anchor.diff:     (y1 :initarg :y1 :type vec3)
      (y2 :initarg :y2 :type vec3)))
   
  -;; TODO: make super-class rectangle and sub-class pic-object. Seperating rendering
  +;; TODO: make super-class rectangle and sub-class pic-object. Separating rendering
   ;;       representation from interior representation and proxy geometry for collision
   ;;       tests
   ;; UPDATE: maybe go with the superclass: rectangle, subclass: pic-object approach, which
   ;;         will contain a representation slot (e.g. filled with sphere, rectangle), an
  -;;         <animation> slot a proxy geometry, a cached geometry (past timesteam when no
  +;;         <animation> slot a proxy geometry, a cached geometry (past time step when no
   ;;         collision orientation)
   (defclass rectangle (parallelogram)
  -  ;; TODO: instead of vec2 provide as seperate x1-x x1-y ? So that
  +  ;; TODO: instead of vec2 provide as separate x1-x x1-y ? So that
     ;;       transforming into 1d-array is easier (to pass into foreign-array)
     (;; center-radius representation
      (center-point :initarg :center-point :type vec3)
  
positive.code:     ;;VBO yes, this is necessary, because it associates the vbo
      ;;with the vao
  
      ;;postion
      (gl:bind-buffer :array-buffer position-vbo)
      (%gl:enable-vertex-attrib-array 0)
      (%gl:vertex-attrib-pointer 0 3 :float :false 0 0)
  
negative.code: 	 (y2 (vec3+ center-point-vec3 (vec3 rx ry rz))))
      (values x1 x2 y1 y2)))
  
  ;; TODO: add texture coordinate to initialisation, providing a texture.png
  ;; and a fixed texture coordinate with texatl:with-sprite ?
  ;; UPDATE: maybe go with the superclass: rectangle, subclass: pic-object which will contain
  ;;         a representation point (e.g. filled with sphere, rectangle), an <animation> slot
  ;;         a proxy geometry, a cached geometry (past time step when no collision orientation)
  (defun make-rectangle-c (&optional
  			   (center-point (vec3 0.0 0.0 0.0))
  			   (radius (vec3 50.0 50.0 0.0)))
  