// ARKit face tracking to blend shape weights
func mapARKitToAvatar(faceAnchor: ARFaceAnchor) -> [String: Float] {
    let arkit = faceAnchor.blendShapes

    // Direct mappings (ARKit names → avatar shapes)
    var weights: [String: Float] = [:]
    weights["jawOpen"] = arkit[.jawOpen]?.floatValue ?? 0
    weights["mouthSmileLeft"] = arkit[.mouthSmileLeft]?.floatValue ?? 0
    weights["mouthSmileRight"] = arkit[.mouthSmileRight]?.floatValue ?? 0
    weights["eyeBlinkLeft"] = arkit[.eyeBlinkLeft]?.floatValue ?? 0
    weights["eyeBlinkRight"] = arkit[.eyeBlinkRight]?.floatValue ?? 0

    // Derived expressions (combinations)
    let smile = ((weights["mouthSmileLeft"] ?? 0) + (weights["mouthSmileRight"] ?? 0)) / 2
    weights["expression_happy"] = smile

    return weights
}

// Photon PUN2 - efficient avatar sync
public struct AvatarState : INetworkStruct {
    // Pack position: 3 floats → 6 bytes (half precision)
    public Half3 Position;

    // Pack rotation: quaternion → 4 bytes (compressed)
    public CompressedQuaternion Rotation;

    // Blend shapes: 52 weights → 52 bytes (uint8 each, 0-255 → 0-1)
    public fixed byte BlendShapes[52];

    // Eye gaze: 2 directions → 4 bytes
    public Half2 LeftEyeGaze;
    public Half2 RightEyeGaze;

    // Total: ~70 bytes per update (vs 400+ uncompressed)
}

// Simplified SSS for real-time (pre-integrated)
float3 SubsurfaceScattering(float3 normal, float3 light, float3 view,
                            float curvature, float3 skinColor, float melanin) {
    float NdotL = dot(normal, light);

    // Wrap lighting for soft terminator
    float wrap = 0.5;
    float diffuse = saturate((NdotL + wrap) / (1 + wrap));

    // Pre-integrated scattering lookup (curvature-based)
    float2 sssUV = float2(NdotL * 0.5 + 0.5, curvature);
    float3 sss = tex2D(_SSSLookup, sssUV).rgb;

    // Melanin affects scattering color
    float3 scatterColor = lerp(float3(1, 0.4, 0.25), float3(0.8, 0.5, 0.4), melanin);

    return skinColor * diffuse + sss * scatterColor * (1 - diffuse);
}