post effect gives me this strange effect

[JohnSonLi]

public class DungePostEffect : MonoBehaviour {
    public Material mat;
    private void OnRenderImage(RenderTexture src, RenderTexture dest)
    {
        Graphics.Blit(src, dest, mat);
    }
}

shader code of mat

Properties {
        _MainTex ("Base (RGB)", 2D) = "white" {}
        _Multiple ("sub", Range(0,1)) = 1
    }
    SubShader {
        Pass
        {
           alphaTest gequal .001
           Blend SrcAlpha OneMinusSrcAlpha
           
            LOD 200
            CGPROGRAM
            #include "UnityCG.cginc"
           sampler2D _MainTex;
           float _Multiple;
           
           struct verData
           {
               float4 vertex:POSITION;
               float2 texcoord:TEXCOORD0;
           };
           
           struct v2f
           {
               float4 pos:SV_POSITION;
               float2 uv:TEXCOORD0;
           };
           
           v2f vert(verData v)
           {
               v2f o;
               o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
               o.uv = v.texcoord;
               return o;
           }
           
           half4 frag(v2f i):COLOR
           {
               half4 col = tex2D(_MainTex,i.uv);
               return half4(col.r*_Multiple,col.g*_Multiple,col.b*_Multiple,col.a);
           }
           
           #pragma vertex vert
           #pragma fragment frag
           
            ENDCG
        }

anyone tells me where the problem is?

[JohnSonLi]

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