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8 - Using OpenGL

This chapter discusses using FLTK for your OpenGL applications.

Using OpenGL in FLTK

The easiest way to make an OpenGL display is to subclass Fl_Gl_Window. Your subclass must implement a draw() method which uses OpenGL calls to draw the display. Your main program should call redraw() when the display needs to change, and (somewhat later) FLTK will call draw().

With a bit of care you can also use OpenGL to draw into normal FLTK windows. This allows you to use Gouraud shading for drawing your widgets. To do this you use the gl_start() and gl_finish() functions around your OpenGL code.

You must include FLTK's <FL/gl.h> header file. It will include the file <GL/gl.h>, define some extra drawing functions provided by FLTK, and include the <windows.h> header file needed by WIN32 applications.

Making a Subclass of Fl_Gl_Window

To make a subclass of Fl_Gl_Window, you must provide:

  • A class definition.
  • A draw() method.
  • A handle() method if you need to receive input from the user.

If your subclass provides static controls in the window, they must be redrawn whenever the FL_DAMAGE_ALL bit is set in the value returned by damage(). For double-buffered windows you will need to surround the drawing code with the following code to make sure that both buffers are redrawn:

    #ifndef MESA
    #endif // !MESA
    ... draw stuff here ...
    #ifndef MESA
    #endif // !MESA

If you are using the Mesa graphics library, the call to glDrawBuffer() is not required and will slow down drawing considerably. The preprocessor instructions shown above will optimize your code based upon the graphics library used.

Defining the Subclass

To define the subclass you just subclass the Fl_Gl_Window class:

    class MyWindow : public Fl_Gl_Window {
      void draw();
      int handle(int);
      MyWindow(int X, int Y, int W, int H, const char *L)
        : Fl_Gl_Window(X, Y, W, H, L) {}

The draw() and handle() methods are described below. Like any widget, you can include additional private and public data in your class (such as scene graph information, etc.)

The draw() Method

The draw() method is where you actually do your OpenGL drawing:

    void MyWindow::draw() {
      if (!valid()) {
        ... set up projection, viewport, etc ...
        ... window size is in w() and h().
        ... valid() is turned on by FLTK after draw() returns
      ... draw ...

The handle() Method

The handle() method handles mouse and keyboard events for the window:

    int MyWindow::handle(int event) {
      switch(event) {
      case FL_PUSH:
        ... mouse down event ...
        ... position in Fl::event_x() and Fl::event_y()
        return 1;
      case FL_DRAG:
        ... mouse moved while down event ...
        return 1;
      case FL_RELEASE:   
        ... mouse up event ...
        return 1;
      case FL_FOCUS :
      case FL_UNFOCUS :
        ... Return 1 if you want keyboard events, 0 otherwise
        return 1;
      case FL_KEYBOARD:
        ... keypress, key is in Fl::event_key(), ascii in Fl::event_text()
        ... Return 1 if you understand/use the keyboard event, 0 otherwise...
        return 1;
      case FL_SHORTCUT:
        ... shortcut, key is in Fl::event_key(), ascii in Fl::event_text()
        ... Return 1 if you understand/use the shortcut event, 0 otherwise...
        return 1;
        // pass other events to the base class...
        return Fl_Gl_Window::handle(event);

When handle() is called, the OpenGL context is not set up! If your display changes, you should call redraw() and let draw() do the work. Don't call any OpenGL drawing functions from inside handle()!

You can call some OpenGL stuff like hit detection and texture loading functions by doing:

      case FL_PUSH:
        make_current(); // make OpenGL context current
        if (!valid()) {
          ... set up projection exactly the same as draw ...
          valid(1); // stop it from doing this next time
        ... ok to call NON-DRAWING OpenGL code here, such as hit
        detection, loading textures, etc...

Your main program can now create one of your windows by doing new MyWindow(...). You can also use FLUID by:

  1. Putting your class definition in a MyWindow.H file.
  2. Creating a Fl_Box widget in FLUID.
  3. In the widget panel fill in the "class" field with MyWindow. This will make FLUID produce constructors for your new class.
  4. In the "Extra Code" field put #include "MyWindow.H", so that the FLUID output file will compile.

You must put glwindow->show() in your main code after calling show() on the window containing the OpenGL window.

Using OpenGL in Normal FLTK Windows

You can put OpenGL code into an Fl_Widget::draw() method or into the code for a boxtype or other places with some care.

Most importantly, before you show any windows, including those that don't have OpenGL drawing, you must initialize FLTK so that it knows it is going to use OpenGL. You may use any of the symbols described for Fl_Gl_Window::mode() to describe how you intend to use OpenGL:


You can then put OpenGL drawing code anywhere you can draw normally by surrounding it with:

    ... put your OpenGL code here ...

gl_start() and gl_finish() set up an OpenGL context with an orthographic projection so that 0,0 is the lower-left corner of the window and each pixel is one unit. The current clipping is reproduced with OpenGL glScissor() commands. These functions also synchronize the OpenGL graphics stream with the drawing done by other X, WIN32, or FLTK functions.

The same context is reused each time. If your code changes the projection transformation or anything else you should use glPushMatrix() and glPopMatrix() functions to put the state back before calling gl_finish().

You may want to use Fl_Window::current()->h() to get the drawable height so that you can flip the Y coordinates.

Unfortunately, there are a bunch of limitations you must adhere to for maximum portability:

  • You must choose a default visual with Fl::gl_visual().
  • You cannot pass FL_DOUBLE to Fl::gl_visual().
  • You cannot use Fl_Double_Window or Fl_Overlay_Window.

Do not call gl_start() or gl_finish() when drawing into an Fl_Gl_Window!

OpenGL Drawing Functions

FLTK provides some useful OpenGL drawing functions. They can be freely mixed with any OpenGL calls, and are defined by including <FL/gl.H> which you should include instead of the OpenGL header <GL/gl.h>.

void gl_color(Fl_Color)

Sets the current OpenGL color to a FLTK color. For color-index modes it will use fl_xpixel(c), which is only right if this window uses the default colormap!

void gl_rect(int x, int y, int w, int h)
void gl_rectf(int x, int y, int w, int h)

Outlines or fills a rectangle with the current color. If Fl_Gl_Window::ortho() has been called, then the rectangle will exactly fill the pixel rectangle passed.

void gl_font(Fl_Font fontid, int size)

Sets the current OpenGL font to the same font you get by calling fl_font().

int gl_height()
int gl_descent()
float gl_width(const char *)
float gl_width(const char *, int n)
float gl_width(uchar)

Returns information about the current OpenGL font.

void gl_draw(const char *)
void gl_draw(const char *, int n)

Draws a nul-terminated string or an array of n characters in the current OpenGL font at the current raster position.

void gl_draw(const char *, int x, int y)
void gl_draw(const char *, int n, int x, int y)
void gl_draw(const char *, float x, float y)
void gl_draw(const char *, int n, float x, float y)

Draws a nul-terminated string or an array of n characters in the current OpenGL font at the given position.

void gl_draw(const char *, int x, int y, int w, int h, Fl_Align)

Draws a string formatted into a box, with newlines and tabs expanded, other control characters changed to ^X, and aligned with the edges or center. Exactly the same output as fl_draw().

Speeding up OpenGL

Performance of Fl_Gl_Window may be improved on some types of OpenGL implementations, in particular MESA and other software emulators, by setting the GL_SWAP_TYPE environment variable. This variable declares what is in the backbuffer after you do a swapbuffers.

  • setenv GL_SWAP_TYPE COPY

    This indicates that the back buffer is copied to the front buffer, and still contains it's old data. This is true of many hardware implementations. Setting this will speed up emulation of overlays, and widgets that can do partial update can take advantage of this as damage() will not be cleared to -1.


    This indicates that nothing changes the back buffer except drawing into it. This is true of MESA and Win32 software emulation and perhaps some hardware emulation on systems with lots of memory.

  • All other values for GL_SWAP_TYPE, and not setting the variable, cause FLTK to assume that the back buffer must be completely redrawn after a swap.

This is easily tested by running the gl_overlay demo program and seeing if the display is correct when you drag another window over it or if you drag the window off the screen and back on. You have to exit and run the program again for it to see any changes to the environment variable.

Using OpenGL Optimizer with FLTK

OpenGL Optimizer is a scene graph toolkit for OpenGL available from Silicon Graphics for IRIX and Microsoft Windows. It allows you to view large scenes without writing a lot of OpenGL code.

OptimizerWindow Class Definition

To use OpenGL Optimizer with FLTK you'll need to create a subclass of Fl_Gl_Widget that includes several state variables:

    class OptimizerWindow : public Fl_Gl_Window {
      csContext *context_; // Initialized to 0 and set by draw()...
      csDrawAction *draw_action_; // Draw action...
      csGroup *scene_; // Scene to draw...
      csCamara *camera_; // Viewport for scene...
      void draw();
      OptimizerWindow(int X, int Y, int W, int H, const char *L)
        : Fl_Gl_Window(X, Y, W, H, L) {
          context_ = (csContext *)0;
          draw_action_ = (csDrawAction *)0;
          scene_ = (csGroup *)0;
          camera_ = (csCamera *)0;
      void scene(csGroup *g) { scene_ = g; redraw(); }
      void camera(csCamera *c) {
        camera_ = c;
        if (context_) {

The camera() Method

The camera() method sets the camera (projection and viewpoint) to use when drawing the scene. The scene is redrawn after this call.

The draw() Method

The draw() method performs the needed initialization and does the actual drawing:

    void OptimizerWindow::draw() {
      if (!context_) {
        // This is the first time we've been asked to draw; create the
        // Optimizer context for the scene...
    #ifdef WIN32
        context_ = new csContext((HDC)fl_getHDC());
        context_ = new csContext(fl_display, fl_visual);
        context_->makeCurrent(fl_display, fl_window);
    #endif // WIN32
        ... perform other context setup as desired ...   
        // Then create the draw action to handle drawing things...
        draw_action_ = new csDrawAction;
        if (camera_) {
      } else {
    #ifdef WIN32
        context_->makeCurrent(fl_display, fl_window);
    #endif // WIN32
      if (!valid()) {
        // Update the viewport for this context...
        context_->setViewport(0, 0, w(), h());
      // Clear the window...
      context_->clear(csContext::COLOR_CLEAR | csContext::DEPTH_CLEAR,
                      0.0f,		// Red
    		  0.0f,		// Green
    		  0.0f,		// Blue
    		  1.0f);	// Alpha
      // Then draw the scene (if any)...
      if (scene_)

The scene() Method

The scene() method sets the scene to be drawn. The scene is a collection of 3D objects in a csGroup. The scene is redrawn after this call.

User Comments [ Add Comment ]

From greg.ercolano, 06:42 Nov 18, 2005 (score=3)

If it's the case that FLTK widgets can't be children of an Fl_Gl_Window, that should probably be mentioned here.
Reply ]

From Anonymous, 11:01 Sep 10, 2004 (score=1)

gl_start and gl_end are deprecated and don't work properly on OSX
Reply ]


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