Post processing fragment shader: draw shape in world space?

Hi,

I have little experience with shaders, but I'm trying to set up a post-processing shader. For that, I'm simply calling Graphics.Blit(...) on the Camera's OnRenderImage(...) function. No problem there.

(I have edited the post from here on to rewrite the question to be a bit more specific)

Currently, I am simply trying to draw a circle at a position in the world (currently hard-coded at (0, 0, 2.5)). Here's my shader so far:

 Properties
     {
         _MainTex("", 2D) = "white" {}
     }
     SubShader
     {
         ZTest Always Cull Off ZWrite Off Fog{Mode Off}
 
         Pass
         {
             CGPROGRAM
 #pragma vertex vert
 #pragma fragment frag
 #include "UnityCG.cginc"
 
             struct v2f
             {
                 float4 pos : POSITION;
                 half2 uv : TEXCOORD0;
             };
  
             v2f vert(appdata_img v)
             {
                 v2f o;
                 o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
                 o.uv  = MultiplyUV(UNITY_MATRIX_TEXTURE0, v.texcoord.xy);
 
                 return o;
             }
 
             sampler2D _MainTex;
 
             fixed4 frag(v2f i) : COLOR
             {
                 fixed4 orgCol = tex2D(_MainTex, i.uv);
 
                 fixed4 col = orgCol;
 
                 // Circle at (0, 0, 2.5) in the world
                 float3 camToCenter = float3(0.0f, 0.0f, 2.5f) - _WorldSpaceCameraPos;
                 float3 camToCenterN = normalize(camToCenter);
 
                 // Camera base vectors
                 float3 cameraRightW = mul((float3x3)unity_CameraToWorld, float3(1, 0, 0));
                 float3 cameraUpW    = mul((float3x3)unity_CameraToWorld, float3(0, 1, 0));
                 float3 cameraFwdW   = mul((float3x3)unity_CameraToWorld, float3(0, 0, 1));
 
                 // Screen size
                 float scrW = _ScreenParams.x;
                 float scrH = _ScreenParams.y;
 
                 // Ray through current pixel
                 // w = width of screen in world space
                 // h = height of screen in world space
                 // w = 2 * tan(0.5 * fovHor), assuming screen is 1 unit in front of camera (in world space)
                 // fovHor = 2 * atan(1 / unity_CameraProjection._m11)
                 // => w = 2 / unity_CameraProjection._m11
                 float w = 2.0f / unity_CameraProjection._m11;
                 float h = w * scrH / scrW;
                 float3 ray = cameraFwdW + (i.uv.x - 0.5f) * w * cameraRightW
                                         + (i.uv.y - 0.5f) * h * cameraUpW;
                 ray = normalize(ray);
 
                 // Make circle of radius 1 (in world space)
                 float alpha = acos(dot(ray, camToCenterN));
                 int makeRed = (abs(length(camToCenter) * sin(alpha)) < 1.0f);
                 col.r =  makeRed;
                 col.g = (makeRed ? 0.0f : col.g);
                 col.b = (makeRed ? 0.0f : col.b);
 
                 // PROBLEM: resulting circle is:
                 //   -> Not of radius 1 (appears to be 1.5)
                 //   -> Not at (0, 0, 2.5) when not looking exactly at (0, 0, 2.5)
 
                 return col;
             }
             ENDCG
         }
     }

The problem I am having: This is supposed to draw a red circle of radius 1 at (0, 0, 2.5). When I look at (0, 0, 2.5), there is a red circle there, and I can move the camera around and it still works.

But when I am not looking exactly at (0, 0, 2.5), the circle moves a bit away as the camera rotates. Moreover, the radius of the circle isn't actually 1. When I add a sphere at (0, 0, 2.5), it is smaller; I need to give it a radius of 1.5.

I noticed that a bunch of things aren't quite as easy as in regular shaders, since this shader is applied as a post process.