Shader "Custom/TransparentShadowCollector"
{
    Properties
    {
        _ShadowIntensity ("Shadow Intensity", Range (0, 1)) = 0.6
    }
 
 
    SubShader
    {
 
        Tags {"Queue"="AlphaTest" }
 
        Pass
        {
            Tags {"LightMode" = "ForwardBase" }
            Cull Back
            Blend SrcAlpha OneMinusSrcAlpha
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #pragma multi_compile_fwdbase
 
            #include "UnityCG.cginc"
            #include "AutoLight.cginc"
            uniform float _ShadowIntensity;
 
            struct v2f
            {
                float4 pos : SV_POSITION;
                LIGHTING_COORDS(0,1)
            };
            v2f vert(appdata_base v)
            {
                v2f o;
                o.pos = UnityObjectToClipPos (v.vertex);
                TRANSFER_VERTEX_TO_FRAGMENT(o);
               
                return o;
            }
            fixed4 frag(v2f i) : COLOR
            {
                float attenuation = LIGHT_ATTENUATION(i);  //光照衰减
                return fixed4(0,0,0,(1-attenuation)*_ShadowIntensity);
            }
            ENDCG
        }
 
    }
    Fallback "VertexLit"
}

1. 为顶点/片段着色器增加阴影

   include "AutoLight.cginc"

   "LightMode" = "ForwardBase"
   LIGHTING_COORDS(0,1)

   define LIGHTING_COORDS(idx1,idx2) float3 _LightCoord : TEXCOORD##idx1; SHADOW_COORDS(idx)

   TRANSFER_VERTEX_TO_FRAGMENT(o);
   2.LIGHT_ATTENUATION 光照衰减
   在没有物体遮挡的情况下光照衰减为0