Question Unity adding unwanted shadow attenuation to transparent surface shader

Hello

I'm currently working on a transparent surface shader, that should render shadows based on provided textures. These are the provided textures:


The first two textures are shadows of a scene after the sphere has been inserted, and the last two are the scene before the sphere has been inserted

I want to add the first two textures together(let's call the combined texture for after), and the last two textures together(let's call the combined texture for prev). With the texture combined, I first added 0.01 to both textures(to avoid division by 0), then multiplied both textures by newLa(which is_La/9). Lastly, I divide the two textures with each other. This should result in a texture containing only the shadows of the inserted sphere, where the shadow colour is determined by the _La value. The idea is further described in these papers:

• https://www.scitepress.org/Papers/2020/89430/89430.pdf
• https://vbn.aau.dk/ws/portalfiles/portal/4968814/icpr06.pdf

But the shadows' colour is not determined by the _La value, but instead, unity adds colour to the shadows themselves or so I assume. This I don't understand why because I don't use any of the functions in the AutoLight.cginc file and I don't obtain the light colour of the scene.

Any help in making the shadow take colour from the _La variable is much welcome

This is the code I'm running

Code (CSharp):

1. Shader "Custom/ShadowReceiver"
2. {
3.     Properties
4.     {
5.         _La("Ambient light from hemisphere", Vector) = (0, 0, 0, 0)
6.         _ShadowMask ("Sun texture", 2D) = "white" {}
7.         _AmbientMask ("Ambient texture", 2D) = "white" {}
8.         _AllHemiShadowMask ("After ambient texture", 2D) = "white" {}
9.         _AllSunShadowMask ("After ambient texture", 2D) = "white" {}
10.     }
11.     SubShader
12.     {
13.         Tags { "Queue" = "Geometry" }
14.        
15.         Pass
16.         {
17.             Cull Back
18.             ZTest LEqual
19.             ZWrite Off
20.             Blend dstColor Zero
21.             CGPROGRAM
22.          
23.             #pragma vertex vert
24.             #pragma fragment frag
25.             #include "UnityCG.cginc"
26.            
27.             uniform vector _La;
28.  
29.             sampler2D _ShadowMask;
30.             float4 _ShadowMask_ST;
31.            
32.             sampler2D _AllSunShadowMask;
33.             float4 _AllSunShadowMask_ST;
34.  
35.             sampler2D _AmbientMask;
36.             sampler2D _AllHemiShadowMask;
37.            
38.             float4 _AmbientMask_ST;
39.             float4 _AllHemiShadowMask_ST;
40.            
41.             struct v2f
42.             {
43.                 float4 pos : SV_POSITION;
44.                 float2 uv : TEXCOORD0;
45.                 float4 wPos: TEXCOORD1;
46.                 float4 viewPos: TEXCOORD2;
47.             };
48.             v2f vert(appdata_base v)
49.             {
50.                 v2f o;
51.                 o.pos = UnityObjectToClipPos (v.vertex);
52.                 o.wPos = mul(unity_ObjectToWorld, v.vertex);
53.                 o.viewPos = ComputeScreenPos(o.pos);
54.                 o.uv = v.texcoord;
55.          
56.                 return o;
57.             }
58.             fixed4 frag(v2f input) : COLOR
59.             {
60.                 float2 textureCoordinateAll = input.viewPos.xy / input.viewPos.w;
61.                 textureCoordinateAll = TRANSFORM_TEX(textureCoordinateAll, _AllSunShadowMask);
62.                 fixed4 afterSun = tex2D(_AllSunShadowMask,textureCoordinateAll);
63.                
64.                 float2 textureCoordinate = input.viewPos.xy / input.viewPos.w;
65.                 textureCoordinate = TRANSFORM_TEX(textureCoordinate, _ShadowMask);
66.                 fixed4 prevSun = tex2D(_ShadowMask, textureCoordinate);
67.  
68.                 float2 textureCoordinateHemi = input.viewPos.xy / input.viewPos.w;
69.                 float2 textureCoordinateHemiAll = input.viewPos.xy / input.viewPos.w;
70.                
71.                 textureCoordinateHemi = TRANSFORM_TEX(textureCoordinateHemi, _AmbientMask);
72.                 textureCoordinateHemiAll = TRANSFORM_TEX(textureCoordinateHemiAll, _AllHemiShadowMask);
73.                
74.                 float4 prevHemi = tex2D(_AmbientMask, textureCoordinateHemi);
75.                 float4 afterHemi = tex2D(_AllHemiShadowMask, textureCoordinateHemiAll);
76.  
77.                 fixed4 after = (afterSun + afterHemi) - 1;
78.                 fixed4 prev = prevHemi + prevSun - 1;
79.  
80.                 fixed4 newLa = _La/9;
81.                
82.                 return ((after+0.01) * newLa)/((prev+0.01) * newLa);
83.                
84.             }
85.             ENDCG
86.         }
87.     }
88.     FallBack "Vertex"
89. }