尝试MatCap类型shader

听说MatCap能在低端机上做出很漂亮的pbr效果，就尝试了一下。

MatCap全称MaterailCapture，里面存的是光照信息，通过法线的xy分量去采样matcap，得到在该方向法线的光照信息，因为不是实时光照，所以性能好，好不好看就看你的Matcap做得好不好了。下面是简单的matcap尝试：

Shader "Unlit/SimpleMatCapShader"

{

    Properties

    {

        _MainTex ("Texture", 2D) = "white" {}

        _MatCap("MatCap", 2D) = "white" {}

        _MatCapFactor("MatCapFactor", Range(,)) =

        _EnvTex("EnvTex (CubeMap)", Cube) = "_SkyBox" {}

        _EnvFactor("EnvStrength", Range(,)) = 0.8

    }

    SubShader

    {

        Tags { "RenderType"="Opaque" }

        LOD 

        Pass

        {

            CGPROGRAM

            #pragma vertex vert

            #pragma fragment frag

            #include "UnityCG.cginc"

            struct appdata

            {

                float4 vertex : POSITION;

                float2 uv : TEXCOORD0;

                float3 normal : NORMAL;

            };

            struct v2f

            {

                float4 twoUv : TEXCOORD0;//主纹理uv存xy,matCapUv存在zw,这算一种优化

                float4 vertex : SV_POSITION;

                float3 RefDir : TEXCOORD1;

            };

            sampler2D _MainTex;

            float4 _MainTex_ST;

            sampler2D _MatCap;

            half _MatCapFactor;

            samplerCUBE _EnvTex;

            half _EnvFactor;

            v2f vert (appdata v)

            {

                v2f o;

                o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);

                o.twoUv.xy = TRANSFORM_TEX(v.uv, _MainTex);

                //matcap存的其实是：法线中xy分量作为uv，对应的光照颜色信息，即用xy去采样就好，注：xy必须是归一化法线中的分量，故z才没必要

                o.twoUv.zw = UnityObjectToWorldNormal(v.normal).xy;

                o.twoUv.zw = o.twoUv.zw * 0.5 + 0.5;//(-1,1)->(0,1)

                float3 wolrdN = UnityObjectToWorldNormal(v.normal);

                o.RefDir = reflect(-WorldSpaceViewDir(v.vertex), wolrdN);

                return o;

            }

            fixed4 frag (v2f i) : SV_Target

            {

                // sample the texture

                fixed4 col = tex2D(_MainTex, i.twoUv.xy);

                fixed4 mapCapCol = tex2D(_MatCap, i.twoUv.zw);

                fixed4 reflection = texCUBE(_EnvTex, i.RefDir);

                col.rgb = col.rgb * mapCapCol.rgb * _MatCapFactor + reflection.rgb * _EnvFactor;

                return col;

            }

            ENDCG

        }

    }

}