hz-unity-code-review

name	hz-unity-code-review
license	Apache-2.0
description	Reviews Unity code targeting Meta Quest and Horizon OS for performance issues, rendering best practices, and common VR pitfalls. Use during code review or when diagnosing Quest performance problems in Unity projects.
allowed-tools	Bash(metavr:*) Bash(hzdb:*)

Unity Code Review for Meta Quest

When to Use

Use this skill when reviewing Unity C# code or project settings that target Meta Quest headsets. This includes:

• Reviewing scripts for VR performance issues
• Checking rendering pipeline configuration and settings
• Ensuring adherence to Quest-specific best practices
• Identifying common VR development pitfalls
• Validating input handling for controllers, hands, and eye tracking
• Auditing memory usage and GC allocation patterns

Key Review Areas

1. Rendering Pipeline Configuration

Quest applications must use the Universal Render Pipeline (URP) with specific settings optimized for mobile VR. The Built-in Render Pipeline is not recommended for new Quest projects.

Critical settings to verify:

• Single-pass multiview must be enabled (Player Settings > XR Plug-in Management > Oculus > Stereo Rendering Mode)
• Vulkan should be the primary graphics API
• Linear color space is required for correct lighting
• HDR should be disabled in URP asset settings
• Post-processing should be minimal or disabled

2. Draw Call Budgets and Batching

Quest has draw call budgets that vary by workload complexity. Every draw call has CPU overhead that directly impacts frame timing.

Metric	Quest 2 / Quest Pro	Quest 3 / Quest 3S
Draw calls (busy simulation)	80-200	200-300
Draw calls (medium simulation)	200-300	400-600
Draw calls (light simulation)	400-600	700-1000
Triangles per frame	750K-1M	1M-2M
SetPass calls	< 50	< 80

Enable and verify:

• Static batching for non-moving geometry
• GPU instancing for repeated objects
• SRP batcher for URP materials
• Dynamic batching for small meshes (< 300 vertices)

3. Shader Complexity

Mobile GPUs on Quest cannot handle desktop-class shaders. Review all materials for:

• Use of URP/Lit or URP/Simple Lit instead of Standard shader
• Custom shaders that minimize texture samples and ALU operations
• Avoidance of real-time shadows where possible (bake instead)
• No screen-space effects (SSAO, SSR, screen-space shadows)

4. Memory Management

GC allocations cause frame hitches and must be eliminated from hot paths.

// BAD: Allocates every frame
void Update() {
    string label = "Score: " + score.ToString();
    var enemies = FindObjectsOfType<Enemy>();
    var filtered = enemies.Where(e => e.IsAlive).ToList();
}

// GOOD: Zero allocations in Update
private StringBuilder _sb = new StringBuilder(32);
private List<Enemy> _enemyCache = new List<Enemy>();
private Enemy[] _enemyArray;

void Start() {
    _enemyArray = FindObjectsOfType<Enemy>();
}

void Update() {
    _sb.Clear();
    _sb.Append("Score: ");
    _sb.Append(score);
}

5. Input Handling

Quest supports multiple input modalities. Code should handle:

• Controllers: Use Unity's Input System Package for new projects (recommended); OVRInput is maintained for legacy support
• Hand tracking: OVRHand and OVRSkeleton for hand pose data
• Eye tracking: OVREyeGaze (Quest Pro / Quest 3, requires permission)
• Graceful switching between controller and hand tracking modes

6. Physics Configuration

Physics simulation is expensive on mobile. Review for:

• Physics timestep set to match target frame rate (72/90/120 Hz)
• Simplified collision meshes (use primitives, not mesh colliders)
• Reduced solver iterations (4-6 is usually sufficient)
• Layer-based collision matrix to minimize pair checks
• Rigidbody sleep thresholds configured appropriately

7. Audio Setup

Audio is often overlooked but can impact performance:

• Compress audio clips (Vorbis for music, ADPCM for short SFX)
• Use streaming load type for clips longer than 1 second
• Limit simultaneous audio sources (target < 16)
• Spatialize audio using Meta's audio SDK for 3D positioning

Quick Review Checklist

Area	Target	Notes
Draw calls	80-200 (busy) to 400-600 (light)	Use batching, instancing, atlasing
Triangles	750K-1M/frame	Use LODs, occlusion culling
Texture resolution	Max 2K, 4K sparingly	ASTC compression required
Shader	URP mobile shaders	No Standard shader, no screen-space effects
Rendering mode	Single-pass multiview	Must be enabled in XR settings
FFR	Enabled (High or HighTop)	Fixed foveated rendering reduces edge fragment cost
MSAA	4x quality / 2x perf	Free on tile-based GPU when configured correctly
Target frame rate	72 Hz minimum	90 Hz recommended, 120 Hz for smooth experiences
GC allocations	0 B/frame in steady state	No allocations in Update/LateUpdate/FixedUpdate
Audio sources	< 16 simultaneous	Use pooling for audio sources

What to Look For in Code

GC-Heavy Patterns

// Flag these patterns in code review:
Camera.main                          // Calls FindWithTag internally
GameObject.Find("name")             // Linear search every call
GetComponent<T>() in Update         // Cache the result
new List<T>() in Update             // Allocates on heap
string + string in Update           // Creates new string objects
foreach on non-List collections     // Enumerator allocation
LINQ queries (.Where, .Select)      // Multiple allocations
Boxing (int -> object)              // Heap allocation
delegate/lambda in hot paths        // Closure allocation

Update() Misuse

// BAD: Empty Update still has overhead
void Update() { }

// BAD: Logic that doesn't need per-frame execution
void Update() {
    SavePlayerPrefs();  // Should be event-driven
}

// GOOD: Use events, coroutines, or InvokeRepeating for non-per-frame logic
void OnScoreChanged(int newScore) {
    UpdateScoreUI(newScore);
}

Camera.main Anti-Pattern

// BAD: Camera.main uses FindWithTag internally
void Update() {
    transform.LookAt(Camera.main.transform);
}

// GOOD: Cache the reference
private Transform _cameraTransform;

void Start() {
    _cameraTransform = Camera.main.transform;
}

void Update() {
    transform.LookAt(_cameraTransform);
}

Find Calls in Hot Paths

// BAD: Expensive search every frame
void Update() {
    var player = GameObject.FindWithTag("Player");
    var rb = player.GetComponent<Rigidbody>();
}

// GOOD: Cache in Awake/Start or use dependency injection
private Rigidbody _playerRb;

void Awake() {
    _playerRb = GameObject.FindWithTag("Player").GetComponent<Rigidbody>();
}

Using metavr for Validation

You can use the metavr tool to validate builds and check device-side behavior. Invoke via metavr <args> (published as the npm package metavr; if metavr is not on PATH, run npx -y metavr <args>) — no install required.

# Check connected Quest device
metavr device list

# Install and run a build
metavr app install path/to/build.apk
metavr app launch com.company.app

# Check device logs for errors
metavr adb logcat --tag Unity

# Monitor GPU performance
metavr perf capture

Use device-side profiling to validate that code review findings translate to real performance improvements.

Reference Documents

For detailed guidance on specific topics, see the following reference documents:

• Performance Checklist — comprehensive performance targets and optimization strategies
• Rendering Best Practices — Quest-specific rendering configuration and shader guidelines
• Input Handling — controller, hand tracking, and eye tracking implementation
• Common Pitfalls — frequently encountered mistakes and their fixes