addshadow and screenpos

Hello, I've been struggling for a while with a bug on my surface shader and succeed to isolate a strange behavior I can't understand. Sometimes shadows are rendered on part of my mesh that are transparent.

For exemple with this simple code

Code (CSharp):

1. Shader "Custom/CutoutShadowSurface"
2. {
3.     Properties
4.     {
5.         _MainTex ("Albedo (RGB)", 2D) = "white" {}
6.         _Cutout("Cutout", Range(0, 1)) = 0.5
7.     }
8.     SubShader
9.     {
10.         Tags { "RenderType"="Opaque" }
11.         LOD 200
12.  
13.         CGPROGRAM
14.         // Physically based Standard lighting model, and enable shadows on all light types
15.         #pragma surface surf SimpleLambert alphatest:_Cutout addshadow
16.  
17.         // Use shader model 3.0 target, to get nicer looking lighting
18.         #pragma target 3.0
19.  
20.         sampler2D _MainTex;
21.  
22.         struct Input
23.         {
24.             float2 uv_MainTex;
25.             float4 screenPos;
26.         };
27.  
28.         half _Glossiness;
29.         half _Metallic;
30.         fixed4 _Color;
31.  
32.         // Add instancing support for this shader. You need to check 'Enable Instancing' on materials that use the shader.
33.         // See https://docs.unity3d.com/Manual/GPUInstancing.html for more information about instancing.
34.         // #pragma instancing_options assumeuniformscaling
35.         UNITY_INSTANCING_BUFFER_START(Props)
36.             // put more per-instance properties here
37.         UNITY_INSTANCING_BUFFER_END(Props)
38.  
39.         half4 LightingSimpleLambert(SurfaceOutput s, half3 lightDir, half atten) {
40.             half NdotL = dot(s.Normal, lightDir);
41.             half4 c;
42.             c.rgb = s.Albedo /** _LightColor0.rgb * (NdotL * atten)*/;
43.             c.a = s.Alpha;
44.             return c;
45.         }
46.  
47.         void surf (Input IN, inout SurfaceOutput o)
48.         {
49.             float3 screenPos = IN.screenPos.xyz / IN.screenPos.w;
50.             fixed4 c = tex2D (_MainTex, screenPos);
51.             o.Albedo = float3(1, 1, 1);
52.             o.Alpha = c.r;
53.         }
54.         ENDCG
55.     }
56.     FallBack "Diffuse"
57. }
58.  

when I use some "dither noise" texture I found on internet I get this result:

but when I use the default unity knob texture I got no shadows at all:

When I use the UV texture coordinates instead of screenPos I don't have this problem.

 

Yep, the

screenPos

Input

is broken for Surface Shader generated shadow casters. You have to calculate it yourself if you want to use the screen position for shadows.
https://forum.unity.com/threads/try...ow-doesnt-work-correctly.912515/#post-6004391

 

Thanks! It works like a charm now