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)
Set the current 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)
Outline or fill 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)
Set 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)
Return information about the current OpenGL font.
void gl_draw(const char *) void gl_draw(const char *, int n)
Draw 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)
Draw 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)
Draw 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().