#version 330

uniform vec2 resolution;

uniform vec3 camera_forward;
uniform vec3 camera_right;
uniform vec3 camera_up;

uniform vec3 sun_direction;

uniform float aspect_ratio;
uniform float tan_half_fovy;

out vec4 finalColor;

void
main()
{
	/*
	 * Reconstruct a world-space direction for this screen pixel.
	 */
	vec2 screen = gl_FragCoord.xy / resolution;
	vec2 ndc = screen * 2.0 - 1.0;

	vec3 view_direction = normalize(
		camera_forward +
		camera_right * ndc.x * aspect_ratio * tan_half_fovy +
		camera_up * ndc.y * tan_half_fovy
	);

	vec3 moon_direction = -sun_direction;

	/*
	 * Vertical sky gradient.
	 */
	float height = smoothstep(
		-0.20,
		0.80,
		view_direction.y
	);

	/*
	 * Day/night transition based on the sun's elevation.
	 */
	float day_amount = smoothstep(
		-0.10,
		0.12,
		sun_direction.y
	);

	float night_amount = 1.0 - day_amount;

	vec3 night_horizon = vec3(
		0.055,
		0.045,
		0.110
	);

	vec3 night_zenith = vec3(
		0.005,
		0.008,
		0.025
	);

	vec3 day_horizon = vec3(
		0.620,
		0.780,
		0.920
	);

	vec3 day_zenith = vec3(
		0.080,
		0.280,
		0.620
	);

	vec3 night_sky = mix(
		night_horizon,
		night_zenith,
		height
	);

	vec3 day_sky = mix(
		day_horizon,
		day_zenith,
		height
	);

	vec3 sky = mix(
		night_sky,
		day_sky,
		day_amount
	);

	/*
	 * Sunrise and sunset colouring.
	 */
	float horizon =
		1.0 -
		smoothstep(
			0.0,
			0.35,
			abs(view_direction.y)
		);

	float twilight =
		1.0 -
		smoothstep(
			0.02,
			0.28,
			abs(sun_direction.y)
		);

	float sun_alignment = dot(
		view_direction,
		sun_direction
	);

	float sun_facing = max(
		sun_alignment,
		0.0
	);

	/*
	 * Avoid pow(): x^4 is sufficient for the broad sunset effect.
	 */
	float sun_facing_2 = sun_facing * sun_facing;
	float sun_facing_4 = sun_facing_2 * sun_facing_2;

	vec3 sunset_colour = vec3(
		0.65,
		0.16,
		0.035
	);

	sky +=
		sunset_colour *
		horizon *
		twilight *
		sun_facing_4;

	/*
	 * Sun halo.
	 *
	 * The values are precalculated cosine values. Since both
	 * view_direction and sun_direction are normalized, their dot
	 * product is the cosine of the angle between them.
	 *
	 * Wide halo:
	 *     outer edge: 18 degrees
	 *     inner edge: 3 degrees
	 *
	 * Core halo:
	 *     outer edge: 7 degrees
	 *     inner edge: 1 degree
	 */
	float sun_wide = smoothstep(
		0.9510565,
		0.9986295,
		sun_alignment
	);

	float sun_core = smoothstep(
		0.9925462,
		0.9998477,
		sun_alignment
	);

	/*
	 * Moon halo.
	 *
	 * Because the moon is directly opposite the sun in this model,
	 * its alignment is the negative sun alignment.
	 *
	 * Wide halo:
	 *     outer edge: 14 degrees
	 *     inner edge: 2 degrees
	 *
	 * Core halo:
	 *     outer edge: 5 degrees
	 *     inner edge: 0.5 degrees
	 */
	float moon_alignment = -sun_alignment;

	float moon_wide = smoothstep(
		0.9702957,
		0.9993908,
		moon_alignment
	);

	float moon_core = smoothstep(
		0.9961947,
		0.9999619,
		moon_alignment
	);

	/*
	 * Fade each halo near and below the horizon.
	 *
	 * These ranges are deliberately broader than the symbol fade so
	 * the surrounding light disappears gradually.
	 */
	float sun_above_horizon = smoothstep(
		-0.12,
		0.04,
		sun_direction.y
	);

	float moon_above_horizon = smoothstep(
		-0.10,
		0.04,
		moon_direction.y
	);

	/*
	 * Warm the sun halo around sunrise and sunset.
	 */
	vec3 daylight_glow = vec3(
		1.00,
		0.82,
		0.38
	);

	vec3 sunset_glow = vec3(
		1.00,
		0.25,
		0.035
	);

	vec3 sun_glow_colour = mix(
		daylight_glow,
		sunset_glow,
		twilight
	);

	vec3 moon_glow_colour = vec3(
		0.32,
		0.46,
		0.82
	);

	/*
	 * Broad component blends the sprite into the sky.
	 * Core component makes the centre appear luminous.
	 */
	float sun_glow =
		(
			sun_wide * 0.13 +
			sun_core * 0.32
		) *
		sun_above_horizon;

	float moon_glow =
		(
			moon_wide * 0.08 +
			moon_core * 0.17
		) *
		moon_above_horizon *
		night_amount;

	/*
	 * Soft screen-like blending. This brightens the sky without
	 * immediately clipping bright daytime colours to white.
	 */
	sky +=
		(1.0 - sky) *
		sun_glow_colour *
		sun_glow;

	sky +=
		(1.0 - sky) *
		moon_glow_colour *
		moon_glow;

	finalColor = vec4(
		clamp(sky, 0.0, 1.0),
		1.0
	);
}