LambertVertexLighting.shader

/**
 * 頂点ライティング
*/
Shader "VertexLighting"
{
    SubShader
    {
        Tags { "RenderType"="Opaque"}
        Pass
        {
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            
            #include "UnityCG.cginc"
    
            struct appdata
            {
                float4 vertex : POSITION;
                float3 normal : NORMAL;
            };
    
            struct v2f
            {
                float4 vertex : SV_POSITION;
                float luminance : TEXCOORD0;
            };
    
            float3 _UserWorldSpaceLightDir;
            fixed4 _LightColor0;
            
            v2f vert (appdata v)
            {
                v2f o;
                o.vertex = UnityObjectToClipPos(v.vertex);
                float4 invLightDir = mul(UNITY_MATRIX_M, -_UserWorldSpaceLightDir);
                // 各頂点の拡散反射輝度を算出 
                float luminance = dot(v.normal, normalize(invLightDir));
                o.luminance = luminance;
                return o;
            }
            
            fixed4 frag (v2f i) : SV_Target
            {
                return float4(i.luminance, i.luminance, i.luminance, 1) * _LightColor0;
            }
            ENDCG
        }
    }
}

VertexLighting.cs

using UnityEngine;

[RequireComponent(typeof(MeshRenderer))]
public class VertexLighting : MonoBehaviour
{
    private Transform _lightTransform;
    private Material _material;
    private static readonly int UserWorldSpaceLightDir0Id = Shader.PropertyToID("_UserWorldSpaceLightDir");

    void Awake()
    {
        var directionalLight = FindObjectOfType<Light>();
        if (directionalLight != null)
            _lightTransform = directionalLight.transform;
        var meshRenderer = GetComponent<MeshRenderer>();
        _material = meshRenderer.material;
    }

    void Update()
    {
        if (_lightTransform != null)
        {
            // ライトのオイラー角から方向ベクトルを算出
            var direction = Quaternion.Euler(_lightTransform.eulerAngles) * Vector3.forward;
            // ライトベクトルをシェーダにわたす
            _material.SetVector(UserWorldSpaceLightDir0Id, direction);
        }
    }
}