Unity adding unwanted shadow attenuation to transparent surface shader

[reptinias]

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

Shader "Custom/ShadowReceiver"
{
    Properties
    {
        _La("Ambient light from hemisphere", Vector) = (0, 0, 0, 0)
        _ShadowMask ("Sun texture", 2D) = "white" {}
        _AmbientMask ("Ambient texture", 2D) = "white" {}
        _AllHemiShadowMask ("After ambient texture", 2D) = "white" {}
        _AllSunShadowMask ("After ambient texture", 2D) = "white" {}
    }
    SubShader
    {
        Tags { "Queue" = "Geometry" }
       
        Pass
        {
            Cull Back
            ZTest LEqual
            ZWrite Off
            Blend dstColor Zero
            CGPROGRAM
          
            #pragma vertex vert
            #pragma fragment frag
            #include "UnityCG.cginc"
           
            uniform vector _La;

            sampler2D _ShadowMask;
            float4 _ShadowMask_ST;
           
            sampler2D _AllSunShadowMask;
            float4 _AllSunShadowMask_ST;

            sampler2D _AmbientMask;
            sampler2D _AllHemiShadowMask;
           
            float4 _AmbientMask_ST;
            float4 _AllHemiShadowMask_ST;
           
            struct v2f
            {
                float4 pos : SV_POSITION;
                float2 uv : TEXCOORD0;
                float4 wPos: TEXCOORD1;
                float4 viewPos: TEXCOORD2;
            };
            v2f vert(appdata_base v)
            {
                v2f o;
                o.pos = UnityObjectToClipPos (v.vertex);
                o.wPos = mul(unity_ObjectToWorld, v.vertex);
                o.viewPos = ComputeScreenPos(o.pos);
                o.uv = v.texcoord;
          
                return o;
            }
            fixed4 frag(v2f input) : COLOR
            {
                float2 textureCoordinateAll = input.viewPos.xy / input.viewPos.w;
                textureCoordinateAll = TRANSFORM_TEX(textureCoordinateAll, _AllSunShadowMask);
                fixed4 afterSun = tex2D(_AllSunShadowMask,textureCoordinateAll);
               
                float2 textureCoordinate = input.viewPos.xy / input.viewPos.w;
                textureCoordinate = TRANSFORM_TEX(textureCoordinate, _ShadowMask);
                fixed4 prevSun = tex2D(_ShadowMask, textureCoordinate);

                float2 textureCoordinateHemi = input.viewPos.xy / input.viewPos.w;
                float2 textureCoordinateHemiAll = input.viewPos.xy / input.viewPos.w;
               
                textureCoordinateHemi = TRANSFORM_TEX(textureCoordinateHemi, _AmbientMask);
                textureCoordinateHemiAll = TRANSFORM_TEX(textureCoordinateHemiAll, _AllHemiShadowMask);
               
                float4 prevHemi = tex2D(_AmbientMask, textureCoordinateHemi);
                float4 afterHemi = tex2D(_AllHemiShadowMask, textureCoordinateHemiAll);

                fixed4 after = (afterSun + afterHemi) - 1;
                fixed4 prev = prevHemi + prevSun - 1;

                fixed4 newLa = _La/9;
               
                return ((after+0.01) * newLa)/((prev+0.01) * newLa);
               
            }
            ENDCG
        }
    }
    FallBack "Vertex"
}