uniform sampler2D tex;

float lin_interp(float x, float domain_low, float domain_hi, float range_low, float range_hi) {
  if ((x >= domain_low) && (x <= domain_hi)) {
    x = (x - domain_low) / (domain_hi - domain_low);
    x = range_low + x * (range_hi - range_low);
  }
  return x;
}

float pvp_adjust_3(float x) {
  // red
  x = lin_interp(x, 0.00, 0.125, -0.050, 0.090);
  // orange
  x = lin_interp(x, 0.125, 0.25,  0.090, 0.167);
  // yellow
  x = lin_interp(x, 0.25, 0.375,  0.167, 0.253);
  // chartreuse
  x = lin_interp(x, 0.375, 0.50,  0.253, 0.383);
  // green
  x = lin_interp(x, 0.50, 0.625,  0.383, 0.500);
  // teal
  x = lin_interp(x, 0.625, 0.75,  0.500, 0.667);
  // blue
  x = lin_interp(x, 0.75, 0.875,  0.667, 0.800);
  // purple
  x = lin_interp(x, 0.875, 1.00,  0.800, 0.950);
  return(x);
}

vec3 hsv2rgb(float h, float s, float v)
{
  float i, f, p, q, t, r, g, b;
  int ii;
  if (s == 0.0) {
    // Ignore hue
    r = v;
    g = v;
    b = v;
  } else {
    /* Apply physiological mapping: red, yellow, green and blue should
       be equidistant. */
    h = pvp_adjust_3(h);
    h = h - floor(h);
    h = h * 6.0;
    i = floor(h); ii = int(i);
    f = h - i;
    p = v*(1.0 - s);  // The "low-flat" curve
    q = v*(1.0 - (s*f));  // The "falling" curve
    t = v*(1.0 - (s*(1.0 - f)));  // The "rising" curve
    switch(ii) {
      case 0:  // red point
        r = v; g = t; b = p; // RED max, GRN rising, BLU min
        break;
      case 1:  // yellow point
        r = q; g = v; b = p; // RED falling, GRN max, BLU min
        break;
      case 2:  // green point
        r = p; g = v; b = t; // RED min, GRN max, BLU rising
        break;
      case 3:  // cyan point
        r = p; g = q; b = v; // RED min, GRN falling, BLU max
        break;
      case 4:  // blue point
        r = t; g = p; b = v; // RED rising, GRN min, BLU max
        break;
      case 5:  // magenta point
      default:
        r = v; g = p; b = q; // RED max, GRN min, BLU falling
        break;
    }
  }
  return vec3(r, g, b);
}

void main(void) {
  float n = texture2D(tex, gl_TexCoord[0].st).x;
  float h = n;
  float s = 1.0;
  float v = clamp(0.0, 1.0, 0.25 + log(h + 1.0));
  gl_FragColor = vec4(hsv2rgb(frac(h), s, v), 1.0);
}