Unity Combining Passes in Shader

Question

I have a shader which uses multiple passes to determine the thickness of an object at each point. I want to use this thickness value in order to sample from a texture, which will determine the color of the screen at that point. However, since the calculation is performed in two passes, I don't have any way to access the one value which results at the end, which is what I would be using to index the uv for the texture. Thanks for any help in advance!

{
Properties
{
    _MainTex ("Texture", 2D) = "white" {}
}
SubShader
{
    Pass
    {
        Cull Back ZWrite On ZTest Always
        CGPROGRAM
        #pragma vertex vert
        #pragma fragment frag

        #include "UnityCG.cginc"

        struct appdata
        {
            float4 vertex : POSITION;
            float3 normal : NORMAL;
        };

        struct v2f
        {
            float2 depth : DEPTH;
            float4 vertex : SV_POSITION;
        };

        v2f vert (appdata v)
        {
            v2f o;
            o.vertex = UnityObjectToClipPos(v.vertex);
            o.depth = -UnityObjectToViewPos(v.vertex).z * 200 * _ProjectionParams.w;
            return o;
        }

        sampler2D _MainTex;

        fixed4 frag (v2f i) : SV_Target
        {
            float c=1-i.depth;
            fixed4 col = fixed4(c,c,c,1);
            return col;
        }
        ENDCG
    }
    Pass
    {
        Cull Front ZWrite On ZTest Always
        BlendOp RevSub
        Blend One One
        CGPROGRAM
        #pragma vertex vert
        #pragma fragment frag

        #include "UnityCG.cginc"

        struct appdata
        {
            float4 vertex : POSITION;
        };

        struct v2f
        {
            float2 depth : DEPTH;
            float4 vertex : SV_POSITION;
        };

        v2f vert (appdata v)
        {
            v2f o;
            o.vertex = UnityObjectToClipPos(v.vertex);
            o.depth = -UnityObjectToViewPos(v.vertex).z * 200* _ProjectionParams.w;
            return o;
        }

        sampler2D _MainTex;

        fixed4 frag (v2f i) : SV_Target
        {
            float c = 1-i.depth;
            //fixed4 col = tex2D(_MainTex,c.xx);
            fixed4 col = fixed4(c,c,c,1);
            return col;
        }
        ENDCG
    }
}
}

Answer 1

Use a grab pass

{
Properties
{
    _MainTex ("Texture", 2D) = "white" {}
}
SubShader
{
    Pass
    {
        Cull Back ZWrite On ZTest Always
        CGPROGRAM
        #pragma vertex vert
        #pragma fragment frag

        #include "UnityCG.cginc"

        struct appdata
        {
            float4 vertex : POSITION;
            float3 normal : NORMAL;
        };

        struct v2f
        {
            float2 depth : DEPTH;
            float4 vertex : SV_POSITION;
        };

        v2f vert (appdata v)
        {
            v2f o;
            o.vertex = UnityObjectToClipPos(v.vertex);
            o.depth = -UnityObjectToViewPos(v.vertex).z * 200 * _ProjectionParams.w;
            return o;
        }

        sampler2D _MainTex;

        fixed4 frag (v2f i) : SV_Target
        {
            float c=1-i.depth;
            fixed4 col = fixed4(c,c,c,1);
            return col;
        }
        ENDCG
    }

    GrabPass { "_GrabTexture" }        

    Pass
    {
        Cull Front ZWrite On ZTest Always
        BlendOp RevSub
        Blend One One
        CGPROGRAM
        #pragma vertex vert
        #pragma fragment frag

        #include "UnityCG.cginc"

        struct appdata
        {
            float4 vertex : POSITION;
        };

        struct v2f
        {
            float2 depth : DEPTH;
            float4 vertex : SV_POSITION;
            float4 grabUv : TEXCOORD0;
        };

        v2f vert (appdata v)
        {
            v2f o;
            o.vertex = UnityObjectToClipPos(v.vertex);
            o.depth = -UnityObjectToViewPos(v.vertex).z * 200* _ProjectionParams.w;
            o.grabUv = ComputeGrabScreenPos (o.vertex);
            return o;
        }

        sampler2D _MainTex;
        sampler2D _GrabTexture;

        fixed4 frag (v2f i) : SV_Target
        {
            fixed4 lastPassColor = tex2Dproj(_GrabTexture, i.grabUv);
            float c = 1-i.depth;
            fixed4 col = tex2D(_MainTex,c.xx);
            //fixed4 col = fixed4(c,c,c,1);
            return col;
        }
        ENDCG
    }
}
}