How to apply a shader to only one side of an object in Unity3D

Question

So I have this rather simple surface shader that essentially cuts a hole in the object based on another texture.

Shader "Custom/NewSurfaceShader" {
    Properties {
        _MainTex("Texture (RGB)", 2D) = "white" {}
    _SliceGuide("Slice Guide (RGB)", 2D) = "white" {}
    _SliceAmount("Slice Amount", Range(0.0, 1.0)) = 0.5
    }
    SubShader {
        Tags { "RenderType"="Opaque" }
        Cull OFF
        CGPROGRAM

#pragma surface surf Lambert addshadow
        struct Input {
        float2 uv_MainTex;
        float2 uv_SliceGuide;
        float _SliceAmount;
    };
    sampler2D _MainTex;
    sampler2D _SliceGuide;
    float _SliceAmount;



    void surf(Input IN, inout SurfaceOutput o) {

        clip(tex2D(_SliceGuide, IN.uv_SliceGuide).rgb - _SliceAmount);
        o.Albedo = tex2D(_MainTex, IN.uv_MainTex).rgb;
    }
    ENDCG
    }
        Fallback "Diffuse"
}

This produces the following effect.

Now as cool as this looks I want to only apply it to, let's say, one side of the cube and its opposite side. Any suggestions on how do I go about doing this will be appreciated.

Answer 1

I have an idea:

• Shoot Raycast from camera (if first person).
• Use textureCoord to find the UV coordinates.
• Pass the coordinates into the shader, and use them to draw the decal.

This should take care of one side of the object, the part facing the raycast. I can suggest a way to detect the other side of the object. But i do not know if this is efficient:

Suggestion: Shoot a 'backtrace' ray starting from quite a distance from the original ray:

You use the ray info from ray i described above and you see in the picture below (1st Ray), find the distance between the origin of the 1st Ray, and its hitPoint's (Hitpoint A) position. Then add some distance to that, and use that point in space to shoot a ray (2nd Ray) back to the 1st Ray's origin. Find the Hitpoint on the opposide side of the object and apply textureCoord again -> pass to shader -> use for second decal.

Keep in mind that there can be other objects hitting the rayCast that are in between the backtrace's origin and hitpoint on your desired object. A RaycastAll and a comparison should fix that possible problem.