Implement KHR_materials_sheen Gltf extension

src/ThreeDModels/Format/Gltf/Extensions/KHR_materials_sheen.cs

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+using System.Text.Json;
+using ThreeDModels.Format.Gltf.Elements;
+using ThreeDModels.Format.Gltf.IO;
+using static ThreeDModels.Format.Gltf.IO.Utf8JsonReaderHelpers;
+
+namespace ThreeDModels.Format.Gltf.Extensions;
+
+/// <summary>
+/// Represents an object that defines the sheen material model.
+/// </summary>
+public class KHR_materials_sheen : IGltfProperty
+{
+    /// <summary>
+    /// Color of the sheen layer (in linear space).
+    /// </summary>
+    public required float[] SheenColorFactor { get; set; }
+    /// <summary>
+    /// The sheen color (RGB) texture.
+    /// </summary>
+    public TextureInfo? SheenColorTexture { get; set; }
+    /// <summary>
+    /// The sheen layer roughness of the material.
+    /// </summary>
+    public float SheenRoughnessFactor { get; set; }
+    /// <summary>
+    /// The sheen roughness (Alpha) texture.
+    /// </summary>
+    public TextureInfo? SheenRoughnessTexture { get; set; }
+    public Dictionary<string, object?>? Extensions { get; set; }
+    public object? Extras { get; set; }
+}
+
+public class KhrMaterialsSheenExtension : IGltfExtension
+{
+    public string Name => nameof(KHR_materials_sheen);
+    public static readonly float[] Default_SheenColorFactor = [0.0f, 0.0f, 0.0f];
+    public const float Default_SheenRoughnessFactor = 0.0f;
+
+    public object? Read(ref Utf8JsonReader jsonReader, GltfReaderContext context, Type parentType)
+    {
+        if (parentType != typeof(Material))
+        {
+            throw new InvalidDataException("KHR_materials_sheen must be used in a Material.");
+        }
+        float[]? sheenColorFactor = null;
+        TextureInfo? sheenColorTexture = null;
+        float? sheenRoughnessFactor = null;
+        TextureInfo? sheenRoughnessTexture = null;
+        Dictionary<string, object?>? extensions = null;
+        object? extras = null;
+        if (jsonReader.TokenType == JsonTokenType.PropertyName && jsonReader.Read())
+        {
+        }
+        if (jsonReader.TokenType == JsonTokenType.Null)
+        {
+            return null;
+        }
+        else if (jsonReader.TokenType != JsonTokenType.StartObject)
+        {
+            throw new InvalidDataException("Failed to find start of property.");
+        }
+        while (jsonReader.Read())
+        {
+            if (jsonReader.TokenType == JsonTokenType.EndObject)
+            {
+                break;
+            }
+            var propertyName = jsonReader.GetString();
+            if (propertyName == nameof(sheenColorFactor))
+            {
+                sheenColorFactor = ReadFloatList(ref jsonReader, context)?.ToArray();
+            }
+            else if (propertyName == nameof(sheenColorTexture))
+            {
+                sheenColorTexture = TextureInfoSerialization.Read(ref jsonReader, context);
+            }
+            else if (propertyName == nameof(sheenRoughnessFactor))
+            {
+                sheenRoughnessFactor = ReadFloat(ref jsonReader);
+            }
+            else if (propertyName == nameof(sheenRoughnessTexture))
+            {
+                sheenRoughnessTexture = TextureInfoSerialization.Read(ref jsonReader, context);
+            }
+            else if (propertyName == nameof(extensions))
+            {
+                extensions = ExtensionsSerialization.Read<KHR_materials_sheen>(ref jsonReader, context);
+            }
+            else if (propertyName == nameof(extras))
+            {
+                extras = JsonSerialization.Read(ref jsonReader, context);
+            }
+            else
+            {
+                throw new InvalidDataException($"Unknown property: {propertyName}");
+            }
+        }
+        if (sheenColorFactor != null && sheenColorFactor.Length != 3)
+        {
+            throw new InvalidDataException("KHR_materials_sheen.sheenColorFactor must have 3 elements.");
+        }
+        return new KHR_materials_sheen()
+        {
+            SheenColorFactor = sheenColorFactor ?? Default_SheenColorFactor,
+            SheenColorTexture = sheenColorTexture,
+            SheenRoughnessFactor = sheenRoughnessFactor ?? Default_SheenRoughnessFactor,
+            SheenRoughnessTexture = sheenRoughnessTexture,
+            Extensions = extensions,
+            Extras = extras,
+        };
+    }
+}