Add gl_image example program to illustrate drawing images to an Fl_Gl_Window.

documentation/src/examples.dox

@@ -123,6 +123,12 @@ you build FLTK, unlike those in the 'test' directory shown below.
 <td> \ref examples_valuators       </td>
 <td> \ref examples_windowfocus     </td>
 </tr>
+<tr>
+<td colspan=6 align=center><b>New in FLTK 1.5</b></td>
+</tr>
+<tr>
+<td> \ref examples_gl_image        </td>
+</tr>
 </table>
 
 
@@ -370,6 +376,12 @@ and double-buffered rendering, and take over the entire
 screen. More information in the source code.
 
 
+\subsection examples_gl_image gl_image
+
+\par
+\c gl_image shows how to draw most images supported by FLTK
+in an Fl_Gl_Window. Fl_Bitmap and depth 1 and 2 Fl_RGB_Image aren't supported yet.
+
 \subsection examples_gl_overlay gl_overlay
 
 \par

test/CMakeLists.txt

@@ -273,6 +273,7 @@ if(OPENGL_FOUND)
   if(NOT FLTK_BUILD_SHARED_LIBS)
     fl_create_example(glpuzzle glpuzzle.cxx "${GLDEMO_LIBS}")
   endif()
+  fl_create_example(gl_image gl_image.cxx "${GLDEMO_LIBS};fltk::images")
   fl_create_example(gl_overlay gl_overlay.cxx "${GLDEMO_LIBS}")
   fl_create_example(shape shape.cxx "${GLDEMO_LIBS}")
 endif(OPENGL_FOUND)

test/gl_image.cxx

@@ -0,0 +1,111 @@
+//
+// OpenGL image-drawing test program for the Fast Light Tool Kit (FLTK).
+//
+// Copyright 2026 by Bill Spitzak and others.
+//
+// This library is free software. Distribution and use rights are outlined in
+// the file "COPYING" which should have been included with this file.  If this
+// file is missing or damaged, see the license at:
+//
+//     https://www.fltk.org/COPYING.php
+//
+// Please see the following page on how to report bugs and issues:
+//
+//     https://www.fltk.org/bugs.php
+//
+
+#include <FL/Fl_Gl_Window.H>
+#include <FL/Fl_Shared_Image.H>
+#include <FL/Fl_File_Chooser.H>
+#include <FL/Fl_Menu_Bar.H>
+#include <FL/gl.h>
+
+class image_background_box : public Fl_Widget {
+public:
+  image_background_box(int x, int y, int w, int h) : Fl_Widget(x, y, w, h, NULL) {}
+  void draw() {
+    gl_color(FL_WHITE);
+    gl_rectf(x(), y(), w(), h());
+    gl_color(color());
+    int startX;
+    for (int Y = y(); Y < y()+h(); Y += 5) {
+      startX = Y % 10;
+      for (int X = startX; X < w(); X += 10) {
+        gl_rectf(X, Y, 5, 5);
+      }
+    }
+    if (image()) image()->draw((w() - image()->w())/2, y() + (h() - image()->h())/2);
+  }
+};
+
+
+void chooser_cb(Fl_Widget *, class Gl_Image_Window *mainwin);
+
+void close_cb(Fl_Widget *, void *) {
+  Fl::first_window()->hide();
+}
+
+
+Fl_Menu_Item items[] = {
+  { "File", 0, 0, 0, FL_SUBMENU},
+  { "Choose image file…", 0, (Fl_Callback*)chooser_cb},
+  { "Quit", 0, close_cb},
+  {0},
+  {0},
+};
+
+
+class Gl_Image_Window : public Fl_Gl_Window {
+  const Fl_Image *img_;
+  image_background_box *box_;
+public:
+  Gl_Image_Window(int w, int h, const char *t) : Fl_Gl_Window(w, h, t) {
+    img_ = NULL;
+    begin();
+    Fl_Menu_Bar *bar = new Fl_Menu_Bar(0, 0, w, 30);
+    bar->menu(items);
+    items[1].user_data(this);
+    box_ = new image_background_box(0, 30, w, h - 30);
+    box_->color(FL_GRAY);
+    end();
+    resizable(box_);
+  }
+
+  void set_image(const Fl_Image *img) {
+    if (img_) ((Fl_Image*)img_)->release();
+    img_ = img;
+    box_->image((Fl_Image*)img);
+    ((Fl_Image*)img)->scale(box_->w(), box_->h());
+    redraw();
+  }
+
+  void resize(int x, int y, int w, int h) override {
+    Fl_Gl_Window::resize(x,y,w,h);
+    if (img_) ((Fl_Image*)img_)->scale(box_->w(), box_->h());
+  }
+
+  void draw() override {
+    Fl_Gl_Window::draw(); // Draw FLTK child widgets.
+  }
+};
+
+
+void chooser_cb(Fl_Widget *, Gl_Image_Window *mainwin) {
+  char *fname = fl_file_chooser("select image file", "*.{png,jpg,gif,svg,svgz}", NULL, 0);
+  if (fname) {
+    Fl_Shared_Image *shared = Fl_Shared_Image::get(fname);
+    if (!shared->fail()) {
+      mainwin->copy_label(fname);
+      mainwin->set_image(shared);
+    }
+  }
+}
+
+
+int main(int argc, char **argv) {
+  Fl::use_high_res_GL(1);
+  fl_register_images();
+  Gl_Image_Window mainwin(600, 600, "GL Image Viewer");
+  mainwin.show(argc, argv);
+  return Fl::run();
+}