Reflection HLSL

 https://en.wikibooks.org/wiki/Cg_Programming/Unity/Reflecting_Surfaces

resnel equation

// Upgrade NOTE: replaced 'mul(UNITY_MATRIX_MVP,*)' with 'UnityObjectToClipPos(*)'

// Upgrade NOTE: replaced 'mul(UNITY_MATRIX_MVP,*)' with 'UnityObjectToClipPos(*)'

// Upgrade NOTE: replaced 'mul(UNITY_MATRIX_MVP,*)' with 'UnityObjectToClipPos(*)'

Shader "Custom/fresnel"
{
    Properties
    {
        _Color ("Color", Color) = (1,1,1,1)
        _MainTex ("Albedo (RGB)", 2D) = "white" {}
        _FresnelColor("Fresnel Color", Color) = (1,1,1,1)
        _FresnelBias("Fresnel Bias", Float) = 0
        _FresnelScale("Fresnel Scale", Float) = 1
        _FresnelPower("Fresnel Power", Float) = 1
        _ReflectionCubemap("Reflection (HDR)",CUBE)=""{}
    }
    SubShader
        {
            Tags
            {
                "Queue" = "Geometry"
                "IgnorePorjector" = "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 : POSITION;
                   half2 uv : TEXCOORD0;
                   float fresnel : TEXCOORD1;
                   float3 worldPos : TEXCOORD2;
                   float3 normalDir : TEXCOORD3;
                };

               sampler2D _MainTex;
               float4 _MainTex_ST; //MainTex zw
               samplerCUBE _ReflectionCubemap;
               half4 _ReflectionCubemap_HDR;
               fixed4 _Color;
               fixed4 _FresnelColor;
               fixed _FresnelBias;
               fixed _FresnelScale;
               fixed _FresnelPower;

               float3 customObjSpaceViewDir(float4 v)
               {
                   float3 objSpaceCameraPos = mul(unity_WorldToObject, float4(_WorldSpaceCameraPos.xyz, 1)).xyz; //local camera position
                   return objSpaceCameraPos - v.xyz; // from camera to v.pos direction
               }

               //UNITY_MATRIX_MVP  = GL.GetGPUProjectionMatrix(Camera.main.projectionMatrix, false) * Camera.main.worldToCameraMatrix * debugObject.renderer.localToWorldMatrix;
               //UNITY_MATRIX_MVP  = UNITY_MATRIX_P * _WorldSpaceCameraPos * unity_ObjectToWorld;
               //float4 customUNITY_MATRIX_MVP()
               //{
               //    return mul(mul(UNITY_MATRIX_M, UNITY_MATRIX_V), UNITY_MATRIX_P).xyzw;
               //    //mul(UNITY_MATRIX_P, float4(_WorldSpaceCameraPos.xyz, 1);// *unity_ObjectToWorld).xyzw;
               //}

               float4 customUnityObjectToClipPos(float3 pos)
               {
                   return UnityObjectToClipPos(float4(pos, 1));  // == mul(UNITY_MATRIX_MVP, float4(pos, 1))
               }

               // #degine TRANSFORM_TEX(tex,name) (tex.xy * name##_ST.xy + name##_ST.zw) // scale and offset

               v2f vert(appdata_t  v)
               {
                   v2f o;
                   o.pos = customUnityObjectToClipPos(v.pos);//UnityObjectToClipPos(v.pos);
                   o.uv = TRANSFORM_TEX(v.uv, _MainTex);
                   o.normalDir = normalize(mul(float4(v.normal, 0.0), unity_WorldToObject).xyz);

                   o.worldPos = mul(unity_ObjectToWorld, v.pos);

                   float3 i = normalize(customObjSpaceViewDir(v.pos));
                   //o.fresnel = dot(i, v.normal);//_FresnelBias + _FresnelScale * pow(1 + dot(i, v.normal), _FresnelPower);
                   // dot(customObjSpaceViewDir, v.normal) => object to camera lookat dot product
                   o.fresnel = _FresnelBias + _FresnelScale * pow(1 + dot(i, v.normal), _FresnelPower);
                   return o;
               }

               fixed4 frag(v2f i) : SV_Target
               {
                   half3 viewDir = normalize(_WorldSpaceCameraPos - i.worldPos);
                   float3 reflectDir = reflect(viewDir, normalize(i.normalDir));
                   half4 ref = texCUBE(_ReflectionCubemap, reflectDir * float3(1,-1,-1));
                   fixed4 c = tex2D(_MainTex, i.uv) * _Color;
                   //return i.fresnel;// lerp(c, _FresnelColor, 1 - i.fresnel);
                   return lerp(c, _FresnelColor * ref, clamp(1 -i.fresnel,0,1));
               }
               ENDCG
             }
        }
}