Read Quest 3 mesh data and render random color in Unity.

Quest3Mesh.cs

using System.Collections.Generic;
using System.IO;
using Newtonsoft.Json;
using Newtonsoft.Json.Linq;
using UnityEngine;

/// <summary>
/// Attach this script to an empty GameObject in an empty scene, add quest3JsonFilePath in the inspector and press Play.
/// </summary>
public class Quest3Mesh : MonoBehaviour
{
    // public fields for inspector
    public string quest3JsonFilePath = null;

    /// <summary>
    /// On Play, load the file from quest3JsonFilePath and parse
    /// </summary>
    void Start()
    {
        // load json
        try
        {
            using (StreamReader r = new StreamReader(quest3JsonFilePath))
            {
                string json = r.ReadToEnd();
                JObject jObj = JObject.Parse(json);
                LoadMeshObject(jObj, "global mesh");
            }
        }
        catch (JsonReaderException e)
        {
            using (StreamReader r = new StreamReader(quest3JsonFilePath))
            {
                string json = r.ReadToEnd();
                JArray jArr = JArray.Parse(json);
                for (int i = 0; i < jArr.Count; i++)
                {
                    JObject jObj = (JObject)jArr[i];
                    LoadMeshObject(jObj, (string)jObj.SelectToken($"semanticLabel"));
                }
            }
        }
    }

    static void LoadMeshObject(JObject jObj, string name)
    {
        // create entity
        var gobj = new GameObject();
        gobj.name = name;

        // parse vertices and indices, translating to unity coordinate frame from quest coordinate frame
        var vertices = new List<Vector3>();
        var indices = new List<int>();
        var v = 0;
        while (jObj.SelectToken($"vertices.{v}") != null)
        {
            vertices.Add(new Vector3(
                (float)jObj.SelectToken($"vertices.{v + 0}"),
                (float)jObj.SelectToken($"vertices.{v + 1}"),
                -(float)jObj.SelectToken($"vertices.{v + 2}")
            ));
            v += 3;
        }
        var i = 0;
        while (jObj.SelectToken($"indices.{i}") != null)
        {
            indices.Add((int)jObj.SelectToken($"indices.{i}"));
            i++;
        }
        var x = 0;
        while (jObj.SelectToken($"polygon.[{x}]") != null)
        {
            vertices.Add(new Vector4(
                (float)jObj.SelectToken($"polygon.[{x}].x"),
                (float)jObj.SelectToken($"polygon.[{x}].y"),
                -(float)jObj.SelectToken($"polygon.[{x}].z"),
                (float)jObj.SelectToken($"polygon.[{x}].w")
            ));
            indices.Add(x);
            x++;
        }
        if (jObj.SelectToken($"polygon.[2]") != null)
        {
            indices.Add(2);
        }

        // add mesh geometry
        var mesh = new Mesh();
        if (vertices.Count > 0)
        {
            mesh.SetVertices(vertices);
        }
        if (indices.Count > 0)
        {
            indices.Reverse();
            mesh.SetTriangles(indices, 0);
        }
        mesh.RecalculateNormals();
        mesh.RecalculateBounds();
        var filter = gobj.AddComponent<MeshFilter>();
        filter.sharedMesh = mesh;

        // add material/color
        var mr = gobj.AddComponent<MeshRenderer>();
        Color c = Random.ColorHSV();
        mr.material.color = c;

        // compute pose matrix
        var matrices = new List<Vector4>();
        int p = 0;
        while (jObj.SelectToken($"meshPose.{p}") != null)
        {
            matrices.Add(new Vector4(
                (float)jObj.SelectToken($"meshPose.{p + 0}"),
                (float)jObj.SelectToken($"meshPose.{p + 1}"),
                (float)jObj.SelectToken($"meshPose.{p + 2}"),
                (float)jObj.SelectToken($"meshPose.{p + 3}")
            ));
            p += 4;
        }
        while (jObj.SelectToken($"planePose.{p}") != null)
        {
            matrices.Add(new Vector4(
                (float)jObj.SelectToken($"planePose.{p + 0}"),
                (float)jObj.SelectToken($"planePose.{p + 1}"),
                (float)jObj.SelectToken($"planePose.{p + 2}"),
                (float)jObj.SelectToken($"planePose.{p + 3}")
            ));
            p += 4;
        }

        // set pose
        if (matrices.Count > 0)
        {
            FromMatrix(gobj.transform, new Matrix4x4(matrices[0], matrices[1], matrices[2], matrices[3]));
        }

        static void FromMatrix(Transform transform, Matrix4x4 m)
        {
            if (m.ValidTRS())
            {
                transform.localPosition = new Vector3(
                    m.m03,
                    m.m13,
                    -m.m23
                );
                transform.localRotation = new Quaternion(
                    -m.rotation.x,
                    -m.rotation.y,
                    m.rotation.z,
                    m.rotation.w
                );
                transform.localScale = m.lossyScale;
            }
        }
    }
}