arfoundation-samples/Assets/Scripts/FaceMaterialSwitcher.cs

using System.Collections.Generic;
using UnityEngine;
using UnityEngine.XR.ARFoundation;
using UnityEngine.XR.ARSubsystems;

namespace UnityEngine.XR.ARFoundation.Samples
{
    public class FaceMaterialSwitcher : MonoBehaviour
    {
        [SerializeField]
        [Tooltip("Materials to use for face meshes.")]
        Material[] m_FaceMaterials;

        /// <summary>
        /// Getter/setter for the Face Materials.
        /// </summary>
        public Material[] faceMaterials
        {
            get { return m_FaceMaterials; }
            set { m_FaceMaterials = value; }
        }

        static int s_CurrentMaterialIndex;
        static Dictionary<TrackableId, Material> s_FaceTracker = new Dictionary<TrackableId, Material>();

        void Start()
        {
            ARFace face = GetComponent<ARFace>();
            Material mat;
            if (!s_FaceTracker.TryGetValue(face.trackableId, out mat))
            {
                s_FaceTracker.Add(face.trackableId, m_FaceMaterials[s_CurrentMaterialIndex]);
                GetComponent<MeshRenderer>().material = m_FaceMaterials[s_CurrentMaterialIndex];
                s_CurrentMaterialIndex = (s_CurrentMaterialIndex + 1) % m_FaceMaterials.Length;
            }
            else
            {
                // Assign the material that was already used for the face's unique id.
                GetComponent<MeshRenderer>().material = mat;
            }
        }
    }
}