// Create a bumpy surface by using procedural noise to generate a height (
// displacement in normal direction).
//
// Inputs:
//   is_moon  whether we're looking at the moon or centre planet
//   s  3D position of seed for noise generation
// Returns elevation adjust along normal (values between -0.1 and 0.1 are
//   reasonable.
float bump_height( bool is_moon, vec3 s)
{
  float b = 
    +0.05*(0.5+0.5*smooth_heaviside(perlin_noise( 1.0*s),10)*2-1)
    +(0.5 + 0.44*float(!is_moon))*(0.5+0.5*smooth_heaviside((
      +(0.6+0.14*float(is_moon))*perlin_noise( 2.0*s)
      +(0.2-0.04*float(is_moon))*perlin_noise( 4.0*s)
      +(0.2-0.1*float(is_moon))*perlin_noise( 8.0*s)
      -0.005*perlin_noise( 64.0*s)
      -0.001*perlin_noise( 128.0*s)
      ),8-float(is_moon)*-s.x*7)*2-1)
    +0.01*(0.5+0.5*smooth_heaviside((
      +0.1*perlin_noise( 16.0*s)
      +0.8*perlin_noise( 32.0*s)
      +0.1*perlin_noise( 64.0*s)
      ),4)*2-1)
    -.5;
  return 0.06*b+0.07;
}