anchor.diff:  
   ;;; Bounding box
   (defstruct bbox
  +  "Axis Aligned Bounding Box."
     valid
     x-min y-min z-min
     x-max y-max z-max)
  
positive.code:                (aref (image-data image) (+ 1 (* index bpp))) g
                 (aref (image-data image) (+ 2 (* index bpp))) b
                 (aref (image-data image) (+ 3 (* index bpp))) a)))))
  
  ;;; 2D Texture
  (defstruct texture
  
negative.code:         (width (- right left))
          (height (- top bottom)))
    (shape-add-vertex/index shape left bottom)
    (shape-add-tex-vertex shape 0.0 0.0)
    (shape-add-vertex/index shape right bottom)
    (shape-add-tex-vertex shape (if tex-width (/ width tex-width) 1.0) 0.0)
    (shape-add-vertex/index shape right top)
    (shape-add-tex-vertex shape (if tex-width (/ width tex-width) 1.0)
                                (if tex-height (/ height tex-height) 1.0))
    (shape-add-vertex/index shape left top)
    (shape-add-tex-vertex shape 0.0 (if tex-height (/ height tex-height) 1.0))
    (shape-add-indices shape 0)
    shape))
  
  