//const vec3  DAYSKY_COLOUR   = vec3( 0.37, 0.54, 0.97 );
//const vec3  NIGHTSKY_COLOUR = vec3( 0.03, 0.05, 0.20 );
//const vec3  SUNSET_COLOUR   = vec3( 1.00, 0.32, 0.01 );
//const vec3  AMBIENT_COLOUR  = vec3( 0.13, 0.17, 0.35 );
//const vec3  SUNSET_AMBIENT  = vec3( 0.25, 0.17, 0.51 );
//const vec3  SUN_COLOUR      = vec3( 1.10, 0.89, 0.35 );

const float SUN_ANGLE       = 0.0001;
const float PI              = 3.14159265;

//struct world_info
//{
//   float time,
//         time_of_day,
//         day_phase,
//         sunset_phase;
//   
//   vec3 sun_dir;
//};

float luminance( vec3 v )
{
   return dot( v, vec3(0.2126, 0.7152, 0.0722) );
}

vec3 clearskies_ambient( vec3 dir )
{
   float sun_azimuth  = g_sunset_phase * (dot( dir.xz, g_sun_dir.xz )*0.4+0.6);
   float sky_gradient = dir.y;
   
   /* Blend phase colours */
   vec3 ambient  = g_daysky_colour.rgb   * (g_day_phase-g_sunset_phase*0.1);
        ambient += g_sunset_colour.rgb   * (1.0-dir.y*0.5)*sun_azimuth;
        ambient += g_nightsky_colour.rgb * (1.0-g_day_phase);
   
   /* Add gradient */
        ambient -= sky_gradient * luminance(ambient);
        
   return ambient;
}

vec3 clearskies_sky( vec3 ray_dir )
{
   ray_dir.y = abs( ray_dir.y );
   vec3 sky_colour  = clearskies_ambient( ray_dir );
   
   /* Sun */
   float sun_theta  = dot( ray_dir, g_sun_dir.xyz );
   float sun_size   = max( 0.0, sun_theta * 0.5 + 0.5 + SUN_ANGLE );
   float sun_shape  = pow( sun_size, 2000.0 );
         sun_shape += sun_size * max(g_sun_dir.y,0.0) * 0.5;
         
   vec3 sun_colour  = mix( vec3(1.0), g_sunset_colour.rgb, g_sunset_phase*0.5 );
        sun_colour *= sun_shape;
   
   vec3 composite   = sky_colour + sun_colour;
   return composite;
}

vec3 clearskies_lighting( vec3 normal, float shadow, vec3 halfview )
{
   float fresnel = 1.0 - abs(dot(normal,halfview));

   vec3  reflect_colour = mix( g_daysky_colour.rgb, g_sunset_colour.rgb, 
                               g_sunset_phase );

   vec3  sky_reflection = 0.5 * fresnel * reflect_colour;
   vec3  light_sun      = max(0.0,dot(normal,g_sun_dir.xyz)*0.75+0.25) 
                           * g_sun_colour.rgb * g_day_phase;

   float scaled_shadow = max( shadow, 1.0 - max(g_sun_dir.y,0.0) );
   vec3 ambient = mix( g_ambient_colour.rgb, g_sunset_ambient.rgb, 
                       g_sunset_phase );

   return ambient + (light_sun + sky_reflection) * shadow;
}