Unity shader學習之陰影

Unity陰影采用的是 shadow map 的技術，即把攝像機放到光源位置上，看不到的地方就有陰影。

前向渲染中，若一光源開啟了陰影，Unity會計算它的陰影映射紋理(shadow map)，它其實是一張深度圖，記錄了從光源位置出發，能看到的場景中距離它最近的表面位置的深度信息。

Unity中采用LightMode為ShadowCaster的Pass來生成這張 shadow map，首先會從本shader中找這個pass，若沒有就從fallback的shader中層層尋找。

然后在fragment shader中，比較該點與shadow map中對應的點的深度，若大於，則處在陰影區域，否則不在陰影區域。

陰影包括投射陰影和接收陰影2個部分。

投射陰影，則需將該物體加入到光源的shadow map的計算中。

接收陰影，則需要對shadow map進行采樣，把采樣結果和最后的光照結果相乘來產生陰影效果。

shader如下：

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

Shader "Custom/Forward Rendering"
{
    Properties
    {
        _MainTex("Main Texture", 2D) = "white" {}
        _Specular("Specular", Color) = (1,1,1,1)
        _Gloss("Gloss", Range(8,256)) = 8
    }

    SubShader
    {
        Pass
        {
            Tags
            {
                "LightMode" = "ForwardBase"
            }

            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #pragma multi_compile_fwdbase

            #include "UnityCg.cginc"
            #include "Lighting.cginc"
            #include "AutoLight.cginc"

            sampler2D _MainTex;
            fixed4 _Specular;
            float _Gloss;

            struct appdata
            {
                float4 vertex : POSITION;
                float2 uv : TEXCOORD0;
                float3 normal : NORMAL;
            };

            struct v2f
            {
                float4 pos : SV_POSITION;
                float2 uv : TEXCOORD0;
                float3 worldNormal : TEXCOORD1;
                float4 worldPos: TEXCOORD2;
                SHADOW_COORDS(3)
            };

            v2f vert(appdata v)
            {
                v2f o;
                o.pos = UnityObjectToClipPos(v.vertex);
                o.uv = v.uv;
                o.worldNormal = UnityObjectToWorldNormal(v.normal);
                o.worldPos = mul(unity_ObjectToWorld, v.vertex);
                TRANSFER_SHADOW(o);
                return o;
            }

            fixed4 frag(v2f i) : SV_TARGET
            {
                fixed4 albedo = tex2D(_MainTex, i.uv);
                fixed4 ambient = albedo * UNITY_LIGHTMODEL_AMBIENT;

                float3 worldLight = normalize(UnityWorldSpaceLightDir(i.worldPos.xyz));
                float3 worldView = normalize(UnityWorldSpaceViewDir(i.worldPos.xyz));
                fixed4 diff = albedo * _LightColor0 * max(0, dot(i.worldNormal, worldLight));

                float3 halfDir = normalize(worldView + worldLight);
                fixed4 spec = albedo * _Specular * pow(max(0, dot(halfDir, i.worldNormal)), _Gloss);

                float shadow = SHADOW_ATTENUATION(i);
                fixed4 col = ambient + (diff + spec) * shadow;
                return col;
            }

            ENDCG
        }

        Pass
        {
            Tags
            {
                "LightMode" = "ForwardAdd"
            }
            Blend One One

            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #pragma multi_compile_fwdadd

            #include "UnityCg.cginc"
            #include "Lighting.cginc"
            #include "AutoLight.cginc"

            sampler2D _MainTex;
            fixed4 _Specular;
            float _Gloss;
            
            struct appdata
            {
                float4 vertex : POSITION;
                float2 uv : TEXCOORD0;
                float3 normal : NORMAL;
            };

            struct v2f
            {
                float4 pos : SV_POSITION;
                float2 uv : TEXCOORD0;
                float3 worldNormal : TEXCOORD1;
                float4 worldPos: TEXCOORD2;
                SHADOW_COORDS(3)
            };

            v2f vert(appdata v)
            {
                v2f o;
                o.pos = UnityObjectToClipPos(v.vertex);
                o.uv = v.uv;
                o.worldNormal = UnityObjectToWorldNormal(v.normal);
                o.worldPos = mul(unity_ObjectToWorld, v.vertex);
                TRANSFER_SHADOW(o);
                return o;
            }

            fixed4 frag(v2f i) : SV_TARGET
            {
                fixed4 albedo = tex2D(_MainTex, i.uv);

                float3 worldLight = normalize(UnityWorldSpaceLightDir(i.worldPos.xyz));
                float3 worldView = normalize(UnityWorldSpaceViewDir(i.worldPos.xyz));
                fixed4 diff = albedo * _LightColor0 * max(0, dot(i.worldNormal, worldLight));

                float3 halfDir = normalize(worldView + worldLight);
                fixed4 spec = albedo * _Specular * pow(max(0, dot(halfDir, i.worldNormal)), _Gloss);

                // 參考 AutoLight.cginc
                float atten;
                #ifdef USING_DIRECTIONAL_LIGHT
                atten = 1;
                #else
                    float4 lightCoord = mul(unity_WorldToLight, i.worldPos);
                    #ifdef POINT
                    atten = tex2D(_LightTexture0, dot(lightCoord.xyz,lightCoord.xyz).xx).UNITY_ATTEN_CHANNEL;
                    #elif SPOT
                    atten = (lightCoord.z > 0) * tex2D(_LightTexture0, lightCoord.xy / lightCoord.w + 0.5).w * tex2D(_LightTextureB0, dot(lightCoord, lightCoord).xx).UNITY_ATTEN_CHANNEL;
                    #endif
                #endif

                float shadow = SHADOW_ATTENUATION(i);

                fixed4 col = (diff + spec) * atten * shadow;
                return col;
            }

            ENDCG
        }
    }

    Fallback "VertexLit"
}

效果如下：