Optimizing Physics Colliders in Complex Game Scenes

A scene with 800 GameObjects, each with a MeshCollider based on real geometry — PhysX spends 8–12 ms per FixedUpdate on Broadphase and Narrowphase checks. That's half the frame budget for physics alone. In VR, where the frame budget is 11 ms (90 FPS), this causes an instant drop. Our team has encountered such projects many times: collider optimization is a task where proper architecture from the start saves weeks of work. Typical savings on a project range from 40 to 60 hours of manual labor. We guarantee stable 90 FPS after improvements. Contact us — we will assess your project and propose an optimization plan.

How PhysX Spends Time and Where the Losses Are

Physics in Unity goes through two stages: Broadphase (quick elimination: which objects are even close to each other?) and Narrowphase (exact intersection check for filtered pairs).

Broadphase uses an AABB tree (Axis-Aligned Bounding Box). If objects are constantly moving, the tree is constantly rebuilt. 200 Rigidbodies with isKinematic = false that move every frame cause a continuous AABB tree rebuild. Solution: objects that are not moving at the moment should be set to isKinematic = true via script — they drop out of the dynamic tree.

Narrowphase is where MeshCollider kills performance. Checking intersection of two arbitrary meshes is O(n×m) per triangles. A player collider against a building collider with 50,000 triangles means thousands of operations per Narrowphase. PhysX does not support non-convex MeshCollider against MeshCollider for dynamic objects at all — only against static.

Why Is MeshCollider So Expensive?

Every triangle in a MeshCollider requires a Narrowphase check. For static objects (walls, floors) this is tolerable, but for dynamic objects it's a disaster. Even with convex = true, physics evaluates all faces of the convex hull. Practice: replacing 10 complex MeshColliders on characters with Compound Collider reduced Physics CPU time by 40% in one VR simulator.

How Compound Collider Reduces Load

Compound Collider — a set of primitives as children of one Rigidbody. The only way to get non-convex dynamic physics without using a non-convex MeshCollider: several convex pieces = complex shape. For VR weapons, detailed objects, robots — standard approach. Replacing one MeshCollider with a Compound of 3–5 primitives reduces physics cost by 50 times while maintaining >90% accuracy.

Practical Replacements and Simplifications

First rule: replace MeshCollider where the player won't notice. A chair with MeshCollider based on real geometry is 200 triangles. The same chair with 2 BoxColliders (legs + seat) is 2 primitives. Collision accuracy is 95% of the original, cost — 50 times lower.

For organic shapes (rocks, barrels, cars), a set of 3–6 ConvexMeshColliders (convex simplified meshes) works many times faster than non-convex and is visually indistinguishable. In Unity: MeshCollider.convex = true + simplified mesh via LOD or manual collision proxy creation in Blender.

Collision Matrix (Layer Collision Matrix). In Physics Settings → Layer Collision Matrix, disable checks between layers that should never interact: Environment vs Environment, UI vs Physics Objects, VFX vs any layer. Each disabled pair means fewer Broadphase pairs. With proper matrix configuration, the number of active pairs can be reduced by 30–50%.

Sleep Threshold. Rigidbody falls asleep when velocity drops below Physics.sleepThreshold. A sleeping Rigidbody does not participate in physics — almost zero cost. The default value (0.005) is too low for complex scenes: objects remain active during micro-vibrations. For VR scenes without fluid/cloth simulation, set it to 0.1–0.2.

How We Perform Optimization: Case Study

From practice: in a VR simulator for industrial equipment, a workshop scene contained 1200 objects, most with MeshCollider from imported CAD models (15,000–80,000 triangles each). Physics CPU time — 14 ms. After replacing all static MeshColliders with convex proxies (automatically via Editor script + VHACD decomposition) and disabling unnecessary Layer Matrix pairs — Physics CPU time dropped to 2.8 ms. Gain — 80% of physics time. Average savings per project range from 40 to 60 hours of development. Cloud computing cost reduction due to less physics reaches 30–50%.

Diagnostic Tools

Physics Profiler in Unity Profiler — shows time for Broadphase, Narrowphase, SimulateAndCollide, UpdateTriggers separately. Here you can see where the losses are.

Physics Debugger (Window → Analysis → Physics Debugger) — visualizes colliders directly in Scene View with color coding by type (Static, Dynamic, Kinematic, Trigger). Allows quickly finding objects with unexpectedly heavy colliders.

PhysicsViewer custom Editor Tool — a script that outputs a list of all MeshColliders in the scene, sorted by triangle count. We write it in 30 minutes, saving hours of searching.

What Is Included in the Work

• Profiling Physics CPU time, identifying bottlenecks
• Full inventory of scene colliders with classification
• Replacing heavy MeshColliders with primitives/Compound while maintaining accuracy
• Configuring Layer Collision Matrix and Sleep Threshold
• Writing Editor scripts for batch replacement and validation automation
• Re-profiling and a report with performance delta
• Providing documentation and training the team to maintain optimization

Work Stages

1. Profiling — Physics Profiler snapshot, identifying bottlenecks.
2. Scene audit — inventory of all colliders, classification by type and cost.
3. Optimization — replacing MeshColliders with primitives/Compound, tuning Layer Matrix, Sleep Threshold.
4. Automation tools — Editor scripts for batch replacement and validation.
5. Re-profiling — comparison with baseline, documentation of changes.

The cost is calculated after profiling and assessing the scope of changes. Contact us — we will assess your project for free and propose an optimization plan. Order profiling today: our team guarantees stable 90 FPS after improvements.

VR and AR Development

When we first launch a project in a VR headset, most teams face the same thing: technically everything works, but in the headset either motion sickness occurs, or hands 'float' with a delay, or the scene looks jerky at the periphery. These are not bugs in the usual sense — they are a consequence of the fact that VR/AR development requires a different approach to render architecture, interaction, and UX from the very beginning of the project. Our experience: over 7 years in game dev, 15+ completed VR/AR projects for Meta Quest, SteamVR, PSVR2, HoloLens. We work with teams that need not just a prototype but a production‑ready application with a stable frame rate.

Platforms and SDKs

We work with all relevant stacks. We use OpenXR as the base layer wherever possible — it provides cross‑platform compatibility between Meta, Valve Index, HP Reverb and other PC VR devices. On top of OpenXR, we build on the XR Interaction Toolkit (Unity) or VR Expansion Plugin (Unreal). Contact us for a stack assessment tailored to your project.

How to minimize motion sickness in VR locomotion?

Locomotion — the main source of motion sickness for inexperienced VR users. According to research, about 70% of users experience discomfort with improper movement settings Oculus Developer Guidelines. Teleportation — standard navigation method when smooth movement is undesirable.

Components from XR Interaction Toolkit: TeleportationArea, TeleportationAnchor, TeleportationProvider. Basic implementation works out of the box, but for production we refine it in four steps:

1. Setting up XRRayInteractor with a curved ray (Bend Ray) — the teleportation arc looks more natural than a straight ray and is perceived better by users.
2. Adding a valid landing zone — a visual indicator changes color when hovering over an obstacle (red/green).
3. Implementing fade transition — smooth screen fade (black fade) before teleportation reduces disorientation.
4. Rotation snapping — after teleportation we offer snap rotation by 45° or 90° instead of smooth, reducing motion sickness risk.

For projects requiring smooth locomotion (action games, simulators), we use comfort settings: vignetting during movement, reducing FOV during acceleration. Settings are available to the user in the menu — different people have different sensitivity thresholds. The difference between kinematic and physics‑based movement: kinematic gives instant hand following but lets objects pass through walls; physics‑based via Joint provides realistic collisions but requires velocity damping and max joint force tuning. We choose based on the type of interaction.

How to make object grabbing in VR physically realistic?

This is the most underestimated part of VR development. Clients often perceive it as 'just hand animation', but in practice it is a complex system where physical correctness, responsiveness, and comfort conflict.

Grab (grabbing)

XR Interaction Toolkit provides three types of Interactable for grabbing:

• XRGrabInteractable — standard grab, object follows controller via physics joint or direct position/rotation
• XRSimpleInteractable — for objects without physical movement (buttons, levers)
• Custom Interactable by inheriting from XRBaseInteractable

Attach Transform — a frequently ignored detail. Each Interactable must have a properly configured Attach Transform (the point where the hand 'attaches'). Without it, the pistol grip will be at the center of the mesh, not where it is held.

For weapons and tools with two‑handed grab — a separate TwoHandGrab system: leading hand determines position, the second — orientation. XR Interaction Toolkit supports this via XRTwoHandGrabInteractable or custom logic with two Attach Points.

Throw (throwing)

Velocity smoothing is critical for realistic throwing because the Rigidbody.velocity at the moment of controller release reflects instantaneous speed, often incorrect due to tracking discretization. The user makes a quick wrist movement — but the object flies half as fast.

Solution: velocity smoothing over the last N frames (typically 5–10 frames, ~80–160 ms at 60 Hz) before release. XR Interaction Toolkit does this via VelocityEstimator. Additionally, we apply a velocity scaling multiplier — a small speed increase (1.2–1.5×) makes throws subjectively more satisfying. Angular velocity (for objects that should spin in flight) is also averaged similarly.

AR: Plane Tracking and Environment Interaction

AR adds a different class of problems — working with real, unpredictable environment. AR Foundation — a cross‑platform layer on top of ARKit and ARCore. Most basic features (plane detection, raycasting, image tracking, face tracking) are available through a unified API.

Plane Detection

ARPlaneManager detects horizontal and vertical planes. Practical nuances:

• Initialization takes time — the user must look around the room while the system builds a map. An explicit onboarding with instruction 'slowly move the camera across surfaces' is needed.
• Planes are unstable — their boundaries and position are updated as data accumulates. Objects placed on a plane need to be attached via parent to ARPlane, not to world coordinates.
• Plane merging — two detected floor segments may merge into one, moving the anchor. For critical anchors, use ARAnchor instead of direct attachment to the plane.

Image tracking (via ARTrackedImageManager) quality directly depends on the quality of reference images. Images with high detail frequency and contrasting edges (like a QR code but stylish) track more reliably than smooth logos. ARCore Geospatial API — for outdoor AR with real‑world coordinate binding (accuracy up to 10 cm in well‑mapped areas).

Optimization for VR: Frame Rate and Comfort

VR requires stable high frame rate. About 60% of development time in mobile VR goes to optimization, not functionality — retrofit costs twice as much as proper architecture from the first sprint.

Single Pass Instanced Rendering

The main render optimization in VR. Without it, the scene is rendered twice (once per eye), doubling draw calls. Single Pass Instanced renders both eyes in one pass via instancing: geometry is processed once, the shader gets two view/projection matrices through GPU instancing. Enabled in Unity via XR Plug-in Management > Rendering Mode: Single Pass Instanced. Important: custom shaders must support SPI — standard URP/HDRP shaders support it, custom HLSL requires modifications (UNITY_SETUP_STEREO_EYE_INDEX_POST_VERTEX and related macros). Applying this technique reduces draw calls by 40–50%, making it twice as efficient as naive double rendering.

Foveated Rendering

On Meta Quest, Fixed Foveated Rendering (FFR) is available — reducing resolution at the periphery where visual acuity is lower. Configured via OVRManager or Meta XR SDK:

OVRManager.fixedFoveatedRenderingLevel = OVRManager.FixedFoveatedRenderingLevel.High;
OVRManager.useDynamicFixedFoveatedRendering = true;

Dynamic FFR automatically increases the level when frame rate drops — more convenient than fixed in scenes with variable load.

IPD and Comfort Settings

IPD (Inter‑Pupillary Distance) — affects depth perception. At the programmable level on most devices, only reading IPD is available (OVRPlugin.GetSystemDisplayFrequency), physical adjustment is on the headset. For applications requiring precise positioning (medical simulators, training), we account for IPD in scene scale calculations.

Haptics

Haptic feedback — an underestimated tool. Even a simple vibration response when grabbing an object or hitting significantly increases the sense of presence. On average, integrating haptic patterns takes 30–80 hours per project.

XR Haptics via OpenXR:

var hapticImpulse = new UnityEngine.XR.HapticCapabilities();
InputDevice device = InputDevices.GetDeviceAtXRNode(XRNode.RightHand);
device.SendHapticImpulse(0, amplitude: 0.5f, duration: 0.1f);

For complex patterns (tactile 'texture' of a surface when touched, increasing vibration when drawing a bowstring) we use Meta Haptics Studio — allows designing haptic clips visually. This can reduce time spent on manual haptic tuning by about 30%.

What does VR/AR application development include?

When ordering a turnkey project, we provide the following deliverables:

• Architectural document with stack description, render logic, and interaction system
• Working prototype (MVP) for testing on target device
• Integration of necessary SDKs (Meta XR, OpenXR, AR Foundation, etc.)
• Optimization for target frequencies 72/90/120 Hz with draw call and FPS profiling
• Testing on physical hardware (Quest, SteamVR, HoloLens) with user involvement
• Full documentation for build, deployment, and support
• Training for the client's team (workshop on XR Toolkit)
• Warranty support for 1 month after delivery

What affects cost and timeline?

VR/AR projects are more expensive than regular games of similar scope. Iterations are slower — each fix must be tested in the headset, an emulator does not convey the real experience. Motion sickness forces reworking some conceptual decisions after the first playtest. Optimization takes a significant portion of time — for mobile VR (Quest) up to 60–70% of the cycle. For Quest projects, we start optimization from the first sprint. The cost of basic SDK integration (XR Interaction Toolkit) varies depending on the scope of custom Interactable. Typical budgets for a full Quest project range from $25,000 to $80,000 depending on complexity, number of custom interactions, and depth of optimization. Proper architectural planning from sprint one typically saves 40% on later rework compared to fixing performance bottlenecks retroactively.

Get a consultation on your project — we will assess the task, stack, and timelines. Order turnkey VR/AR application development with a guaranteed stable frame rate.