A Unity shader that detects flipped polygons.

00_FlipCheck.shader.md

FlipCheck.shader

Shader "FlipCheck"
{
    SubShader
    {
        Cull Off

        Pass
        {
            CGPROGRAM

            #pragma vertex Vertex
            #pragma fragment Fragment

            #include "UnityCG.cginc"

            struct Attributes
            {
                float4 position : POSITION;
                half3 normal : NORMAL;
            };

            struct Varyings
            {
                float4 position : SV_POSITION;
                half3 normal : NORMAL;
            };

            Varyings Vertex(Attributes input)
            {
                Varyings o;
                o.position = UnityObjectToClipPos(input.position);
                o.normal = UnityObjectToWorldNormal(input.normal);
                return o;
            }

            fixed4 Fragment(Varyings input, fixed vface : VFACE) : SV_Target
            {
                fixed3 c = input.normal / 2 + 0.5;
                c = lerp(c, half3(1, 0, 0), vface < 0);
                c = GammaToLinearSpace(c);
                return fixed4(c, 1);
            }

            ENDCG
        }
    }
}