/* Copyrighted Pixar 1989 */
/* From the RenderMan Companion p.369 */
/* Listing 16.26  Shader mapping object-space coordinates to colors*/

/*
 * glow(): a shader for providing a centered "glow" in a sphere 
 */
surface
glow( float	attenuation = 2 )
{
#ifndef ORIGINAL
	/* This section was added by wave@media.mit.edu -- tal */
   point NN = normalize(N);
   point II = normalize(I);
   float falloff = II.NN;	/* Direct incidence has cosine closer to 1.*/
#else
		/* What is in the book. -- tal */
   float falloff = I.N;	/* Direct incidence has cosine closer to 1.*/
#endif /* ORIGINAL */

   if (falloff < 0) 
   {  /* Back of sphere only */
#ifndef ORIGINAL
		/* This section was added by wave@media.mit.edu -- tal */
      /* Normalize falloff by lengths of I and N */
      falloff = falloff * falloff / (II.II * NN.NN) ;
#else
		/* What is in the book. -- tal */
      falloff = falloff * falloff / (I.I * N.N) ;

#endif /* ORIGINAL */

      falloff = pow(falloff, attenuation);
      Ci = Cs * falloff;
      Oi = falloff;
   } 
   else
     Oi = 0;
}