// NOTE: Shader automatically converted from Godot Engine 4.0.alpha5's PhysicalSkyMaterial.
// And then augmented with a few tweaks

shader_type sky;

uniform float rayleigh : hint_range(0, 64) = 2.0;
uniform vec4 rayleigh_color : hint_color = vec4(0.3, 0.405, 0.6, 1.0);
uniform float mie : hint_range(0, 1) = 0.005;
uniform float mie_eccentricity : hint_range(-1, 1) = 0.8;
uniform vec4 mie_color : hint_color = vec4(0.69, 0.729, 0.812, 1.0);

uniform float turbidity : hint_range(0, 1000) = 10.0;
uniform float sun_disk_scale : hint_range(0, 360) = 1.0;
uniform vec4 ground_color : hint_color = vec4(0.1, 0.07, 0.034, 1.0);
uniform float exposure : hint_range(0, 128) = 0.1;
uniform float dither_strength : hint_range(0, 10) = 1.0;

uniform sampler2D night_sky : hint_black_albedo;

uniform sampler2D samayun : hint_albedo;
uniform float samayun_arc = 45 ;
uniform vec3 samayun_position = vec3( 0.0, 0.5, 0.0 );

uniform sampler2D zabr : hint_albedo;
uniform float zabr_arc = 15 ;
uniform vec3 zabr_position = vec3( 0.0, 0.7, 0.0 );

uniform sampler2D stigi : hint_albedo;
uniform float stigi_arc = 8 ;
uniform vec3 stigi_position = vec3( 0.0, 0.8, 0.0 );

const vec3 UP = vec3( 0.0, 1.0, 0.0 );

// Sun constants
const float SUN_ENERGY = 1000.0;

// Optical length at zenith for molecules.
const float rayleigh_zenith_size = 8.4e3;
const float mie_zenith_size = 1.25e3;

float henyey_greenstein(float cos_theta, float g) {
	const float k = 0.0795774715459;
	return k * (1.0 - g * g) / (pow(1.0 + g * g - 2.0 * g * cos_theta, 1.5));
}

// From: https://www.shadertoy.com/view/4sfGzS credit to iq
float hash(vec3 p) {
	p  = fract( p * 0.3183099 + 0.1 );
	p *= 17.0;
	return fract(p.x * p.y * p.z * (p.x + p.y + p.z));
}

// Function to create projection plane for texture at a specific position
vec2 place_object(vec3 position, vec3 eyedir){
	//We define a local plane tangent to the skydome at the given position
	//We work with everything normalized
	vec3 n1 = normalize(cross(position,vec3(0.0,1.0,0.0))) ;
	vec3 n2 = normalize(cross(position,n1)) ;
	//We project EYEDIR on this plane with an approximate correction for projection
	float x = dot(eyedir,n1) * 0.89 ;
	float y = dot(eyedir,n2) * 0.89 ;
	return vec2(x, y);
}

void sky() {
	if (LIGHT0_ENABLED) {
		float zenith_angle = clamp( dot(UP, normalize(LIGHT0_DIRECTION)), -1.0, 1.0 );
		float sun_energy = max(0.0, 1.0 - exp(-((PI * 0.5) - acos(zenith_angle)))) * SUN_ENERGY * LIGHT0_ENERGY;
		float sun_fade = 1.0 - clamp(1.0 - exp(LIGHT0_DIRECTION.y), 0.0, 1.0);

		// Rayleigh coefficients.
		float rayleigh_coefficient = rayleigh - ( 1.0 * ( 1.0 - sun_fade ) );
		vec3 rayleigh_beta = rayleigh_coefficient * rayleigh_color.rgb * 0.0001;
		// mie coefficients from Preetham
		vec3 mie_beta = turbidity * mie * mie_color.rgb * 0.000434;

		// Optical length.
		float zenith = acos(max(0.0, dot(UP, EYEDIR)));
		float optical_mass = 1.0 / (cos(zenith) + 0.15 * pow(93.885 - degrees(zenith), -1.253));
		float rayleigh_scatter = rayleigh_zenith_size * optical_mass;
		float mie_scatter = mie_zenith_size * optical_mass;

		// Light extinction based on thickness of atmosphere.
		vec3 extinction = exp(-(rayleigh_beta * rayleigh_scatter + mie_beta * mie_scatter));

		// In scattering.
		float cos_theta = dot(EYEDIR, normalize(LIGHT0_DIRECTION));

		float rayleigh_phase = (3.0 / (16.0 * PI)) * (1.0 + pow(cos_theta * 0.5 + 0.5, 2.0));
		vec3 betaRTheta = rayleigh_beta * rayleigh_phase;

		float mie_phase = henyey_greenstein(cos_theta, mie_eccentricity);
		vec3 betaMTheta = mie_beta * mie_phase;

		vec3 Lin = pow(sun_energy * ((betaRTheta + betaMTheta) / (rayleigh_beta + mie_beta)) * (1.0 - extinction), vec3(1.5));
		// Hack from https://github.com/mrdoob/three.js/blob/master/examples/jsm/objects/Sky.js
		Lin *= mix(vec3(1.0), pow(sun_energy * ((betaRTheta + betaMTheta) / (rayleigh_beta + mie_beta)) * extinction, vec3(0.5)), clamp(pow(1.0 - zenith_angle, 5.0), 0.0, 1.0));

		// Hack in the ground color.
		Lin  *= mix(ground_color.rgb, vec3(1.0), smoothstep(-0.1, 0.1, dot(UP, EYEDIR)));

		// Solar disk and out-scattering.
		float sunAngularDiameterCos = cos(LIGHT0_SIZE * sun_disk_scale);
		float sunAngularDiameterCos2 = cos(LIGHT0_SIZE * sun_disk_scale*0.5);
		float sundisk = smoothstep(sunAngularDiameterCos, sunAngularDiameterCos2, cos_theta);
		vec3 L0 = (sun_energy * 1900.0 * extinction) * sundisk * LIGHT0_COLOR;
		L0 += texture(night_sky, SKY_COORDS).xyz * extinction;

		vec3 color = (Lin + L0) * 0.04;
		COLOR = pow(color, vec3(1.0 / (1.2 + (1.2 * sun_fade))));
		COLOR *= exposure;
		// Make optional, eliminates banding.
		COLOR += (hash(EYEDIR * 1741.9782) * 0.08 - 0.04) * 0.016 * dither_strength;
	} else {
		// There is no sun, so display night_sky and nothing else.
		COLOR = texture(night_sky, SKY_COORDS).xyz * 0.04;
		COLOR *= exposure;
	}
	// Calculate respective scales of celestial objects
	float samayun_scale = radians(samayun_arc) ;
	float zabr_scale = radians(zabr_arc) ;
	float stigi_scale = radians(stigi_arc) ;
	
	// Calculate respective plane with UV to place celestial object textures
	vec2 samayun_uv = place_object(samayun_position, EYEDIR) ;
	vec2 zabr_uv = place_object(zabr_position, EYEDIR) ;
	vec2 stigi_uv = place_object(stigi_position, EYEDIR) ;
	
	// Adding Samayun
	if (length(EYEDIR - normalize(samayun_position)) < samayun_scale / 2.0) { // we are in the area of the sky where samayun is placed
		// If zabr is nearer at this place, do nothing
		if (length(EYEDIR - normalize(zabr_position)) < zabr_scale / 2.0 && length(zabr_position) < length(samayun_position)){
			} else { // // Add samayun to the sky
				COLOR += texture(samayun,  samayun_uv / samayun_scale + vec2(0.5)).rgb * texture(samayun, samayun_uv / samayun_scale + vec2(0.5)).a;
			}
	}
	// Adding zabr
	if (length(EYEDIR - normalize(zabr_position)) < zabr_scale / 2.0) { // we are in the area of the sky where zabr is placed
			// If samayun is nearer at this place, do nothing
		if (length(EYEDIR - normalize(samayun_position)) < samayun_scale / 2.0 && length(samayun_position) < length(zabr_position)){
			} else { // Add zabr to the sky
				COLOR += texture(zabr,  zabr_uv / zabr_scale + vec2(0.5)).rgb * texture(zabr, zabr_uv / zabr_scale + vec2(0.5)).a;
			}
		}
	// Adding stigi
	if (length(EYEDIR - normalize(stigi_position)) < stigi_scale / 2.0) { // we are in the area of the sky where stigi is placed
			// If samayun is nearer at this place, do nothing
		if (length(EYEDIR - normalize(samayun_position)) < samayun_scale / 2.0 && length(samayun_position) < length(stigi_position)){
			} else { // Add stigi to the sky
				COLOR += texture(stigi,  stigi_uv / stigi_scale + vec2(0.5)).rgb * texture(stigi, stigi_uv / stigi_scale + vec2(0.5)).a;
			}
		}
}