Add initial prototype.

4-high/gel/applet/demo/sprite/chains_2d/chains_2d.gpr

@@ -0,0 +1,17 @@
+with
+     "gel",
+     "lace_shared";
+
+project Chains_2D
+is
+   for Object_Dir use "build";
+   for Exec_Dir   use ".";
+   for Main       use ("launch_chains_2d.adb");
+   for Languages  use ("Ada");
+
+   package Ide      renames Lace_shared.Ide;
+   package Builder  renames Lace_shared.Builder;
+   package Compiler renames Lace_shared.Compiler;
+   package Binder   renames Lace_shared.Binder;
+
+end Chains_2D;

4-high/gel/applet/demo/sprite/chains_2d/launch_chains_2d.adb

@@ -0,0 +1,73 @@
+with
+     gel.Window.sdl,
+     gel.Applet.gui_world,
+     gel.Forge,
+     gel.Sprite,
+     gel.Joint,
+
+     Physics,
+     openGL.Palette;
+
+pragma unreferenced (gel.Window.sdl);
+
+
+procedure launch_Chains_2d
+--
+--  Creates a chain of balls in a 2D space.
+--
+is
+   use gel.Forge,
+       gel.Applet,
+       gel.Math,
+       opengl.Palette;
+
+   the_Applet : gel.Applet.gui_World.view := new_gui_Applet ("Chains 2D",
+                                                             1536, 864,
+                                                             space_Kind => physics.Box2D);
+
+   the_Ground : constant gel.Sprite.view  := new_rectangle_Sprite (the_Applet.gui_World,
+                                                                   Mass   =>   0.0,
+                                                                   Width  => 100.0,
+                                                                   Height =>   1.0,
+                                                                   Color  => apple_Green);
+begin
+   the_Applet.gui_World .Gravity_is    ([0.0, -10.0,   0.0]);
+   the_Applet.gui_Camera.Site_is       ([0.0, -30.0, 100.0]);
+   the_Applet.Renderer  .Background_is (Grey);
+   the_Applet.enable_simple_Dolly      (in_World => gui_World.gui_world_Id);
+
+   the_Ground.Site_is ([0.0, -40.0, 0.0]);
+   the_Applet.gui_World.add (the_Ground, and_Children => False);
+
+   --  Add joints.
+   --
+   declare
+      ball_Count    : constant := 39;
+      the_root_Ball : constant gel.Sprite.view  :=                     new_circle_Sprite (the_Applet.gui_World, Mass =>  0.0);
+      the_Balls     : constant gel.Sprite.views := [1 .. ball_Count => new_circle_Sprite (the_Applet.gui_World, Mass =>  1.0)];
+
+      Parent    : gel.Sprite.view := the_root_Ball;
+      new_Joint : gel.Joint .view;
+   begin
+      for i in the_Balls'Range
+      loop
+         the_Balls (i).Site_is ([Real (-i),  0.0,  0.0]);
+
+         Parent.attach_via_Hinge (the_Child  => the_Balls (i),
+                                  pivot_Axis => [0.0, 0.0, 1.0],
+                                  low_Limit  => to_Radians (-180.0),
+                                  high_Limit => to_Radians ( 180.0),
+                                  new_joint  => new_Joint);
+         Parent := the_Balls (i);
+      end loop;
+
+      the_Applet.gui_World.add (the_root_Ball, and_Children => True);
+   end;
+
+   while the_Applet.is_open
+   loop
+      the_Applet.freshen;     -- Handle any new events, evolve physics and update the screen.
+   end loop;
+
+   gel.Applet.gui_world.free (the_Applet);
+end launch_Chains_2d;