Add initial prototype.

3-mid/opengl/source/lean/model/opengl-model-billboard.ads

@@ -0,0 +1,57 @@
+package openGL.Model.billboard
+--
+--  Models a rectangle capable of displaying an image.
+--
+is
+   type Item   is abstract new Model.item with private;
+
+   type Plane  is (xy, xz, yz);
+
+   type Size_t is
+      record
+         Width  : Real;
+         Height : Real;
+      end record;
+
+   type Coordinates is array (1 .. 4) of Coordinate_2D;
+
+
+   ---------
+   --- Forge
+   --
+
+   default_Size : constant Size_t;
+
+   procedure define (Self : out Item;   Size : Size_t := default_Size);
+
+
+   --------------
+   --- Attributes
+   --
+
+   function Size   (Self : in Item) return Size_t;
+   function Width  (Self : in Item) return Real;
+   function Height (Self : in Item) return Real;
+
+
+
+private
+
+   type Item is abstract new Model.item with
+      record
+         Plane  : billboard.Plane := xy;
+         Size   : Size_t;
+      end record;
+
+
+   subtype site_Id is Index_t range 1 .. 4;
+   subtype Sites   is Vector_3_array (site_Id'Range);
+
+   function vertex_Sites (for_Plane     : in Plane;
+                          Width, Height : in Real) return Sites;
+
+   Normal       : constant Vector_3 := [0.0,  0.0,  1.0];
+   default_Size : constant Size_t   := (Width  => 1.0,
+                                        Height => 1.0);
+
+end openGL.Model.billboard;