How to get view space position of a pixel

Hello, I am need to get the screen space of a pixel or vertex in a shader.

The reason is that I have a renderTexture screen grab, and I want to get the pixel that aligns with the same pixel on the renderTexture.

Here is a test screenshot mapping the x/y coordinates to r/g:

As you can see the y channel aligns at the bottom of the screen, x is nowhere, This changes depending on the distance of the camera from the pixel. Any ideas?

Here is the test shader:

 Shader "Tests/screenParams" {
     Properties {
     }
     SubShader {
         Pass {
             CGPROGRAM
             #include "UnityCG.cginc"
             #pragma vertex vert
             #pragma fragment frag
 
             struct vertexInput{
                 float4 vertex : POSITION;
             };
 
             struct vertexOutput{
                 float4 pos : SV_POSITION;
             };
 
             struct fragmentInput{
                 float4 pos : SV_POSITION;
             };
 
             vertexOutput vert(vertexInput i)
             {
                 vertexOutput o;
                 o.pos = mul(UNITY_MATRIX_MVP, i.vertex);
                 return o;
             }
 
             float4 frag(fragmentInput i): COLOR
             {
                 float4 texCoords = ComputeScreenPos(i.pos);
                 return float4(texCoords.x, texCoords.y, 0.5, 1.0);
             }
             ENDCG
         }
     }
 }