背景

记录下在unity中如果想实现阴影，有哪些路子可以选择，目前看有两种
1.经典的shadowmap
2.planar projection

ShadowMap方式

如果开启renderer组件的cast shadows为on，开启平行光的light组件的shadow type，那么就会在物体shader中寻找LightMode=ShadowCaster的Pass进行渲染
场景有两个物体，平面和球体，使用unity内置的函数
平面的shader为

Shader "Unlit/plane_shader3"
{
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}
    }
    SubShader
    {
        Tags
        {
            "RenderType"="Opaque"
        }
        LOD 100

        Pass
        {
            Tags
            {
                "LightMode"="Forwardbase"
            }

            CGPROGRAM
            #pragma multi_compile_fwdbase;
            #pragma vertex vert
            #pragma fragment frag
            #include "UnityCG.cginc"
            #include "AutoLight.cginc"

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

            struct v2f
            {
                float2 uv : TEXCOORD0;
                float4 pos : SV_POSITION;
                SHADOW_COORDS(3) // float4 _ShadowCoord : TEXCOORD3;
            };

            sampler2D _MainTex;
            float4 _MainTex_ST;

            v2f vert(appdata v)
            {
                v2f o;
                o.pos = UnityObjectToClipPos(v.vertex);
                o.uv = TRANSFORM_TEX(v.uv, _MainTex);
                TRANSFER_SHADOW(o) //  a._ShadowCoord = ComputeScreenPos(a.pos);
                return o;
            }

            fixed4 frag(v2f i) : SV_Target
            {
                fixed shadow = SHADOW_ATTENUATION(i);
                fixed4 col = tex2D(_MainTex, i.uv);
                fixed4 col2 = fixed4(col.xyz * shadow, 1.0);
                return col2;
            }
            ENDCG
        }
        // 下面是需要写入深度图的物体添加这个pass
        Pass
        {
            Tags
            {
                "LightMode"="ShadowCaster"
            }

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

            struct v2f
            {
                V2F_SHADOW_CASTER;
            };

            v2f vert(appdata_base v)
            {
                v2f o;
                TRANSFER_SHADOW_CASTER_NORMALOFFSET(o)
                return o;
            }

            float4 frag(v2f i) : SV_Target
            {
                SHADOW_CASTER_FRAGMENT(i)
            }
            ENDCG
        }
    }
    //    FallBack "Specular"
}

球体的shader为

Shader "Unlit/sphere_shader"
{
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}
    }
    SubShader
    {
        Tags
        {
            "RenderType"="Opaque"
        }
        LOD 100

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

            #include "UnityCG.cginc"

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

            struct v2f
            {
                float2 uv : TEXCOORD0;
                float4 vertex : SV_POSITION;
            };

            sampler2D _MainTex;
            float4 _MainTex_ST;

            v2f vert(appdata v)
            {
                v2f o;
                o.vertex = UnityObjectToClipPos(v.vertex);
                o.uv = TRANSFORM_TEX(v.uv, _MainTex);
                return o;
            }

            fixed4 frag(v2f i) : SV_Target
            {
                // sample the texture
                fixed4 col = tex2D(_MainTex, i.uv);
                // apply fog
                return col;
            }
            ENDCG
        }

        // 下面是需要写入深度图的物体添加这个pass
        Pass
        {
            Name "ShadowCaster"
            Tags
            {
                "LightMode"="ShadowCaster"
            }

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

            struct v2f
            {
                V2F_SHADOW_CASTER;
            };

            v2f vert(appdata_base v)
            {
                v2f o;
                TRANSFER_SHADOW_CASTER_NORMALOFFSET(o)
                return o;
            }

            float4 frag(v2f i) : SV_Target
            {
                SHADOW_CASTER_FRAGMENT(i)
            }
            ENDCG
        }
    }
}

planar projection 平面投影方式

球体和立方体挂载下面这个shader，就是多渲染一个阴影投影到y=0的平面上

// shader，放在需要显示阴影的对象上
Shader "Custom/PlanarShadow1" {
	Properties{
	_Instensity("Shininess", Range(2, 4)) = 2.0  //光照强度
 
	_Diffuse("Diffuse Color",Color) = (1,1,1,1)
    //纹理贴图
	_MainTex("Main Tex",2D) = "white"{}
	//控制纹理颜色
	_Color("Color",Color) = (1,1,1,1)
	}
 
		SubShader{
//光照计算
	    Pass{
			Tags{"LightMode" = "ForwardBase"}
			    CGPROGRAM
	            #include "Lighting.cginc"
	            #pragma vertex vert
	            #pragma fragment frag
 
		//fixed4 _Diffuse;
		fixed4 _Color;
		sampler2D _MainTex;
		float4 _MainTex_ST;
		fixed4 _Specular;
		half _Gloss;
 
		struct a2v {
			float4 vertex:POSITION;//告诉unity把模型空间下的顶点坐标填充给vertex
			float3 normal:NORMAL;
			//纹理坐标  然后还要贴图，就可以取到该坐标的颜色，然后替换漫反射的颜色
			float4 texcoord:TEXCOORD0;
		};
		struct v2f {
			float4 svPos:SV_POSITION;
			float3 worldNormal:TEXCOORD0;
			float4 worldVertex:TEXCOORD1;
			float2 uv:TEXCOORD2;
		};
 
		v2f vert(a2v v) {
			v2f f;
			f.svPos = UnityObjectToClipPos(v.vertex);
			f.worldNormal = UnityObjectToWorldNormal(v.normal);
			f.worldVertex = mul(v.vertex, unity_WorldToObject);
			f.uv = v.texcoord.xy * _MainTex_ST.xy + _MainTex_ST.zw;
			return f;
		}
		fixed4 frag(v2f f) :SV_Target{
			//法线的方向
			fixed3 normalDir = normalize(f.worldNormal);
		    //光线的方向
			fixed3 lightDir = normalize(WorldSpaceLightDir(f.worldVertex));
			//返回的颜色值代替漫反射的颜色
			fixed3 texColor = tex2D(_MainTex, f.uv.xy)*_Color.rgb;
 
			fixed3 diffuse = _LightColor0.rgb * texColor * max(dot(normalDir, lightDir), 0);
			//反射光的方向
			//fixed3 reflectDir = normalize(reflect(-lightDir, normalDir));
			//视野方向
			fixed3 viewDir = normalize(UnityWorldSpaceViewDir(f.worldVertex));
			//blinn-Phong光照模型   计算平分线
			fixed3 halfDir = normalize(lightDir + viewDir);
 
			//漫反射光照+上环境光+纹理颜色
			fixed3 tempColor = diffuse + UNITY_LIGHTMODEL_AMBIENT.rgb*texColor;
 
			return fixed4(tempColor, 1);
 
		}
 
		 ENDCG
	   }
 
 
 
	//计算阴影
	Pass
	{
		Tags{"LightMode" = "ForwardBase"}
		  Stencil          //加个模板
			{
				Ref 0
				Comp equal
				Pass incrWrap
				Fail keep
				ZFail keep
			}
			ZWrite off
 
	//	Blend DstColor SrcColor
		Blend Srcalpha OneminusSrcAlpha
		Offset -1, -1		//使阴影在平面之上  
		CGPROGRAM
		#pragma vertex vert
		#pragma fragment frag
		#include "UnityCG.cginc"
 
		float4x4 _World2Ground;  //阴影接收平面（世界空间到模型空间的转换矩阵）
		float4x4 _Ground2World;	 //阴影接收平面（模型空间到世界空间的转换矩阵）
		float _Instensity;
 
		struct v2f {
			float4 pos:SV_POSITION;
			float atten : TEXCOORD0;
		};
 
	   v2f vert(float4 vertex:POSITION)
	   {
		   float3 litDir;
		   litDir = WorldSpaceLightDir(vertex);//世界空间主光照相对于当前物体的方向
		   litDir = mul(_World2Ground,float4(litDir,0)).xyz;//光源方向转换到接受阴影的平面空间
		   litDir = normalize(litDir);// 归一
		   float4 vt;
		   vt = mul(unity_ObjectToWorld,vertex); //将当前物体转换到世界空间
		   vt = mul(_World2Ground,vt); // 将物体在世界空间的矩阵转换到地面空间
		   vt.xz = vt.xz - (vt.y / litDir.y)*litDir.xz;// 用三角形相似计算沿光源方向投射后的XZ
		   vt.y = 0;// 使阴影保持在接受平面上
		   vt = mul(_Ground2World, vt); // 阴影顶点矩阵返回到世界空间
		   vt = mul(unity_WorldToObject, vt); // 返回到物体的坐标
		   v2f o;
		   o.pos = UnityObjectToClipPos(vt);//输出到裁剪空间
		   o.atten = distance(vertex, vt) / _Instensity;// 根据物体顶点到阴影的距离计算衰减
		   return o;
	   }
 
	   float4 frag(v2f i) :COLOR
	   {
		   return float4(0.3, 0.3, 0.3, 0.5);//一个灰色的阴影出来了
			//return smoothstep(0,1,i.atten / 2);
		}
 
		ENDCG
	   }
	}
}

参考

1.shadowmap https://www.bilibili.com/read/cv13646967/
2.平面投影
https://zhuanlan.zhihu.com/p/42781261
https://blog.csdn.net/cgy56191948/article/details/84990842