Alpha channel from render texture by screen position

Hello everyone!
I'm trying to write a shader that imitates the coloring of a picture with a brush. It looks like this. There is a "Brush" game object on the stage, which leaves behind a trail of particles. These particles are rendered into a render texture using the second camera. I plan to use this texture to get the alpha channel. And there is a paintable picture that has a material with my shader. It is assumed that the shader in the fragment function should take the alpha channel at the screen position from the render texture.

In the shader graph, the logic looks like this
And everything works great

But when I tried to write this shader myself, problems started. I can't to correctly calculate the screen position and translate it to the uv of render texture.

Here is the shader code

Code (CSharp):

1.  
2. Shader "Unlit/DrawShader2"
3. {
4.     Properties
5.     {
6.         [NoScaleOffset]_AlphaTexture("AlphaTexture", 2D) = "white" {}
7.         [NoScaleOffset]_MainTex("MainTex", 2D) = "white" {}
8.         _Alpha("Alpha", Float) = 1
9.     }
10.     SubShader
11.     {
12.         Tags { "RenderType"="Transparent"}
13.         Blend SrcAlpha OneMinusSrcAlpha, One OneMinusSrcAlpha
14.         Cull Off
15.         ZWrite Off
16.         LOD 200
17.  
18.         Pass
19.         {
20.             CGPROGRAM
21.             #pragma vertex vert
22.             #pragma fragment frag
23.  
24.             #include "UnityCG.cginc"
25.  
26.             struct appdata
27.             {
28.                 float4 vertex : POSITION;
29.                 float2 uv : TEXCOORD0;
30.             };
31.  
32.             struct v2f
33.             {
34.                 float2 uv : TEXCOORD0;
35.                 float4 vertex : SV_POSITION;
36.                 float4 screenPos: TEXCOORD1;
37.             };
38.  
39.             sampler2D _MainTex;
40.             sampler2D _AlphaTexture;
41.             float4 _MainTex_ST;
42.             float4 _AlphaTexture_ST;
43.             float4 _AlphaTexture_TexelSize;
44.  
45.             v2f vert (appdata v)
46.             {
47.                 v2f o;
48.                 o.vertex = UnityObjectToClipPos(v.vertex);
49.                 o.uv = TRANSFORM_TEX(v.uv, _MainTex);
50.                 o.screenPos = ComputeScreenPos(o.vertex);
51.                 return o;
52.             }
53.  
54.             fixed4 frag (v2f i) : SV_Target
55.             {
56.                 float2 screenUV = i.screenPos.xy/i.screenPos.zw;
57.                
58.                 fixed4 col = tex2D(_MainTex, i.uv);
59.                 fixed4 alphaCol = tex2D(_AlphaTexture, screenUV);
60.                 col.w = col.w * alphaCol.x;
61.                 return col;
62.             }
63.             ENDCG
64.         }
65.     }
66. }

here is result:



what i'm doing wrong?