anchor.diff:  
   (defparameter *dark-color* (glm:vec4 0.2 0.2 0.2 1.0))
   (defparameter *light-color* (glm:vec4 1.0))
  +(defparameter *draw-dark-p* NIL)
  +
   
   
   (defun draw ()
  
positive.code: 
  	     (when (sdl2:scancode= (sdl2:scancode-value keysym) :scancode-b)
  	       ;; break rotation
  	       (setf *rotate-light-p* (not *rotate-light-p*)))	     
  
  	     (when (sdl2:scancode= (sdl2:scancode-value keysym) :scancode-space)
  	       ;; toggle color on cylinder
  
negative.code:     (gl:uniformfv (camera-space-light-pos-unif p-white-prog) light-pos-camera-space)
      (gl:uniformf (light-attenuation-unif p-white-prog) *light-attenuation*)
      (gl:uniformf (shininess-factor-unif p-white-prog) *shininess-factor*)
      (gl:uniformfv (base-diffuse-color-unif p-white-prog)
  		  (if *draw-dark-p* *dark-color* *light-color*))
  
  
      
      (gl:use-program (the-program p-color-prog))
      (gl:uniformfv (light-intensity-unif p-color-prog) (glm:vec4 0.8 0.8 0.8 1.0))
  