Getting the world position of the pixel in the fragment shader.

Question

Hello guys !
I'm struggling with a custom shader :

 Shader "Custom/HaloEffect" {
     Properties{
         _Position("Position", Vector) = (.0, .0, .0)
         _HaloColor("Halo Color", Color) = (1.0, 1.0, 1.0, 1.0)
     }
 
     SubShader {
         Tags { "Queue"="Transparent" "Render"="Transparent" "IgnoreProjector"="True"}
         LOD 200
         
         ZWrite Off
         Blend SrcAlpha OneMinusSrcAlpha
 
         Pass{
             CGPROGRAM
 
             #pragma target 3.0
             #pragma vertex vert
             #pragma fragment frag
 
             #include "UnityCG.cginc"
 
             struct appdata {
                 float4 vertex : POSITION;
                 
             };
 
             struct v2f {
                 float4 vertex : SV_POSITION;
                 float4 color : COLOR0;
             };
 
             uniform float3 _Position;
             uniform float4 _HaloColor;
             
 
             v2f vert(appdata v) {
                 v2f o;
 
                 o.color = _HaloColor;
                 o.color.a = 1 - distance(mul(unity_ObjectToWorld, v.vertex), _Position) ;
                 o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
 
                 return o;
             }
 
             fixed4 frag(v2f i) : SV_Target {
                 return i.color;
             }
 
             ENDCG
         }
 
     
     }
     FallBack "Diffuse"
 }

The desired behaviour would have been to smootly set the alpha around the world coordinates named "_Position" on the mesh that would execute this shader. In this version the results are pretty near of what I expected. But the distances calculus are operated only on the vertices and the final interpolation is weird.

As you can see here the _Position is between 4 vertices of the plane:
alt text

And here is on one vertex: alt text

I think It does the right work regarding what I wrote. But, what I wished was a kind of a smooth circle.
I think I can managed to obtain this by doing the distance calculus in the Fragment Shader.
But I dont know how to get the world coordinates of the current pixel.
I try to compute an inverse of the MVP Matrix applied to the SV_POSITION, but it turns really bad.
Is the SV_POSITION is the same that gl_FragCoord ?
If I'am totally on the wrong way I wish you guys would tell me.
Please help me find a solution.

Answer 1

  Shader "Custom/HaloEffect" {
      Properties{
          _Position("Position", Vector) = (.0, .0, .0)
          _HaloColor("Halo Color", Color) = (1.0, 1.0, 1.0, 1.0)
      }
  
      SubShader {
          Tags { "Queue"="Transparent" "Render"="Transparent" "IgnoreProjector"="True"}
          LOD 200
          
          ZWrite Off
          Blend SrcAlpha OneMinusSrcAlpha
  
          Pass{
              CGPROGRAM
  
              #pragma target 3.0
              #pragma vertex vert
              #pragma fragment frag
  
              #include "UnityCG.cginc"
  
              struct appdata {
                  float4 vertex : POSITION;
                  
              };
  
              struct v2f {
                  float4 vertex : SV_POSITION;
                  float3 worldPos : TEXCOORD0;
              };
  
              uniform float3 _Position;
              uniform float4 _HaloColor;
              
  
              v2f vert(appdata v) {
                  v2f o;
  
                  o.worldPos = mul (unity_ObjectToWorld, v.vertex);
                  o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
  
                  return o;
              }
  
              fixed4 frag(v2f i) : SV_Target {
                  fixed4 col = _HaloColor;
                  col.a = 1 - distance(i.worldPos), _Position) ;
                  return col;
              }
  
              ENDCG
          }
  
      
      }
      FallBack "Diffuse"
  }

Does it not work to just move the calculations into the fragment shader?

Getting the world position of the pixel in the fragment shader.

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