Fresnel 프레넬

 프레넬 방정식(Fresnel equation) : 네이버 블로그 (naver.com)

반사와 투과가 되는 양을 표현한 방정식

Fresnel equations - Wikipedia

fresnel equation

	Properties
	{
		_MainTex ("Base (RGB) Trans (A)", 2D) = "white" {}
		_Color ("Tint", Color) = (1,1,1,1)
		_FresnelColor ("Fresnel Color", Color) = (1,1,1,1)
		_FresnelBias ("Fresnel Bias", Float) = 0
		_FresnelScale ("Fresnel Scale", Float) = 1
		_FresnelPower ("Fresnel Power", Float) = 1
	}

	SubShader
	{
		Tags
		{
			"Queue"="Geometry"
			"IgnoreProjector"="True"
			"RenderType"="Opaque"
		}

		Cull Back

		Pass
		{
			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			#pragma target 2.0

			#include "UnityCG.cginc"

			struct appdata_t
			{
				float4 pos : POSITION;
				float2 uv : TEXCOORD0;
				half3 normal : NORMAL;
			};

			struct v2f
			{
				float4 pos : SV_POSITION;
				half2 uv : TEXCOORD0;
				float fresnel : TEXCOORD1;
			};

			sampler2D _MainTex;
			float4 _MainTex_ST;
			fixed4 _Color;
			fixed4 _FresnelColor;
			fixed _FresnelBias;
			fixed _FresnelScale;
			fixed _FresnelPower;

			v2f vert(appdata_t v)
			{
				v2f o;
				o.pos = UnityObjectToClipPos(v.pos);
				o.uv = TRANSFORM_TEX(v.uv, _MainTex);

				float3 i = normalize(ObjSpaceViewDir(v.pos));
				o.fresnel = _FresnelBias + _FresnelScale * pow(1 + dot(i, v.normal), _FresnelPower);
				return o;
			}

			fixed4 frag(v2f i) : SV_Target
			{
				fixed4 c = tex2D(_MainTex, i.uv) * _Color;
                return lerp(c, _FresnelColor, 1 - i.fresnel);
			}
			ENDCG
		}
	}
    

resnel equation

        // CookTorrance referenced
        float fresnel_val = 0.2;
        float roughness_val = .06;
        float k = 0.01;

        wVertex = mul(unity_WorldToObject, wVertex);
        float3 viewpos = -mul(UNITY_MATRIX_MV, wVertex).xyz;

        float3 col = float3(0, 0, 0);
        for (int i = 0; i < 2; i++)
        {
            // View vector, light direction, and Normal in model-view space
            float3 toLight = unity_LightPosition[i].xyz;
            float3 L = normalize(toLight);
            float3 V = normalize(viewpos);
            float3 N = mul(UNITY_MATRIX_MV, float4(normal, 0));
            N = normalize(N);

            // Half vector from view to light vector
            float3 H = normalize(V + L);

            //Dot products
            float NdotL = max(dot(N, L), 0);
            float NdotV = max(dot(N, V), 0);
            float NdotH = max(dot(N, H), 1.0e-7);
            float VdotH = max(dot(V, H), 0);

            // model the geometric attenuation of the surface
            float geo_numerator = 2 * NdotH;
            float geo_b = (geo_numerator * NdotV) / VdotH;
            float geo_c = (geo_numerator * NdotL) / VdotH;
            float geometric = 2 * NdotH / VdotH;
            geometric = min(1, max(0, min(geo_b, geo_c)));

            //calculate the roughness of the model
            float r2 = roughness_val * roughness_val;
            float NdotH2 = NdotH * NdotH;
            float NdotH2_r = 1 / (NdotH2 * r2);
            float roughness_exp = (NdotH2 - 1) * NdotH2_r;
            float roughness = exp(roughness_exp) * NdotH2_r / (4 * NdotH2);

            // Calculate the fresnel value
            float fresnel = pow(1.0 - VdotH, 5.0);
            fresnel *= 1 - fresnel_val;
            fresnel += fresnel_val;

        }