Starfield

/*
 * StarField.osl by Thomas Dinges
 * from https://github.com/sambler/osl-shaders
 * 
 * original script from -
 * http://www.renderman.org/RMR/RMRShaders.html
 *
 * Starfield.sl -- Surface shader for a star field.
 *
 * 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?
 *   Width            how wide is the point spread function?  i.e. larger
 *                      values make "wider" stars, but the look less round.
 *
 * AUTHOR: written by Larry Gritz, 1992
 *
 * HISTORY:
 *      28 Dec 1992 -- written by lg for the "Timbre Trees" video (saucer)
 *      12 Jan 1994 -- recoded by lg in correct shading language.
 *
 * modified  12 Jan 1994 by Larry Gritz
 * converted to OSL by Thomas Dinges Dec 2012
 * modified 1/4/2020 by Saul Espinosa for Redshift
 * This file is licensed under Apache 2.0 license
 */
shader StarField
[[  string help = "Generates procedural stars",
    string label = "Starfield" ]]
(
        float Intensity = 0.5
            [[
            float min = 0,
            float max = 100
            ]],
        float Frequency = 100.0
            [[
            float min = 0,
            float max = 500
            ]],
        float Width = 0.2
            [[
            float min = 0,
            float max = 1
            ]],
        output float Fac = 1.0 )
{
  point PP;
  float val;
  PP = Frequency * P;
  /* Use a noise value, but only keep the "tips" of the blotches */
  val = smoothstep (1 - Width, 1, noise(PP));
  /* scale it by the intensity and sharpen it by squaring */
  Fac = Intensity * val*val;
}