Is surface shader lighting bugged for sprite backsides? Normals are correctly reversed

edit: had a brainwrong on this one, this surface shader i was stripping down wasn't reversing the normalmaps in the first place.

I'll hijack my own thread to ask, then: Is there a simple way to do reversed normals in a surface shader?


Using localscale.x = -1, I've flipped a sprite with a simple surface shader material, and directly beneath a spotlight there appears to be an issue (reversed sprite is on the left):


To look into this, I set the o.Emission of the surface shader to equal the normal, so that we can clearly see the normalmap and ensure it's behaving correctly:


Does anyone know what might be going on here? Both sprites should be lit from the top, but the reversed sprite is being lit as though its y normal was reversed.

Here's the code of the shader, simply using a fresh surface shader with added bumpmap property which unpacks for the normals.

Code (CSharp):

1. Shader "Custom/normaltest" {
2.     Properties {
3.         _Color ("Color", Color) = (1,1,1,1)
4.         _MainTex ("Albedo (RGB)", 2D) = "white" {}
5.         _BumpMap("Normal map", 2d) = "white" {}
6.     }
7.     SubShader {
8.         Tags { "RenderType"="Opaque" "Queue" = "Transparent" }
9.         LOD 200
10.         Cull Off
11.         ZWrite Off
12.         Blend SrcAlpha OneMinusSrcAlpha
13.         ZTest Off
14.  
15.         CGPROGRAM
16.         // Physically based Standard lighting model, and enable shadows on all light types
17.         #pragma surface surf Standard fullforwardshadows
18.  
19.         // Use shader model 3.0 target, to get nicer looking lighting
20.         #pragma target 3.0
21.  
22.         sampler2D _MainTex;
23.         sampler2D _BumpMap;
24.  
25.         struct Input {
26.             float2 uv_MainTex;
27.         };
28.  
29.         half _Glossiness;
30.         half _Metallic;
31.         fixed4 _Color;
32.  
33.         // Add instancing support for this shader. You need to check 'Enable Instancing' on materials that use the shader.
34.         // See https://docs.unity3d.com/Manual/GPUInstancing.html for more information about instancing.
35.         // #pragma instancing_options assumeuniformscaling
36.         UNITY_INSTANCING_BUFFER_START(Props)
37.             // put more per-instance properties here
38.         UNITY_INSTANCING_BUFFER_END(Props)
39.  
40.         void surf (Input IN, inout SurfaceOutputStandard o) {
41.             // Albedo comes from a texture tinted by color
42.             fixed4 c = tex2D (_MainTex, IN.uv_MainTex) * _Color;
43.             o.Albedo = c.rgb;
44.             float4 BumpMap = tex2D(_BumpMap, IN.uv_MainTex);
45.             float3 n = UnpackNormal(BumpMap);
46.             o.Normal = n;
47.             o.Emission = n;
48.             o.Alpha = c.a;
49.             clip(o.Alpha - 0.05);
50.         }
51.         ENDCG
52.     }
53.     FallBack "Diffuse"
54. }
55.  

This might be driving me slightly mad, anyone have any ideas?