/*  
 * ClearStarPlane - modified from Larry Gritz's LGStarfield
 *
 * Modification makes the parts clear where they were black.
 *
 * DESCRIPTION:
 *   Makes a star field.  Best when used as a surface shader for the inside
 *   of a large sphere.
 * 
 * PARAMETERS:
 *   frequency          frequency of the stars
 *   intensity          how bright are the stars?
 *   pswidth            how wide is the point spread function?  i.e. larger
 *                      values make "wider" stars, but the look less round.
 *
 * AUTHOR: Nick Meshes
 *
 * 
 */

surface
ClearStarPlane ( float intensity = 1;
	    float frequency = 1;
	    float pswidth = 0.01)
{
  point PP;
  float val;

  /* Shade in shader space */
  PP = frequency * transform ("shader", P);

  /* Use a noise value, but only keep the "tips" of the blotches */
  val = smoothstep (1-pswidth, 1, noise(PP));

  /* scale it by the intensity and sharpen it by squaring */
  Oi = Os;
  Ci = intensity * val*val;
}