Shader "UltraCombos/Skybox/AtmosphereSkybox"
{
	Properties
	{
		_Exposure("Exposure", Range(0,10)) = 1
	}
	SubShader
	{
		Tags{ "Queue" = "Background" "RenderType" = "Background" "PreviewType" = "Skybox" }
		Cull Off ZWrite Off

		Pass
		{
			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			
			#include "UnityCG.cginc"

			struct v2f
			{
				float4 vertex : SV_POSITION;
				float3 rayDir : TEXCOORD0;
			};
			
			v2f vert (appdata_base v)
			{
				v2f o;
				UNITY_SETUP_INSTANCE_ID(v);
				UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO(o);
				o.vertex = UnityObjectToClipPos(v.vertex);

				float3 eyeRay = normalize(mul((float3x3)unity_ObjectToWorld, v.vertex.xyz));
				o.rayDir = eyeRay;

				return o;
			}

			float2 rsi(float3 r0, float3 rd, float sr) {
				// ray-sphere intersection that assumes
				// the sphere is centered at the origin.
				// No intersection when result.x > result.y
				float a = dot(rd, rd);
				float b = 2.0 * dot(rd, r0);
				float c = dot(r0, r0) - (sr * sr);
				float d = (b*b) - 4.0*a*c;
				if (d < 0.0) return float2(1e5, -1e5);
				return float2(
					(-b - sqrt(d)) / (2.0*a),
					(-b + sqrt(d)) / (2.0*a)
				);
			}

			float3 atmosphere(float3 r, float3 r0, float3 pSun, float iSun, float rPlanet, float rAtmos, 
							  float3 kRlh, float kMie, float shRlh, float shMie, float g) 
			{
				const int iSteps = 16;
				const int jSteps = 8;

				// Normalize the sun and view directions.
				pSun = normalize(pSun);
				r = normalize(r);

				// Calculate the step size of the primary ray.
				float2 p = rsi(r0, r, rAtmos);
				if (p.x > p.y) return float3(0, 0, 0);
				p.y = min(p.y, rsi(r0, r, rPlanet).x);
				float iStepSize = (p.y - p.x) / float(iSteps);

				// Initialize the primary ray time.
				float iTime = 0.0;

				// Initialize accumulators for Rayleigh and Mie scattering.
				float3 totalRlh = float3(0, 0, 0);
				float3 totalMie = float3(0, 0, 0);

				// Initialize optical depth accumulators for the primary ray.
				float iOdRlh = 0.0;
				float iOdMie = 0.0;

				// Calculate the Rayleigh and Mie phases.
				float mu = dot(r, pSun);
				float mumu = mu * mu;
				float gg = g * g;
				float pRlh = 3.0 / (16.0 * UNITY_PI) * (1.0 + mumu);
				float pMie = 3.0 / (8.0 * UNITY_PI) * ((1.0 - gg) * (mumu + 1.0)) / (pow(1.0 + gg - 2.0 * mu * g, 1.5) * (2.0 + gg));

				// Sample the primary ray.
				for (int i = 0; i < iSteps; i++) {

					// Calculate the primary ray sample position.
					float3 iPos = r0 + r * (iTime + iStepSize * 0.5);

					// Calculate the height of the sample.
					float iHeight = length(iPos) - rPlanet;

					// Calculate the optical depth of the Rayleigh and Mie scattering for this step.
					float odStepRlh = exp(-iHeight / shRlh) * iStepSize;
					float odStepMie = exp(-iHeight / shMie) * iStepSize;

					// Accumulate optical depth.
					iOdRlh += odStepRlh;
					iOdMie += odStepMie;

					// Calculate the step size of the secondary ray.
					float jStepSize = rsi(iPos, pSun, rAtmos).y / float(jSteps);

					// Initialize the secondary ray time.
					float jTime = 0.0;

					// Initialize optical depth accumulators for the secondary ray.
					float jOdRlh = 0.0;
					float jOdMie = 0.0;

					// Sample the secondary ray.
					for (int j = 0; j < jSteps; j++) {

						// Calculate the secondary ray sample position.
						float3 jPos = iPos + pSun * (jTime + jStepSize * 0.5);

						// Calculate the height of the sample.
						float jHeight = length(jPos) - rPlanet;

						// Accumulate the optical depth.
						jOdRlh += exp(-jHeight / shRlh) * jStepSize;
						jOdMie += exp(-jHeight / shMie) * jStepSize;

						// Increment the secondary ray time.
						jTime += jStepSize;
					}

					// Calculate attenuation.
					float3 attn = exp(-(kMie * (iOdMie + jOdMie) + kRlh * (iOdRlh + jOdRlh)));

					// Accumulate scattering.
					totalRlh += odStepRlh * attn;
					totalMie += odStepMie * attn;

					// Increment the primary ray time.
					iTime += iStepSize;

				}

				// Calculate and return the final color.
				return iSun * (pRlh * kRlh * totalRlh + pMie * kMie * totalMie);
			}
			
			half _Exposure;

			fixed4 frag (v2f i) : SV_Target
			{
				half3 color = atmosphere(
					normalize(i.rayDir),              // normalized ray direction
					float3(0,6372e3,0),               // ray origin
					_WorldSpaceLightPos0.xyz,         // position of the sun
					22.0,                             // intensity of the sun
					6371e3,                           // radius of the planet in meters
					6471e3,                           // radius of the atmosphere in meters
					float3(5.5e-6, 13.0e-6, 22.4e-6), // Rayleigh scattering coefficient
					21e-6,                            // Mie scattering coefficient
					8e3,                              // Rayleigh scale height
					1.2e3,                            // Mie scale height
					0.758                             // Mie preferred scattering direction
				);

			#if !defined(UNITY_COLORSPACE_GAMMA)
				color.rgb = GammaToLinearSpace(color.rgb);
			#endif

				color = 1.0 - exp(-1.0 * color * _Exposure);

				return half4(color, 1.0);
			}
			ENDCG
		}
	}
}