FLTK 1.4.0
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GLUT Compatibility

This appendix describes the GLUT compatibility header file supplied with FLTK.

FLTK's GLUT compatibility is based on the original GLUT 3.7 and the follow-on FreeGLUT 2.4.0 libraries.

Using the GLUT Compatibility Header File

You should be able to compile existing GLUT source code by including <FL/glut.H> instead of <GL/glut.h>. This can be done by editing the source, by changing the -I switches to the compiler, or by providing a symbolic link from GL/glut.h to FL/glut.H.

All files calling GLUT procedures must be compiled with C++. You may have to alter them slightly to get them to compile without warnings, and you may have to rename them to get make to use the C++ compiler.

You must link with the FLTK library. Most of FL/glut.H is inline functions. You should take a look at it (and maybe at test/glpuzzle.cxx in the FLTK source) if you are having trouble porting your GLUT program.

This has been tested with most of the demo programs that come with the GLUT and FreeGLUT distributions.

Known Problems

The following functions and/or arguments to functions are missing, and you will have to replace them or comment them out for your code to compile:

  • glutInitDisplayMode(GLUT_LUMINANCE)
  • glutKeyboardUpFunc(void(*callback)(unsigned char key, int x, int y))
  • glutLayerGet(GLUT_HAS_OVERLAY)
  • glutLayerGet(GLUT_LAYER_IN_USE)
  • glutPushWindow()
  • glutSetColor(), glutGetColor(), glutCopyColormap()
  • glutVideoResize() missing.
  • glutWarpPointer()
  • glutWindowStatusFunc()
  • Spaceball, buttonbox, dials, and tablet functions

Most of the symbols/enumerations have different values than GLUT uses. This will break code that relies on the actual values. The only symbols guaranteed to have the same values are true/false pairs like GLUT_DOWN and GLUT_UP, mouse buttons GLUT_LEFT_BUTTON, GLUT_MIDDLE_BUTTON, GLUT_RIGHT_BUTTON, and GLUT_KEY_F1 thru GLUT_KEY_F12.

The strings passed as menu labels are not copied.

glutPostRedisplay() does not work if called from inside a display function. You must use glutIdleFunc() if you want your display to update continuously.

glutSwapBuffers() does not work from inside a display function. This is on purpose, because FLTK swaps the buffers for you.

glutUseLayer() does not work well, and should only be used to initialize transformations inside a resize callback. You should redraw overlays by using glutOverlayDisplayFunc().

Overlays are cleared before the overlay display function is called. glutLayerGet(GLUT_OVERLAY_DAMAGED) always returns true for compatibility with some GLUT overlay programs. You must rewrite your code so that gl_color() is used to choose colors in an overlay, or you will get random overlay colors.

glutSetCursor(GLUT_CURSOR_FULL_CROSSHAIR) just results in a small crosshair.

The fonts used by glutBitmapCharacter() and glutBitmapWidth() may be different.

glutInit(argc,argv) will consume different switches than GLUT does. It accepts the switches recognized by Fl::args(), and will accept any abbreviation of these switches (such as "-di" for "-display").

Mixing GLUT and FLTK Code

You can make your GLUT window a child of a Fl_Window with the following scheme. The biggest trick is that GLUT insists on a call to show() the window at the point it is created, which means the Fl_Window parent window must already be shown.

  • Don't call glutInit().
  • Create your Fl_Window, and any FLTK widgets. Leave a blank area in the window for your GLUT window.
  • show() the Fl_Window. Perhaps call show(argc,argv).
  • Call window->begin() so that the GLUT window will be automatically added to it.
  • Use glutInitWindowSize() and glutInitWindowPosition() to set the location in the parent window to put the GLUT window.
  • Put your GLUT code next. It probably does not need many changes. Call window->end() immediately after the glutCreateWindow()!
  • You can call either glutMainLoop(), Fl::run(), or loop calling Fl::wait() to run the program.

class Fl_Glut_Window

Class Hierarchy

The Fl_Gl_Window widget sets things up so OpenGL works.
Definition Fl_Gl_Window.H:56
GLUT is emulated using this window class and these static variables (plus several more static variabl...
Definition glut.H:53

Include Files

#include <FL/glut.H>


Each GLUT window is an instance of this class. You may find it useful to manipulate instances directly rather than use GLUT window id's. These may be created without opening the display, and thus can fit better into FLTK's method of creating windows.

The current GLUT window is available in the global variable glut_window.

new Fl_Glut_Window(...) is the same as glutCreateWindow() except it does not show() the window or make the window current.

window->make_current() is the same as glutSetWindow(number). If the window has not had show() called on it yet, some functions that assume an OpenGL context will not work. If you do show() the window, call make_current() again to set the context.

~Fl_Glut_Window() is the same as glutDestroyWindow().


The Fl_Glut_Window class contains several public members that can be altered directly:

member description
display A pointer to the function to call to draw the normal planes.
entry A pointer to the function to call when the mouse moves into or out of the window.
keyboard A pointer to the function to call when a regular key is pressed.
menu[3] The menu to post when one of the mouse buttons is pressed.
mouse A pointer to the function to call when a button is pressed or released.
motion A pointer to the function to call when the mouse is moved with a button down.
overlaydisplay A pointer to the function to call to draw the overlay planes.
passivemotion A pointer to the function to call when the mouse is moved with no buttons down.
reshape A pointer to the function to call when the window is resized.
special A pointer to the function to call when a special key is pressed.
visibility A pointer to the function to call when the window is iconified or restored (made visible.)


Fl_Glut_Window::Fl_Glut_Window(int x, int y, int w, int h, const char *title = 0)
Fl_Glut_Window::Fl_Glut_Window(int w, int h, const char *title = 0)

The first constructor takes 4 int arguments to create the window with a preset position and size. The second constructor with 2 arguments will create the window with a preset size, but the window manager will choose the position according to its own whims.

virtual Fl_Glut_Window::~Fl_Glut_Window()

Destroys the GLUT window.

void Fl_Glut_Window::make_current()

Switches all drawing functions to the GLUT window.

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