Implement KHR_materials_volume Gltf extension

src/ThreeDModels/Format/Gltf/Extensions/KHR_materials_volume.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 parameters for the volume of a material.
+/// </summary>
+public class KHR_materials_volume : IGltfProperty
+{
+    /// <summary>
+    /// The thickness of the volume beneath the surface.
+    /// </summary>
+    public float ThicknessFactor { get; set; }
+    /// <summary>
+    /// Texture that defines the thickness of the volume, stored in the G channel.
+    /// </summary>
+    public TextureInfo? ThicknessTexture { get; set; }
+    /// <summary>
+    /// Average distance that light travels in the medium before interacting with a particle.
+    /// </summary>
+    public float AttenuationDistance { get; set; }
+    /// <summary>
+    /// Color that white light turns into due to absorption when reaching the attenuation distance.
+    /// </summary>
+    public required float[] AttenuationColor { get; set; }
+    public Dictionary<string, object?>? Extensions { get; set; }
+    public object? Extras { get; set; }
+}
+
+public class KhrMaterialsVolumeExtension : IGltfExtension
+{
+    public string Name => nameof(KHR_materials_volume);
+    public const float Default_ThicknessFactor = 0.0f;
+    public const float Default_AttenuationDistance = 0.01f;
+    public static readonly float[] Default_AttenuationColor = [1.0f, 1.0f, 1.0f];
+
+    public object? Read(ref Utf8JsonReader jsonReader, GltfReaderContext context, Type parentType)
+    {
+        if (parentType != typeof(Material))
+        {
+            throw new InvalidDataException("KHR_materials_volume must be used in a Material.");
+        }
+        float? thicknessFactor = null;
+        TextureInfo? thicknessTexture = null;
+        float? attenuationDistance = null;
+        float[]? attenuationColor = 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(thicknessFactor))
+            {
+                thicknessFactor = ReadFloat(ref jsonReader);
+            }
+            else if (propertyName == nameof(thicknessTexture))
+            {
+                thicknessTexture = TextureInfoSerialization.Read(ref jsonReader, context);
+            }
+            else if (propertyName == nameof(attenuationDistance))
+            {
+                attenuationDistance = ReadFloat(ref jsonReader);
+            }
+            else if (propertyName == nameof(attenuationColor))
+            {
+                attenuationColor = ReadFloatList(ref jsonReader, context)?.ToArray();
+            }
+            else if (propertyName == nameof(extensions))
+            {
+                extensions = ExtensionsSerialization.Read<KHR_materials_volume>(ref jsonReader, context);
+            }
+            else if (propertyName == nameof(extras))
+            {
+                extras = JsonSerialization.Read(ref jsonReader, context);
+            }
+            else
+            {
+                throw new InvalidDataException($"Unknown property: {propertyName}");
+            }
+        }
+        if (attenuationColor != null && attenuationColor.Length != 3)
+        {
+            throw new InvalidDataException("KHR_materials_volume.attenuationColor must have 3 elements.");
+        }
+        return new KHR_materials_volume()
+        {
+            ThicknessFactor = thicknessFactor ?? Default_ThicknessFactor,
+            ThicknessTexture = thicknessTexture,
+            AttenuationDistance = attenuationDistance ?? Default_AttenuationDistance,
+            AttenuationColor = attenuationColor ?? Default_AttenuationColor,
+            Extensions = extensions,
+            Extras = extras,
+        };
+    }
+}