Is there any way to do multiple passes on a texture / in a fragment shader?

Question

Is there any way to do multiple passes in a fragment shader? I need to transform the texture in the first step and then in the second step modify parts were affected by the first pass. Is there any way to do this with a single shader? I wrote an example one how I would like to to it but the first pass seems to be fully ignored:

 Shader "Test/FragmentShaderWithMemory" {
     Properties {
         _MainTex ("MainTexture", 2D) = "white" {}
     }
     SubShader {
 
         Pass {
             CGPROGRAM
             #pragma vertex vert_img
             #pragma fragment frag
             #include "UnityCG.cginc"
             
             uniform sampler2D _MainTex;
 
             float3 frag(v2f_img i) : COLOR {
                 float3 pixelColor = tex2D(_MainTex, i.uv);
                 // the red channel to 0 as a test:
                 pixelColor.x = 0; 
                 return pixelColor;
             }
             ENDCG
         }
         
         Pass {
             CGPROGRAM
             #pragma vertex vert_img
             #pragma fragment frag
             #include "UnityCG.cginc"
             
             uniform sampler2D _MainTex;
 
             float3 frag(v2f_img i) : COLOR {
                 float3 pixelColor = tex2D(_MainTex, i.uv);
                 // all pixels should now have red to 0, test by setting all green channels to 1:
                 if (pixelColor.x==0) pixelColor.y = 1; 
                 return pixelColor;
             }
             ENDCG
         }
 
     }
 }

Answer 1

You can actually use the GrabPass {} function as a bit of a hack here to pass information between passes in a shader. In your case, try the following:

      Shader "Test/FragmentShaderWithMemory" {
          Properties {
              _MainTex ("MainTexture", 2D) = "white" {}
          }
          SubShader {
      
              Pass {
                  CGPROGRAM
                  #pragma vertex vert_img
                  #pragma fragment frag
                  #include "UnityCG.cginc"
                  
                  uniform sampler2D _MainTex;
      
                  float3 frag(v2f_img i) : COLOR {
                      float3 pixelColor = tex2D(_MainTex, i.uv);
                      // the red channel to 0 as a test:
                      pixelColor.x = 0; 
                      return pixelColor;
                  }
                  ENDCG
              }
              
              GrabPass { "_MainTex2" }
 
              Pass {
                  CGPROGRAM
                  #pragma vertex vert_img
                  #pragma fragment frag
                  #include "UnityCG.cginc"
                  
                  uniform sampler2D _MainTex2;
      
                  float3 frag(v2f_img i) : COLOR {
                      i.uv.y = 1 - i.uv.y;    //You might have to do this to invert the grab pass texture in some cases where it's flipped
                      float3 pixelColor = tex2D(_MainTex2, i.uv);
                      // all pixels should now have red to 0, test by setting all green channels to 1:
                      if (pixelColor.x==0) pixelColor.y = 1; 
                      return pixelColor;
                  }
                  ENDCG
              }
      
          }
      }

Is there any way to do multiple passes on a texture / in a fragment shader?

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