/*
 * backlit.sl
 *
 * Lawson Wade
 *
 * A surface shader that simulates a translucent
 * plastic surface by allowing itself to be lit
 * from behind
 */

#include "rmannotes.sl"


surface
backlit(
	float     Ka = 1,
	          Kd = 0.5,
	          Ks = 0.5,
	          roughness = 0.1;
	color     specularcolor = 1;
	)
{
  point Nf, V;
  point Nin, Vmirrored;
  color surface_color, layer_color;
  color surface_opac;
  float count;


  /* illumination vectors */

  Nf = faceforward(normalize(N), I);
  V = normalize(-I);

  /*** background layer ***/

  surface_opac = Os;
  surface_color = 0;

  /*** ambient light ***/

  surface_color += Os * Cs * Ka * ambient();

  /*** diffuse light from front ***/

  surface_color += Os * Cs * Kd * diffuse(Nf);

  /*** diffuse light from behind ***/
  
  layer_color = 0;
  Nin = -Nf;
  illuminance(P, Nin, 0.5*PI)
    layer_color += Cl * (normalize(L) . Nin);
  surface_color += Os * layer_color;

  /*** specular light from front ***/

  surface_color += specularcolor * Ks * specular(Nf, V, roughness);

  /*** specular light from behind ***/

  Vmirrored = faceforward(reflect(V, Nf), Nin);
  surface_color += specularcolor * Ks * specular(Nin, Vmirrored, 1.0);


  /* output */

  Oi = surface_opac;
  Ci = surface_color;
}