Add cross-platform support for adding widgets to an OpenGL3-based Fl_Gl_Window.

Under non-macOS platforms, the key is to call glUseProgram(0); after having used OpenGL 3
which allows to then use OpenGL 1 and draw FLTK widgets over the OpenGL3 scene.

Under macOS, this is impossible because macOS GL3 contexts are not compatible
with GL1. The solution implemented here is to create an additional Fl_Gl_Window
placed above and sized as the GL3-based window, to give it a non opaque,
GL1-based context, and to put the FLTK widgets in that additional window.

1 files changed, 31 insertions, 1 deletions

diff --git a/examples/OpenGL3test.cxx b/examples/OpenGL3test.cxx
index 9acbaf38b..26ee1b816 100644
--- a/examples/OpenGL3test.cxx
+++ b/examples/OpenGL3test.cxx
@@ -138,6 +138,13 @@ public:
     GLfloat p[]={0,0};
     glUniform2fv(positionUniform, 1, (const GLfloat *)&p);
     glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
+#ifndef __APPLE__
+    // suggested by https://stackoverflow.com/questions/22293870/mixing-fixed-function-pipeline-and-programmable-pipeline-in-opengl
+    // Switch from GL3-style to GL1-style drawing;
+    // good under Windows, X11 and Wayland; impossible under macOS.
+    glUseProgram(0);
+    Fl_Gl_Window::draw(); // Draw FLTK child widgets.
+#endif
   }
   virtual int handle(int event) {
     static int first = 1;
@@ -161,6 +168,9 @@ public:
       redraw();
     }
 
+    int retval = Fl_Gl_Window::handle(event);
+    if (retval) return retval;
+    
     if (event == FL_PUSH && gl_version_major >= 3) {
       static float factor = 1.1;
       GLfloat data[4];
@@ -175,7 +185,7 @@ public:
       add_output("push  Fl_Gl_Window::pixels_per_unit()=%.1f\n", pixels_per_unit());
       return 1;
     }
-    return Fl_Gl_Window::handle(event);
+    return retval;
   }
   void reset(void) { shaderProgram = 0; }
 };
@@ -218,6 +228,25 @@ void add_output(const char *format, ...)
 }
 
 
+void button_cb(Fl_Widget *, void *) {
+  add_output("run button callback\n");
+}
+
+void add_widgets(Fl_Gl_Window *g) {
+#ifdef __APPLE__
+  g = fl_mac_prepare_add_widgets_to_GL3_win(g);
+#endif
+  Fl::set_color(FL_FREE_COLOR, 255, 0, 0, 140); // partially transparent red
+  g->begin();
+  // Create here widgets to go above the GL3 scene
+  Fl_Button* b = new Fl_Button( 0, 170, 60, 30, "button");
+  b->color(FL_FREE_COLOR);
+  b->box(FL_BORDER_BOX );
+  b->callback(button_cb, NULL);
+  g->end();
+}
+
+
 int main(int argc, char **argv)
 {
   Fl::use_high_res_GL(1);
@@ -225,6 +254,7 @@ int main(int argc, char **argv)
   SimpleGL3Window *win = new SimpleGL3Window(0, 0, 300, 300);
   win->end();
   output_win(win);
+  add_widgets(win);
   topwin->end();
   topwin->resizable(win);
   topwin->label("Click GL panel to reshape");