 12.1 glutDisplayFunc
> 12.2 glutOverlayDisplayFunc
> 12.3 glutReshapeFunc
> 12.4 glutCloseFunc
> 12.5 glutKeyboardFunc
> 12.6 glutSpecialFunc
> 12.7 glutKeyboardUpFunc
> 12.8 glutSpecialUpFunc
> 12.9 glutMouseFunc
> 12.10 glutMotionFunc, glutPassiveMotionFunc
> 12.11 glutVisibilityFunc
> 12.12 glutEntryFunc
> 12.13 glutJoystickFunc
> 12.14 glutSpaceballMotionFunc
> 12.15 glutSpaceballRotateFunc
> 12.16 glutSpaceballButtonFunc
> 12.17 glutButtonBoxFunc
> 12.18 glutDialsFunc
> 12.19 glutTabletMotionFunc
> 12.20 glutTabletButtonFunc

> 12.21 glutMenuStatusFunc
> 12.22 glutWindowStatusFunc

13.0 State Setting and Retrieval Functions

> 13.1 glutSetOption
> 13.2 glutGet
> 13.3 glutDeviceGet
> 13.4 glutGetModifiers
> 13.5 glutLayerGet
> 13.6 glutExtensionSupported
> 13.7 glutGetProcAddress

14.0 Font Rendering Functions

> 14.1 glutBitmapCharacter
> 14.2 glutBitmapString
> 14.3 glutBitmapWidth
> 14.4 glutBitmapLength
> 14.5 glutBitmapHeight
> 14.6 glutStrokeCharacter
> 14.7 glutStrokeString
> 14.8 glutStrokeWidth
> 14.9 glutStrokeLength
> 14.10 glutStrokeHeight

15.0 Geometric Object Rendering Functions

> 15.1 glutWireSphere, glutSolidSphere
> 15.2 glutWireTorus, glutSolidTorus
> 15.3 glutWireCone, glutSolidCone
> 15.4 glutWireCube, glutSolidCube
> 15.5 glutWireTetrahedron, glutSolidTetrahedron
> 15.6 glutWireOctahedron, glutSolidOctahedron
> 15.7 glutWireDodecahedron, glutSolidDodecahedron
> 15.8 glutWireIcosahedron, glutSolidIcosahedron
> 15.9 glutWireRhombicDodecahedron, glutSolidRhombicDodecahedron
> 15.10 glutWireTeapot, glutSolidTeapot

16.0 Game Mode Functions

> 16.1 glutGameModeString
> 16.2 glutEnterGameMode, glutLeaveGameMode
> 16.3 glutGameModeGet

17.0 Video Resize Functions

> 17.1 glutVideoResizeGet
> 17.2 glutSetupVideoResizing, glutStopVideoResizing
> 17.3 glutVideoResize
> 17.4 glutVideoPan

18.0 Color Map Functions

> 18.1 glutSetColor, glutGetColor
> 18.2 glutCopyColormap

19.0 Miscellaneous Functions

> 19.1 glutIgnoreKeyRepeat, glutSetKeyRepeat
> 19.2 glutForceJoystickFunc
> 19.3 glutReportErrors

20.0 Usage Notes

21.0 Implementation Notes

22.0 GLUT State

23.0 "freeglut.h" Header File

24.0 References

25.0 Index

# 2.0 Introduction

# 3.0 Background

The OpenGL programming world owes a tremendous debt to Mr. Mark J. Kilgard for writing the OpenGL Utility Toolkit, or GLUT. The GLUT library of functions allows an application programmer to create, control, and manipulate windows independent of what operating system the program is running on. By hiding the dependency on the operating system from the application programmer, he allowed people to write truly portable OpenGL applications.

Mr. Kilgard copyrighted his library and gave it a rather unusual license. Under his license, people are allowed freely to copy and distribute the libraries and the source code, but they are not allowed to modify it. For a long time this did not matter because the GLUT library worked so well and because Mr. Kilgard was releasing updates on a regular basis. But with the passage of time, people started wanting some slightly different behaviours in their windowing system. When Mr. Kilgard stopped supporting the GLUT library in 1999, having moved on to bigger and better things, this started to become a problem.

In December 1999, Mr. Pawel Olzsta started work on an open-source clone of the GLUT library. This open-source clone, which does not use any of the GLUT source code, has evolved into the present freeglut library. This documentation specifies the application program interface to the freeglut library.

## 3.1 Design Philosophy

## 3.2 Conventions

## 3.3 Terminology

## 3.4 Differences from GLUT 3.7

Since the freeglut library was developed in order to update GLUT, it is natural that there will be some differences between the two. Each function in the API notes any differences between the GLUT and the freeglut function behaviours. The important ones are summarized here.

### 3.4.1 glutMainLoop Behaviour

One of the commonest complaints about the GLUT library was that once an application called "`glutMainLoop`", it never got control back. There was no way for an application to loop in GLUT for a while, possibly as a subloop while a specific window was open, and then return to the calling function. A new function, "`glutMainLoopEvent`", has been added to allow this functionality. Another function, "`glutLeaveMainLoop` ", has also been added to allow the application to tell freeglut to clean up and close down.

### 3.4.2 Action on Window Closure

Another difficulty with GLUT, especially with multiple-window programs, is that if the user clicks on the "x" in the window header the application exits immediately. The application programmer can now set an option, "`GLUT_ACTION_ON_WINDOW_CLOSE`", to specify whether execution should continue, whether GLUT should return control to the main program, or whether GLUT should simply exit (the default).

### 3.4.3 Changes to Callbacks

Several new callbacks have been added and several callbacks which were specific to Silicon Graphics hardware have not been implemented. Most or all of the new callbacks are listed in the GLUT Version 4 "glut.h" header file but did not make it into the documentation. The new callbacks consist of regular and special key release callbacks, a joystick callback, a window status callback, window closure callbacks, a menu closure callback, and a mouse wheel callback. Unsupported callbacks are the three Spaceball callbacks, the ButtonBox callback, and the two Tablet callbacks. If the user has a need for an unsupported callback he should contact the freeglut development team.

### 3.4.4 String Rendering

New functions have been added to render full character strings (including carriage returns) rather than rendering one character at a time. More functions return the widths of character strings and the font heights, in pixels for bitmapped fonts and in OpenGL units for the stroke fonts.

### 3.4.5 Geometry Rendering

Functions have been added to render a wireframe and a solid rhombic dodecahedron, a cylinder, and a Sierpinski sponge.

### 3.4.5 Extension Function Queries

glutGetProcAddress is a wrapper for the glXGetProcAddressARB and wglGetProcAddress functions.

# 4.0 Initialization Functions

## 4.1 glutInit

## 4.2 glutInitWindowPosition, glutInitWindowSize

The "`glutInitWindowPosition` " and "`glutInitWindowSize` " functions specify a desired position and size for windows that freeglut will create in the future.

Usage

`void glutInitWindowPosition ( int x, int y ) ;`
`void glutInitWindowSize ( int width, int height ) ;`

Description

The "`glutInitWindowPosition` " and "`glutInitWindowSize`" functions specify a desired position and size for windows that freeglut will create in the future. The position is measured in pixels from the upper left hand corner of the screen, with "x" increasing to the right and "y" increasing towards the bottom of the screen. The size is measured in pixels. Freeglut does not promise to follow these specifications in creating its windows, it certainly makes an attempt to.

The position and size of a window are a matter of some subtlety. Most windows have a usable area surrounded by a border and with a title bar on the top. The border and title bar are commonly called "decorations." The position of the window unfortunately varies with the operating system. On Linux, it is the coordinates of the upper left-hand corner of its decorations. On Windows, it is the coordinates of the upper left hand corner of its usable interior. For both operating systems, the size of the window is the size of the usable interior.

Windows has some additional quirks which the application programmer should know about. First, the minimum y-coordinate of a window decoration is zero. (This is a feature of freeglut and can be adjusted if so desired.) Second, there appears to be a minimum window width on Windows which is 104 pixels. The user may specify a smaller width, but the Windows system calls ignore it. It is also impossible to make a window narrower than this by dragging on its corner.

Changes From GLUT

For some reason, GLUT is not affected by the 104-pixel minimum window width. If the user clicks on the corner of a window which is narrower than this amount, the window will immediately snap out to this width, but the application can call "`glutReshapeWindow` " and make a window narrower again.

## 4.3 glutInitDisplayMode

## 4.4 glutInitDisplayString

# 5.0 Event Processing Functions

After an application has finished initializing its windows and menus, it enters an event loop. Within this loop, freeglut polls the data entry devices (keyboard, mouse, etc.) and calls the application's appropriate callbacks.

In GLUT, control never returned from the event loop (as invoked by the "`glutMainLoop`" function) to the calling function. This prevented an application from having re-entrant code, in which GLUT could be invoked from within a callback, and it prevented the application from doing any post-processing (such as freeing allocated memory) after GLUT had closed down. Freeglut allows the application programmer to specify more direct control over the event loop by means of two new functions. The first, "`glutMainLoopEvent`", processes a single iteration of the event loop and allows the application to use a different event loop controller or to contain re-entrant code. The second, "`glutLeaveMainLoop`", causes the event loop to exit nicely; this is preferable to the application's calling "`exit`" from within a GLUT callback.

## 5.1 glutMainLoop

The "`glutMainLoop`" function enters the event loop.

Usage

`void glutMainLoop ( void ) ;`

Description

The "`glutMainLoop`" function causes the program to enter the window event loop. An application should call this function at most once. It will call any application callback functions as required to process mouse clicks, mouse motion, key presses, and so on.

Changes From GLUT

In GLUT, there was absolutely no way for the application programmer to have control return from the "`glutMainLoop` " function to the calling function. Freeglut allows the programmer to force this by setting the "`GLUT_ACTION_ON_WINDOW_CLOSE`" option and invoking the "`glutLeaveMainLoop`" function from one of the callbacks. Stopping the program this way is preferable to simply calling "`exit` " from within a callback because this allows freeglut to free allocated memory and otherwise clean up after itself. (I know I just said this, but I think it is important enough that it bears repeating.)

## 5.2 glutMainLoopEvent

The "`glutMainLoopEvent`" function processes a single iteration in the freeglut event loop.

Usage

`void glutMainLoopEvent ( void ) ;`

Description

The "`glutMainLoopEvent` " function causes freeglut to process one iteration's worth of events in its event loop. This allows the application to control its own event loop and still use the freeglut windowing system.

Changes From GLUT

GLUT does not include this function.

## 5.3 glutLeaveMainLoop

The "`glutLeaveMainLoop`" function causes freeglut to stop its event loop.

Usage

`void glutLeaveMainLoop ( void ) ;`

Description

The "`glutLeaveMainLoop` " function causes freeglut to stop the event loop. If the "`GLUT_ACTION_ON_WINDOW_CLOSE`" option has been set to "`GLUT_ACTION_CONTINUE_EXECUTION` ", control will return to the function which called "`glutMainLoop` "; otherwise the application will exit.

If the application has two nested calls to "`glutMainLoop`" and calls "`glutLeaveMainLoop`", the behaviour of freeglut is undefined. It may leave only the inner nested loop or it may leave both loops. If the reader has a strong preference for one behaviour over the other he should contact the freeglut Programming Consortium and ask for the code to be fixed.

Changes From GLUT

GLUT does not include this function.

# 6.0 Window Functions

## 6.1 glutCreateWindow

## 6.2 glutCreateSubwindow

## 6.3 glutDestroyWindow

## 6.4 glutSetWindow, glutGetWindow

## 6.5 glutSetWindowTitle, glutSetIconTitle

## 6.6 glutReshapeWindow

## 6.7 glutPositionWindow

## 6.8 glutShowWindow, glutHideWindow, glutIconifyWindow

## 6.9 glutPushWindow, glutPopWindow

## 6.10 glutFullScreen

# 7.0 Display Functions

## 7.1 glutPostRedisplay

## 7.2 glutPostWindowRedisplay

## 7.3 glutSwapBuffers

# 8.0 Mouse Cursor Functions

## 8.1 glutSetCursor

## 8.2 glutWarpPointer

# 9.0 Overlay Functions

Freeglut does not allow overlays, although it does "answer the mail" with function stubs so that GLUT-based programs can compile and link against freeglut without modification. If the reader needs overlays, he should contact the freeglut Programming Consortium and ask for them to be implemented. He should also be prepared to assist in the implementation.

## 9.1 glutEstablishOverlay

The "`glutEstablishOverlay`" function is not implemented in freeglut .

Usage

`void glutEstablishOverlay ( void ) ;`

Description

The "`glutEstablishOverlay`" function is not implemented in freeglut.

Changes From GLUT

GLUT implements this function.

## 9.2 glutRemoveOverlay

The "`glutRemoveOverlay`" function is not implemented in freeglut .

Usage

`void glutRemoveOverlay ( void ) ;`

Description

The "`glutRemoveOverlay`" function is not implemented in freeglut.

Changes From GLUT

GLUT implements this function.

## 9.3 glutUseLayer

The "`glutUseLayer`" function is not implemented in freeglut .

Usage

`void glutUseLayer ( GLenum layer ) ;`

Description

The "`glutUseLayer`" function is not implemented in freeglut.

Changes From GLUT

GLUT implements this function.

## 9.4 glutPostOverlayRedisplay

The "`glutPostOverlayRedisplay` " function is not implemented in freeglut.

Usage

`void glutPostOverlayRedisplay ( void ) ;`

Description

The "`glutPostOverlayRedisplay` " function is not implemented in freeglut.

Changes From GLUT

GLUT implements this function.

## 9.5 glutPostWindowOverlayRedisplay

The "`glutPostWindowOverlayRedisplay` " function is not implemented in freeglut.

Usage

`void glutPostWindowOverlayRedisplay ( int window ) ;`

Description

The "`glutPostWindowOverlayRedisplay` " function is not implemented in freeglut.

Changes From GLUT

GLUT implements this function.

## 9.6 glutShowOverlay, glutHideOverlay

The "`glutShowOverlay`" and "`glutHideOverlay`" functions are not implemented in freeglut .

Usage

`void glutShowOverlay( void ) ;`
`void glutHideOverlay( void ) ;`

Description

The "`glutShowOverlay`" and "`glutHideOverlay`" functions are not implemented in freeglut .

Changes From GLUT

GLUT implements these functions.

# 10.0 Menu Functions

## 10.1 glutCreateMenu

## 10.2 glutDestroyMenu

## 10.3 glutGetMenu, glutSetMenu

## 10.4 glutAddMenuEntry

## 10.5 glutAddSubMenu

## 10.6 glutChangeToMenuEntry

## 10.7 glutChangeToSubMenu

## 10.8 glutRemoveMenuItem

## 10.9 glutAttachMenu, glutDetachMenu

# 11.0 Global Callback Registration Functions

## 11.1 glutTimerFunc

## 11.2 glutIdleFunc

The "`glutIdleFunc`" function sets the global idle callback. Freeglut calls the idle callback when there are no inputs from the user.

Usage

`void glutIdleFunc ( void (*func) ( void ) ) ;`

`func` The new global idle callback function

Description

The "`glutIdleFunc`" function specifies the function that freeglut will call to perform background processing tasks such as continuous animation when window system events are not being received. If enabled, this function is called continuously from freeglut while no events are received. The callback function has no parameters and returns no value. Freeglut does not change the current window or the current menu before invoking the idle callback; programs with multiple windows or menus must explicitly set the current window and current menu and not rely on its current setting.
The amount of computation and rendering done in an idle callback should be minimized to avoid affecting the program's interactive response. In general, no more than a single frame of rendering should be done in a single invocation of an idle callback.
Calling "`glutIdleFunc`" with a NULL argument disables the call to an idle callback.

Changes From GLUT

Application programmers should note that if they have specified the "continue execution" action on window closure, freeglut will continue to call the idle callback after the user has closed a window by clicking on the "x" in the window header bar. If the idle callback renders a particular window (this is considered bad form but is frequently done anyway), the programmer should supply a window closure callback for that window which changes or disables the idle callback.

# 12.0 Window-Specific Callback Registration Functions

## 12.1 glutDisplayFunc

## 12.2 glutOverlayDisplayFunc

## 12.3 glutReshapeFunc

## 12.4 glutCloseFunc

## 12.5 glutKeyboardFunc

## 12.6 glutSpecialFunc

The "`glutSpecialFunc`" function sets the window's special key press callback. Freeglut calls the special key press callback when the user presses a special key.

Usage

`void glutSpecialFunc ( void (*func) ( int key, int x, int y ) ) ;`

`func` The window's new special key press callback function
`key` The key whose press triggers the callback
`x` The x-coordinate of the mouse relative to the window at the time the key is pressed
`y` The y-coordinate of the mouse relative to the window at the time the key is pressed

Description

The "`glutSpecialFunc`" function specifies the function that freeglut will call when the user presses a special key on the keyboard. The callback function has one argument: the name of the function to be invoked ("called back") at the time at which the special key is pressed. The function returns no value. Freeglut sets the current window to the window which is active when the callback is invoked. "Special keys" are the function keys, the arrow keys, the Page Up and Page Down keys, and the Insert key. The Delete key is considered to be a regular key.
Calling "`glutSpecialUpFunc`" with a NULL argument disables the call to the window's special key press callback.

The "`key` " argument may take one of the following defined constant values:

- `GLUT_KEY_F1, GLUT_KEY_F2, ..., GLUT_KEY_F12` - F1 through F12 keys
- `GLUT_KEY_PAGE_UP, GLUT_KEY_PAGE_DOWN` - Page Up and Page Down keys
- `GLUT_KEY_HOME, GLUT_KEY_END` - Home and End keys
- `GLUT_KEY_LEFT, GLUT_KEY_RIGHT, GLUT_KEY_UP, GLUT_KEY_DOWN` - arrow keys
- `GLUT_KEY_INSERT` - Insert key

Changes From GLUT

None.

## 12.7 glutKeyboardUpFunc

The "`glutKeyboardUpFunc`" function sets the window's key release callback. Freeglut calls the key release callback when the user releases a key.

Usage

`void glutKeyboardUpFunc ( void (*func) ( unsigned char key, int x, int y ) ) ;`

`func` The window's new key release callback function
`key` The key whose release triggers the callback
`x` The x-coordinate of the mouse relative to the window at the time the key is released
`y` The y-coordinate of the mouse relative to the window at the time the key is released

Description

The "`glutKeyboardUpFunc` " function specifies the function that freeglut will call when the user releases a key from the keyboard. The callback function has one argument: the name of the function to be invoked ("called back") at the time at which the key is released. The function returns no value. Freeglut sets the current window to the window which is active when the callback is invoked.
While freeglut checks for upper or lower case letters, it does not do so for non-alphabetical characters. Nor does it account for the Caps-Lock key being on. The operating system may send some unexpected characters to freeglut, such as "8" when the user is pressing the Shift key. Freeglut also invokes the callback when the user releases the Control, Alt, or Shift keys, among others. Releasing the Delete key causes this function to be invoked with a value of 127 for "`key`".
Calling "`glutKeyboardUpFunc`" with a NULL argument disables the call to the window's key release callback.

Changes From GLUT

This function is not implemented in GLUT versions before Version 4. It has been designed to be as close to GLUT as possible. Users who find differences should contact the freeglut Programming Consortium to have them fixed.

## 12.8 glutSpecialUpFunc

The "`glutSpecialUpFunc`" function sets the window's special key release callback. Freeglut calls the special key release callback when the user releases a special key.

Usage

`void glutSpecialUpFunc ( void (*func) ( int key, int x, int y ) ) ;`

`func` The window's new special key release callback function
`key` The key whose release triggers the callback
`x` The x-coordinate of the mouse relative to the window at the time the key is released
`y` The y-coordinate of the mouse relative to the window at the time the key is released

Description

The "`glutSpecialUpFunc` " function specifies the function that freeglut will call when the user releases a special key from the keyboard. The callback function has one argument: the name of the function to be invoked ("called back") at the time at which the special key is released. The function returns no value. Freeglut sets the current window to the window which is active when the callback is invoked. "Special keys" are the function keys, the arrow keys, the Page Up and Page Down keys, and the Insert key. The Delete key is considered to be a regular key.
Calling "`glutSpecialUpFunc`" with a NULL argument disables the call to the window's special key release callback.

The "`key` " argument may take one of the following defined constant values:

- `GLUT_KEY_F1, GLUT_KEY_F2, ..., GLUT_KEY_F12` - F1 through F12 keys
- `GLUT_KEY_PAGE_UP, GLUT_KEY_PAGE_DOWN` - Page Up and Page Down keys
- `GLUT_KEY_HOME, GLUT_KEY_END` - Home and End keys
- `GLUT_KEY_LEFT, GLUT_KEY_RIGHT, GLUT_KEY_UP, GLUT_KEY_DOWN` - arrow keys
- `GLUT_KEY_INSERT` - Insert key

Changes From GLUT

This function is not implemented in GLUT versions before Version 4. It has been designed to be as close to GLUT as possible. Users who find differences should contact the freeglut Programming Consortium to have them fixed.

## 12.9 glutMouseFunc

## 12.10 glutMotionFunc, glutPassiveMotionFunc

## 12.11 glutVisibilityFunc

## 12.12 glutEntryFunc

## 12.13 glutJoystickFunc

## 12.14 glutSpaceballMotionFunc

The "`glutSpaceballMotionFunc`" function is not implemented in freeglut, although the library does "answer the mail" to the extent that a call to the function will not produce an error..

Usage

`void glutSpaceballMotionFunc ( void (* callback)( int x, int y, int z )``) ;`

Description

The "`glutSpaceballMotionFunc` " function is not implemented in freeglut.

Changes From GLUT

GLUT implements this function.

## 12.15 glutSpaceballRotateFunc

The "`glutSpaceballRotateFunc`" function is not implemented in freeglut, although the library does "answer the mail" to the extent that a call to the function will not produce an error..

Usage

`void glutSpaceballRotateFunc ( void (* callback)( int x, int y, int z )``) ;`

Description

The "`glutSpaceballRotateFunc` " function is not implemented in freeglut.

Changes From GLUT

GLUT implements this function.

## 12.16 glutSpaceballButtonFunc

The "`glutSpaceballButtonFunc`" function is not implemented in freeglut, although the library does "answer the mail" to the extent that a call to the function will not produce an error..

Usage

`void glutSpaceballButtonFunc ( void (* callback)( int button, int updown )``) ;`

Description

The "`glutSpaceballButtonFunc` " function is not implemented in freeglut.

Changes From GLUT

GLUT implements this function.

## 12.17 glutButtonBoxFunc

The "`glutSpaceballButtonBoxFunc`" function is not implemented in freeglut, although the library does "answer the mail" to the extent that a call to the function will not produce an error..

Usage

`void glutSpaceballButtonBoxFunc ( void (* callback)( int button, int updown )``) ;`

Description

The "`glutSpaceballButtonBoxFunc` " function is not implemented in freeglut.

Changes From GLUT

GLUT implements this function.

## 12.18 glutDialsFunc

The "`glutDialsFunc`" function is not implemented in freeglut , although the library does "answer the mail" to the extent that a call to the function will not produce an error..

Usage

`void glutDialsFunc ( void (* callback)( int dial, int value )``) ;`

Description

The "`glutDialsFunc`" function is not implemented in freeglut.

Changes From GLUT

GLUT implements this function.

## 12.19 glutTabletMotionFunc

The "`glutTabletMotionFunc`" function is not implemented in freeglut, although the library does "answer the mail" to the extent that a call to the function will not produce an error..

Usage

`void glutTabletMotionFunc ( void (* callback)( int x, int y )``) ;`

Description

The "`glutTabletMotionFunc`" function is not implemented in freeglut.

Changes From GLUT

GLUT implements this function.

## 12.20 glutTabletButtonFunc

The "`glutTabletButtonFunc`" function is not implemented in freeglut, although the library does "answer the mail" to the extent that a call to the function will not produce an error..

Usage

`void glutTabletButtonFunc ( void (* callback)( int button, int updown, int x, int y )``) ;`

Description

The "`glutTabletButtonFunc`" function is not implemented in freeglut.

Changes From GLUT

GLUT implements this function.

## 12.21 glutMenuStatusFunc

## 12.22 glutWindowStatusFunc

# 13.0 State Setting and Retrieval Functions

## 13.1 glutSetOption

## 13.2 glutGet

The following state variables may be queried with "`glutGet`". The returned value is an integer.

These queries are with respect to the current window:

- GLUT_WINDOW_X - window X position
- GLUT_WINDOW_Y - window Y position
- GLUT_WINDOW_WIDTH - window width
- GLUT_WINDOW_HEIGHT - window height
- GLUT_WINDOW_BUFFER_SIZE - number of color or color index bits per pixel
- GLUT_WINDOW_STENCIL_SIZE - number of bits per stencil value
- GLUT_WINDOW_DEPTH_SIZE - number of bits per depth value
- GLUT_WINDOW_RED_SIZE - number of bits per red value
- GLUT_WINDOW_GREEN_SIZE - number of bits per green value
- GLUT_WINDOW_BLUE_SIZE - number of bits per blue value
- GLUT_WINDOW_ALPHA_SIZE - number of bits per alpha value
- GLUT_WINDOW_ACCUM_RED_SIZE - number of red bits in the accumulation buffer
- GLUT_WINDOW_ACCUM_GREEN_SIZE - number of green bits in the accumulation buffer
- GLUT_WINDOW_ACCUM_BLUE_SIZE - number of blue bits in the accumulation buffer
- GLUT_WINDOW_ACCUM_ALPHA_SIZE - number of alpha bits in the accumulation buffer
- GLUT_WINDOW_DOUBLEBUFFER - 1 if the color buffer is double buffered, 0 otherwise
- GLUT_WINDOW_RGBA - 1 if the color buffers are RGB[A], 0 for color index
- GLUT_WINDOW_PARENT - parent window ID
- GLUT_WINDOW_NUM_CHILDREN - number of child windows
- GLUT_WINDOW_COLORMAP_SIZE - number of entries in the window's colormap
- GLUT_WINDOW_NUM_SAMPLES - number of samples per pixel if using multisampling
- GLUT_WINDOW_STEREO - 1 if the window supports stereo, 0 otherwise
- GLUT_WINDOW_CURSOR - current cursor
- GLUT_WINDOW_FORMAT_ID - on Windows, return the pixel format number of the current window

These queries do not depend on the current window.

- GLUT_SCREEN_WIDTH - width of the screen in pixels
- GLUT_SCREEN_HEIGHT - height of the screen in pixels
- GLUT_SCREEN_WIDTH_MM - width of the screen in millimeters
- GLUT_SCREEN_HEIGHT_MM - height of the screen in millimeters
- GLUT_MENU_NUM_ITEMS - number of items in the current menu
- GLUT_DISPLAY_MODE_POSSIBLE - return 1 if the current display mode is supported, 0 otherwise
- GLUT_INIT_WINDOW_X - X position last set by glutInitWindowPosition
- GLUT_INIT_WINDOW_Y - Y position last set by glutInitWindowPosition
- GLUT_INIT_WINDOW_WIDTH - width last set by glutInitWindowSize
- GLUT_INIT_WINDOW_HEIGHT - height last set by glutInitWindowSize
- GLUT_INIT_DISPLAY_MODE - display mode last set by glutInitDisplayMode
- GLUT_ELAPSED_TIME - time (in milliseconds) elapsed since glutInit or glutGet(GLUT_ELAPSED_TIME) was first called
- GLUT_INIT_STATE - ?
- GLUT_VERSION - Return value will be X*10000+Y*100+Z where X is the major version, Y is the minor version and Z is the patch level. This query is only supported in freeglut (version 2.0.0 or later).

## 13.3 glutDeviceGet

## 13.4 glutGetModifiers

## 13.5 glutLayerGet

## 13.6 glutExtensionSupported

## 13.7 glutGetProcAddress

`glutGetProcAddress` returns a pointer to a named GL or freeglut function.

Usage

`void *glutGetProcAddress ( const char *procName ) ;`

`procName` Name of an OpenGL or GLUT function.

Description

`glutGetProcAddress` is useful for dealing with OpenGL extensions. If an application calls OpenGL extension functions directly, that application will only link/run with an OpenGL library that supports the extension. By using a function pointer returned from glutGetProcAddress(), the application will avoid this hard dependency and be more portable and interoperate better with various implementations of OpenGL.

Both OpenGL functions and freeglut functions can be queried with this function.

Changes From GLUT

GLUT does not include this function.

# 14.0 Font Rendering Functions

Freeglut supports two types of font rendering: bitmap fonts, which are rendered using the "`glBitmap`" function call, and stroke fonts, which are rendered as sequences of OpenGL line segments. Because they are rendered as bitmaps, the bitmap fonts tend to render more quickly than stroke fonts, but they are less flexible in terms of scaling and rendering. Bitmap font characters are positioned with calls to the "`glRasterPos*` " functions while stroke font characters use the OpenGL transformations to position characters.

It should be noted that freeglut fonts are similar but not identical to GLUT fonts. At the moment, freeglut fonts do not support the "`" (backquote) and "|" (vertical line) characters; in their place it renders asterisks.

Freeglut supports the following bitmap fonts:

- `GLUT_BITMAP_8_BY_13` - A variable-width font with every character fitting in a rectangle of 13 pixels high by at most 8 pixels wide.
- `GLUT_BITMAP_9_BY_15` - A variable-width font with every character fitting in a rectangle of 15 pixels high by at most 9 pixels wide.
- `GLUT_BITMAP_TIMES_ROMAN_10` - A 10-point variable-width Times Roman font.
- `GLUT_BITMAP_TIMES_ROMAN_24` - A 24-point variable-width Times Roman font.
- `GLUT_BITMAP_HELVETICA_10` - A 10-point variable-width Helvetica font.
- `GLUT_BITMAP_HELVETICA_12` - A 12-point variable-width Helvetica font.
- `GLUT_BITMAP_HELVETICA_18` - A 18-point variable-width Helvetica font.

Freeglut calls "`glRasterPos4v`" to advance the cursor by the width of a character and to render carriage returns when appropriate. It does not use any display lists in it rendering in bitmap fonts.

Freeglut supports the following stroke fonts:

- `GLUT_STROKE_ROMAN` - A proportionally-spaced Roman Simplex font
- `GLUT_STROKE_MONO_ROMAN` - A fixed-width Roman Simplex font

Freeglut does not use any display lists in its rendering of stroke fonts. It calls "`glTranslatef`" to advance the cursor by the width of a character and to render carriage returns when appropriate.

## 14.1 glutBitmapCharacter

The "`glutBitmapCharacter`" function renders a single bitmapped character in the current window using the specified font.

Usage

`void glutBitmapCharacter ( void *font, int character ) ;`

`font` The bitmapped font to use in rendering the character
`character` The ASCII code of the character to be rendered

Description

The "`glutBitmapCharacter` " function renders the given character in the specified bitmap font. Freeglut automatically sets the necessary pixel unpack storage modes and restores the existing modes when it has finished. Before the first call to "`glutBitMapCharacter` " the application program should call "`glRasterPos*`" to set the position of the character in the window. The "`glutBitmapCharacter` " function advances the cursor position as part of its call to "`glBitmap` " and so the application does not need to call "`glRasterPos*`" again for successive characters on the same line.

Changes From GLUT

Nonexistent characters are rendered as asterisks. The rendering position in freeglut is apparently off from GLUT's position by a few pixels vertically and one or two pixels horizontally.

## 14.2 glutBitmapString

The "`glutBitmapString`" function renders a string of bitmapped characters in the current window using the specified font.

Usage

`void glutBitmapString ( void *font, char *string ) ;`

`font` The bitmapped font to use in rendering the character string
`string` String of characters to be rendered

Description

The "`glutBitmapString` " function renders the given character string in the specified bitmap font. Freeglut automatically sets the necessary pixel unpack storage modes and restores the existing modes when it has finished. Before calling "`glutBitMapString`" the application program should call "`glRasterPos*`" to set the position of the string in the window. The "`glutBitmapString`" function handles carriage returns. Nonexistent characters are rendered as asterisks.

Changes From GLUT

GLUT does not include this function.

## 14.3 glutBitmapWidth

The "`glutBitmapWidth`" function returns the width in pixels of a single bitmapped character in the specified font.

Usage

`int glutBitmapWidth ( void *font, int character ) ;`

`font` The bitmapped font to use in calculating the character width
`character` The ASCII code of the character

Description

The "`glutBitmapWidth`" function returns the width of the given character in the specified bitmap font. Because the font is bitmapped, the width is an exact integer.

Changes From GLUT

Nonexistent characters return the width of an asterisk.

## 14.4 glutBitmapLength

The "`glutBitmapLength`" function returns the width in pixels of a string of bitmapped characters in the specified font.

Usage

`int glutBitmapLength ( void *font, char *string ) ;`

`font` The bitmapped font to use in calculating the character width
`string` String of characters whose width is to be calculated

Description

The "`glutBitmapLength` " function returns the width in pixels of the given character string in the specified bitmap font. Because the font is bitmapped, the width is an exact integer: the return value is identical to the sum of the character widths returned by a series of calls to "`glutBitmapWidth` ". The width of nonexistent characters is counted to be the width of an asterisk.

If the string contains one or more carriage returns, freeglut calculates the widths in pixels of the lines separately and returns the largest width.

Changes From GLUT

GLUT does not include this function.

## 14.5 glutBitmapHeight

The "`glutBitmapHeight`" function returns the height in pixels of the specified font.

Usage

`int glutBitmapHeight ( void *font ) ;`

`font` The bitmapped font to use in calculating the character height

Description

The "`glutBitmapHeight` " function returns the height of a character in the specified bitmap font. Because the font is bitmapped, the height is an exact integer. The fonts are designed such that all characters have (nominally) the same height.

Changes From GLUT

GLUT does not include this function.

## 14.6 glutStrokeCharacter

The "`glutStrokeCharacter`" function renders a single stroke character in the current window using the specified font.

Usage

`void glutStrokeCharacter ( void *font, int character ) ;`

`font` The stroke font to use in rendering the character
`character` The ASCII code of the character to be rendered

Description

The "`glutStrokeCharacter` " function renders the given character in the specified stroke font. Before the first call to "`glutStrokeCharacter`" the application program should call the OpenGL transformation (positioning and scaling) functions to set the position of the character in the window. The "`glutStrokeCharacter` " function advances the cursor position by a call to "`glTranslatef` " and so the application does not need to call the OpenGL positioning functions again for successive characters on the same line.

Changes From GLUT

Nonexistent characters are rendered as asterisks.

## 14.7 glutStrokeString

The "`glutStrokeString`" function renders a string of characters in the current window using the specified stroke font.

Usage

`void glutStrokeString ( void *font, char *string ) ;`

`font` The stroke font to use in rendering the character string
`string` String of characters to be rendered

Description

The "`glutStrokeString` " function renders the given character string in the specified stroke font. Before calling "`glutStrokeString`" the application program should call the OpenGL transformation (positioning and scaling) functions to set the position of the string in the window. The "`glutStrokeString` " function handles carriage returns. Nonexistent characters are rendered as asterisks.

Changes From GLUT

GLUT does not include this function.

## 14.8 glutStrokeWidth

The "`glutStrokeWidth`" function returns the width in pixels of a single character in the specified stroke font.

Usage

`int glutStrokeWidth ( void *font, int character ) ;`

`font` The stroke font to use in calculating the character width
`character` The ASCII code of the character

Description

The "`glutStrokeWidth`" function returns the width of the given character in the specified stroke font. Because the font is a stroke font, the width is actually a floating-point number; the function rounds it to the nearest integer for the return value.

Changes From GLUT

Nonexistent characters return the width of an asterisk.

## 14.9 glutStrokeLength

The "`glutStrokeLength`" function returns the width in pixels of a string of characters in the specified stroke font.

Usage

`int glutStrokeLength ( void *font, char *string ) ;`

`font` The stroke font to use in calculating the character width
`string` String of characters whose width is to be calculated

Description

The "`glutStrokeLength` " function returns the width in pixels of the given character string in the specified stroke font. Because the font is a stroke font, the width of an individual character is a floating-point number. Freeglut adds the floating-point widths and rounds the funal result to return the integer value. Thus the return value may differ from the sum of the character widths returned by a series of calls to "`glutStrokeWidth` ". The width of nonexistent characters is counted to be the width of an asterisk.

If the string contains one or more carriage returns, freeglut calculates the widths in pixels of the lines separately and returns the largest width.

Changes From GLUT

GLUT does not include this function.

## 14.10 glutStrokeHeight

The "`glutStrokeHeight`" function returns the height in pixels of the specified font.

Usage

`GLfloat glutStrokeHeight ( void *font ) ;`

`font` The stroke font to use in calculating the character height

Description

The "`glutStrokeHeight` " function returns the height of a character in the specified stroke font. The application programmer should note that, unlike the other freeglut font functions, this one returns a floating-point number. The fonts are designed such that all characters have (nominally) the same height.

Changes From GLUT

GLUT does not include this function.

# 15.0 Geometric Object Rendering Functions

Freeglut includes eighteen routines for generating easily-recognizable 3-d geometric objects. These routines are effectively the same ones that are included in the GLUT library, and reflect the functionality available in the aux toolkit described in the OpenGL Programmer's Guide . They are included to allow programmers to create with a single line of code a three-dimensional object which can be used to test a variety of OpenGL functionality. None of the routines generates a display list for the object which it draws. The functions generate normals appropriate for lighting but, except for the teapon functions, do not generate texture coordinates.

## 15.1 glutWireSphere, glutSolidSphere

The "`glutWireSphere`" and "`glutSolidSphere`" functions draw a wireframe and solid sphere respectively.

Usage

`void glutWireSphere ( GLdouble dRadius, GLint slices, GLint stacks ) ;`

`void glutSolidSphere ( GLdouble dRadius, GLint slices, GLint stacks ) ;`

`dRadius` The desired radius of the sphere

`slices` The desired number of slices (divisions in the longitudinal direction) in the sphere

`stacks` The desired number of stacks (divisions in the latitudinal direction) in the sphere. The number of points in this direction, including the north and south poles, is `stacks+1`

Description

The "`glutWireSphere`" and "`glutSolidSphere`" functions render a sphere centered at the origin of the modeling coordinate system. The north and south poles of the sphere are on the positive and negative Z-axes respectively and the prime meridian crosses the positive X-axis.

Changes From GLUT

None that we know of.

## 15.2 glutWireTorus, glutSolidTorus

The "`glutWireTorus`" and "`glutSolidTorus`" functions draw a wireframe and solid torus (donut shape) respectively.

Usage

`void glutWireTorus ( GLdouble dInnerRadius, GLdouble dOuterRadius, GLint nSides, GLint nRings ) ;`

`void glutSolidTorus ( GLdouble dInnerRadius, GLdouble dOuterRadius, GLint nSides, GLint nRings ) ;`

`dInnerRadius` The desired inner radius of the torus, from the origin to the circle defining the centers of the outer circles

`dOuterRadius` The desired outer radius of the torus, from the center of the outer circle to the actual surface of the torus

`nSides` The desired number of segments in a single outer circle of the torus

`nRings` The desired number of outer circles around the origin of the torus

Description

The "`glutWireTorus`" and "`glutSolidTorus`" functions render a torus centered at the origin of the modeling coordinate system. The torus is circularly symmetric about the Z-axis and starts at the positive X-axis.

Changes From GLUT

None that we know of.

## 15.3 glutWireCone, glutSolidCone

The "`glutWireCone`" and "`glutSolidCone`" functions draw a wireframe and solid cone respectively.

Usage

`void glutWireCone ( GLdouble base, GLdouble height, GLint slices, GLint stacks ) ;`

`void glutSolidCone ( GLdouble base, GLdouble height, GLint slices, GLint stacks ) ;`

`base` The desired radius of the base of the cone

`height` The desired height of the cone

`slices` The desired number of slices around the base of the cone

`stacks` The desired number of segments between the base and the tip of the cone (the number of points, including the tip, is `stacks + 1`)

Description

The "`glutWireCone`" and "`glutSolidCone`" functions render a right circular cone with a base centered at the origin and in the X-Y plane and its tip on the positive Z-axis. The wire cone is rendered with triangular elements.

Changes From GLUT

None that we know of.

## 15.4 glutWireCube, glutSolidCube

The "`glutWireCube`" and "`glutSolidCube`" functions draw a wireframe and solid cube respectively.

Usage

`void glutWireCube ( GLdouble dSize ) ;`

`void glutSolidCube ( GLdouble dSize ) ;`

`dSize` The desired length of an edge of the cube

Description

The "`glutWireCube`" and "`glutSolidCube`" functions render a cube of the desired size, centered at the origin. Its faces are normal to the coordinate directions.

Changes From GLUT

None that we know of.

## 15.5 glutWireTetrahedron, glutSolidTetrahedron

The "`glutWireTetrahedron`" and "`glutSolidTetrahedron`" functions draw a wireframe and solid tetrahedron (four-sided Platonic solid) respectively.

Usage

`void glutWireTetrahedron ( void ) ;`

`void glutSolidTetrahedron ( void ) ;`

Description

The "`glutWireTetrahedron`" and "`glutSolidTetrahedron`" functions render a tetrahedron whose corners are each a distance of one from the origin. The length of each side is 2/3 sqrt(6). One corner is on the positive X-axis and another is in the X-Y plane with a positive Y-coordinate.

Changes From GLUT

None that we know of.

## 15.6 glutWireOctahedron, glutSolidOctahedron

The "`glutWireOctahedron`" and "`glutSolidOctahedron`" functions draw a wireframe and solid octahedron (eight-sided Platonic solid) respectively.

Usage

`void glutWireOctahedron ( void ) ;`

`void glutSolidOctahedron ( void ) ;`

Description

The "`glutWireOctahedron`" and "`glutSolidOctahedron`" functions render an octahedron whose corners are each a distance of one from the origin. The length of each side is sqrt(2). The corners are on the positive and negative coordinate axes.

Changes From GLUT

None that we know of.

## 15.7 glutWireDodecahedron, glutSolidDodecahedron

The "`glutWireDodecahedron`" and "`glutSolidDodecahedron` " functions draw a wireframe and solid dodecahedron (twelve-sided Platonic solid) respectively.

Usage

`void glutWireDodecahedron ( void ) ;`

`void glutSolidDodecahedron ( void ) ;`

Description

The "`glutWireDodecahedron`" and "`glutSolidDodecahedron`" functions render a dodecahedron whose corners are each a distance of sqrt(3) from the origin. The length of each side is sqrt(5)-1. There are twenty corners; interestingly enough, eight of them coincide with the corners of a cube with sizes of length 2.

Changes From GLUT

None that we know of.

## 15.8 glutWireIcosahedron, glutSolidIcosahedron

The "`glutWireIcosahedron`" and "`glutSolidIcosahedron`" functions draw a wireframe and solid icosahedron (twenty-sided Platonic solid) respectively.

Usage

`void glutWireIcosahedron ( void ) ;`

`void glutSolidIcosahedron ( void ) ;`

Description

The "`glutWireIcosahedron`" and "`glutSolidIcosahedron`" functions render an icosahedron whose corners are each a unit distance from the origin. The length of each side is slightly greater than one. Two of the corners lie on the positive and negative X-axes.

Changes From GLUT

None that we know of.

## 15.7 glutWireRhombicDodecahedron, glutSolidRhombicDodecahedron

The "`glutWireRhombicDodecahedron`" and "`glutSolidRhombicDodecahedron` " functions draw a wireframe and solid rhombic dodecahedron (twelve-sided semi-regular solid) respectively.

Usage

`void glutWireRhombicDodecahedron ( void ) ;`

`void glutSolidRhombicDodecahedron ( void ) ;`

Description

The "`glutWireRhombicDodecahedron` " and "`glutSolidRhombicDodecahedron`" functions render a rhombic dodecahedron whose corners are at most a distance of one from the origin. The rhombic dodecahedron has faces which are identical rhombuses (rhombi?) but which have some vertices at which three faces meet and some vertices at which four faces meet. The length of each side is sqrt(3)/2. Vertices at which four faces meet are found at (0, 0, +1) and ( +sqrt(2)/2, +sqrt(2)/2, 0).

Changes From GLUT

GLUT does not include these functions.

## 15.10 glutWireTeapot, glutSolidTeapot

The "`glutWireTeapot`" and "`glutSolidTeapot`" functions draw a wireframe and solid teapot respectively.

Usage

`void glutWireTeapot ( GLdouble dSize ) ;`

`void glutSolidTeapot ( GLdouble dSize ) ;`

`dSize` The desired size of the teapot

Description

The "`glutWireTeapot`" and "`glutSolidTeapot`" functions render a teapot of the desired size, centered at the origin. This is the famous OpenGL teapot [add reference].

Changes From GLUT

None that we know of.

# 16.0 Game Mode Functions

## 16.1 glutGameModeString

## 16.2 glutEnterGameMode, glutLeaveGameMode

## 16.3 glutGameModeGet

# 17.0 Video Resize Functions

## 17.1 glutVideoResizeGet

## 17.2 glutSetupVideoResizing, glutStopVideoResizing

## 17.3 glutVideoResize

## 17.4 glutVideoPan

# 18.0 Color Map Functions

## 18.1 glutSetColor, glutGetColor

## 18.2 glutCopyColormap

# 19.0 Miscellaneous Functions

## 19.1 glutIgnoreKeyRepeat, glutSetKeyRepeat

## 19.2 glutForceJoystickFunc

## 19.3 glutReportErrors

# 20.0 Usage Notes

The following environment variables are recognized by freeglut:

- DISPLAY - specifies a display name.
- GLUT_FPS - specifies a time interval (in milliseconds) for reporting framerate messages to stderr. For example, if FREEGLUT_FPS is set to 5000, every 5 seconds a message will be printed to stderr showing the current frame rate. The frame rate is measured by counting the number of times glutSwapBuffers() is called over the time interval.
- GLUT_ICON - specifies the icon that goes in the upper left-hand corner of the freeglut windows

# 21.0 Implementation Notes

# 22.0 GLUT State

# 23.0 "freeglut.h" Header File

Application programmers who are porting their GLUT programs to freeglut may continue to include `<GL/glut.h>` in their programs. Programs which use the freeglut-specific extensions to GLUT should include `<GL/freeglut.h>`. One possible arrangement is as follows:

    #ifdef FREEGLUT
    #include <GL/freeglut_ext.h>
    #else
    #include <GL/glut.h>
    #endif

Compile-time freeglut version testing can be done as follows:

    #ifdef FREEGLUT_VERSION_2_0
      code specific to freeglut 2.0 or later here
    #endif

In future releases, FREEGLUT_VERSION_2_1, FREEGLUT_VERSION_2_2, etc will be defined. This scheme mimics OpenGL conventions.

The freeglut version can be queried at runtime by calling glutGet(GLUT_VERSION). The result will be X*10000+Y*100+Z where X is the major version, Y is the minor version and Z is the patch level.

This may be used as follows:

    if (glutGet(GLUT_VERSION) < 20001) {
        printf("Sorry, you need freeglut version 2.0.1 or later to run this program.\n");
        exit(1);
    }

# 24.0 References

# 25.0 Index