FLTK + OpenGL - rectangle not drawn

Question

I used OpenGL for a while and now I wanted to use it in conjunction with FLTK. I subclassed Fl_Gl_Window and implemented the draw method:

void MainWindow::draw()
{
    if(!valid())
    {
        // Setup
        glEnable(GL_DEPTH_TEST);
        glViewport(0,0,w(),h()); // At runtime W()=800, h()=600

        glMatrixMode(GL_PROJECTION);
        glLoadIdentity();
        glOrtho(0,w(),0,h(),1,1000);

    }
    gl_color(FL_BLUE);
    gl_rect(0,0,w(),h());
}

I've read that in this way, there's not even the need to clear the color before drawing, and to swap the buffer (it's a double buffered window). Indeed I see that the window is by default black, without needing to call glClear and glColor. However, the rectangle doesn't get drawn and I get a totally black window.

This is how I instantiate the window:

int main(int argc, char **argv)
{
    MainWindow* w= new MainWindow(50,100,width,height,"Test Window");
    w->end();
    w->show(argc,argv);
    return Fl::run();
}

Answer 1

Probable causes of drawn object not showing on the screen:

• object's not attach()-ed to the window object.
• object's coordinates outside the window.