class Fl_Gl_Window


class Fl_Gl_Window -- OpenGL widget

Class Hierarchy

           +----Fl_Pack, Fl_Scroll, Fl_Tabs, Fl_Tile, Fl_Window

Include Files

#include <FL/Fl_Gl_Window.H>


The Fl_Gl_Window widget sets things up so OpenGL works, and also keeps an OpenGL "context" for that window, so that changes to the lighting and projection may be reused between redraws. Fl_Gl_Window also flushes the OpenGL streams and swaps buffers after draw() returns.

OpenGL hardware typically provides some overlay bit planes, which are very useful for drawing UI controls atop your 3D graphics. If the overlay hardware is not provided, FLTK tries to simulate the overlay, This works pretty well if your graphics are double buffered, but not very well for single-buffered.


Fl_Gl_Window::Fl_Gl_Window(int x, 
int y, int w, int h, const char *label = 0)

Creates a new Fl_Gl_Window widget using the given position, size, and label string. The default boxtype is FL_NO_BOX. The default mode is FL_RGB|FL_DOUBLE|FL_DEPTH.

virtual Fl_Gl_Window::~Fl_Gl_Window()

The destructor removes the widget and destroys the OpenGL context associated with it.

virtual void Fl_Gl_Window::draw(void)

Fl_Gl_Window::draw() is a pure virtual method. You must subclass Fl_Gl_Window and provide an implementation for draw(). You may also provide an implementation of draw_overlay() if you want to draw into the overlay planes. You can avoid reinitializing the viewport and lights and other things by checking valid() at the start of draw() and only doing the initialization if it is false.

The draw() method can only use OpenGL calls. Do not attempt to call X, any of the functions in <FL/fl_draw.H>, or glX directly. Do not call gl_start() or gl_finish().

If double-buffering is enabled in the window, the back and front buffers are swapped after this function is completed.

const int Fl_Gl_Window::mode() const
int Fl_Gl_Window::mode(int m)

Set or change the OpenGL capabilites of the window. The value can be any of the following OR'd together:

FL_RGB and FL_SINGLE have a value of zero, so they are "on" unless you give FL_INDEX or FL_DOUBLE.

If the desired combination cannot be done, FLTK will try turning off FL_MULTISAMPLE. If this also fails the show() will call Fl::error() and not show the window.

You can change the mode while the window is displayed. This is most useful for turning double-buffering on and off. Under X this will cause the old X window to be destroyed and a new one to be created. If this is a top-level window this will unfortunately also cause the window to blink, raise to the top, and be de-iconized, and the xid() will change, possibly breaking other code. It is best to make the GL window a child of another window if you wish to do this!

static int Fl_Gl_Window::can_do(int)
int Fl_Gl_Window::can_do() const

Returns non-zero if the hardware supports the given or current OpenGL mode.

char Fl_Gl_Window::valid() const
void Fl_Gl_Window::valid(char i)

Fl_Gl_Window::valid() is turned off when FLTK creates a new context for this window or when the window resizes, and is turned on after draw() is called. You can use this inside your draw() method to avoid unneccessarily initializing the OpenGL context. Just do this:

void mywindow::draw() {
  if (!valid()) {
    ...other initialization...
  ... draw your geometry here ...

You can turn valid() on by calling valid(1). You should only do this after fixing the transformation inside a draw() or after make_current(). This is done automatically after draw() returns.

void Fl_Gl_Window::invalidate()

The invalidate() method turns off valid() and is equivalent to calling value(0).

void Fl_Gl_Window::ortho()

Set the projection so 0,0 is in the lower left of the window and each pixel is 1 unit wide/tall. If you are drawing 2D images, your draw() method may want to call this if valid() is false.

void Fl_Gl_Window::make_current()

The make_current() method selects the OpenGL context for the widget. It is called automatically prior to the draw() method being called and can also be used to implement feedback and/or selection within the handle() method.


The make_overlay_current() method selects the OpenGL context for the widget's overlay. It is called automatically prior to the draw_overlay() method being called and can also be used to implement feedback and/or selection within the handle() method.

void Fl_Gl_Window::swap_buffers()

The swap_buffers() method swaps the back and front buffers. It is called automatically after the draw() method is called.

void Fl_Gl_Window::hide()

Hides the window and destroys the OpenGL context.

int Fl_Gl_Window::can_do_overlay()

Returns true if the hardware overlay is possible. If this is false, FLTK will try to simulate the overlay, with significant loss of update speed. Calling this will cause FLTK to open the display.

void Fl_Gl_Window::redraw_overlay()

This method causes draw_overlay to be called at a later time. Initially the overlay is clear, if you want the window to display something in the overlay when it first appears, you must call this immediately after you show() your window.

virtual void 

You must implement this virtual function if you want to draw into the overlay. The overlay is cleared before this is called. You should draw anything that is not clear using OpenGL. You must use gl_color(i) to choose colors (it allocates them from the colormap using system-specific calls), and remember that you are in an indexed OpenGL mode and drawing anything other than flat-shaded will probably not work.

Both this function and Fl_Gl_Window::draw() should check Fl_Gl_Window::valid() and set the same transformation. If you don't your code may not work on other systems. Depending on the OS, and on whether overlays are real or simulated, the OpenGL context may be the same or different between the overlay and main window.