Question

I am experienced backend programmer, very familiar with languages like Rust, Python, PHP and somewhat familiar with many others (did courses for bunch of stuff like scala, ruby, ocaml, sml, js, really a lot). Also I watch a bunch of youtube tutorials on stuff like game engine programming, reading r/programming and similar forums for a very long time.

Explain to me in detail how collision system works in UE 5.5, the data layout, optimizations, broadphase, narrowphase, which data is checked and how. Are actual numbers for positions i32 or what. I want really deep dive. Top-down overview that goes deeper later on.

Answer

Okay, let's dive deep into the collision system in Unreal Engine 5.5. As an experienced backend programmer, you'll appreciate the separation of concerns and the data-driven nature of the system. We'll go from a high-level overview down to the nitty-gritty details.

Top-Down Overview

At its core, the UE collision system aims to efficiently determine when and how objects in the game world interact physically. It's a multi-stage process designed to minimize the computational cost, especially with a large number of potentially colliding objects.

Here's a simplified breakdown of the main stages:

Setting up Collision: You define collision geometry for your Actors and Components. This involves choosing shapes (boxes, spheres, capsules, meshes) and setting collision properties (channels, responses).
Broadphase Collision Detection: This stage quickly identifies potential collisions between objects. It uses spatial partitioning techniques to drastically reduce the number of pairwise checks needed for precise collision detection.
Narrowphase Collision Detection: This stage performs precise geometric intersection tests on the pairs identified by the broadphase. This is where the actual shape-vs-shape collision checks happen.
Collision Response and Resolution: Once a collision is detected, the engine handles the consequences, which could involve triggering events, applying forces, or stopping movement.
Querying the Collision World: You can also proactively query the collision world to check for overlaps or blocking volumes without actual physical simulation.

Deeper Dive: Data Layout and Key Structures

Understanding the data layout is crucial for comprehending the system's efficiency.

Key Classes and Structures:

FCollisionShape: Represents the geometric shape of a collider. This is an abstract base class with derived classes like FCollisionShape::FBox, FCollisionShape::FSphere, FCollisionShape::FCapsule, and FCollisionShape::FConvex (for convex mesh approximations). For complex meshes, a triangle mesh representation is used.
- Data Held: Dimensions (extents, radius, half-height, etc.), orientation (often implicitly within the transform).
- Important Note: FCollisionShape is a value type, meaning it's copied around.
FBodyInstance: This is the central structure that ties a UPrimitiveComponent (like a UStaticMeshComponent or USphereComponent) to the physics world. It holds the physical properties of the collider.
- Data Held:
  - FCollisionShape: The actual shape of the collider.
  - Transform: World position, rotation, and scale.
  - Collision Enabled State (ECollisionEnabled::Type): Determines if and how this object interacts with the collision system (QueryOnly, PhysicsOnly, CollisionEnabled, NoCollision).
  - Collision Responses: How this object reacts to different collision channels (Overlap, Block, Ignore). Stored as an FCollisionResponseContainer.
  - Physics Material: Properties like friction and restitution.
  - Inertia tensor, mass, etc. (if physics is enabled).
  - Pointers to the broadphase and narrowphase proxies.
UPrimitiveComponent: A component that can have collision. This is the game-facing interface for collision. It owns the FBodyInstance (or an array of them for complex meshes).
- Data Held:
  - Relative Transform: Position, rotation, and scale relative to the owning Actor.
  - Pointer to FBodyInstance (or array of FBodyInstance).
  - Collision Channels and Object Type: Used for filtering.
  - Rendering information, etc.
FCollisionObjectQueryParams and FCollisionQueryParams: Structures used when performing collision queries (line traces, sweeps, overlaps). They specify the query parameters like the starting and ending points, collision shape, and filtering criteria.
FHitResult: A structure returned by collision queries and narrowphase checks, containing detailed information about the collision: point of impact, normal, distance, impacted component, etc.

Data Layout in the Physics Scene:

The physics scene maintains a data structure that organizes all active FBodyInstance objects. This structure is heavily optimized for spatial queries.

Broadphase Collision Detection

The goal of the broadphase is to quickly prune pairs of objects that are unlikely to collide. UE5 uses a Bounding Volume Hierarchy (BVH) as its primary broadphase algorithm.

How BVH Works:

Bounding Volumes: Each movable object is enclosed by a simple bounding volume (typically an Axis-Aligned Bounding Box - AABB).
Hierarchy: These AABBs are organized into a tree-like hierarchy. Leaf nodes contain the AABBs of individual objects, and internal nodes contain the AABBs that encompass the AABBs of their children.
Construction: The BVH is built recursively. Objects are initially grouped, and a bounding box is calculated for each group. This process is repeated until a hierarchy is formed.
Querying: When checking for potential collisions, the engine traverses the BVH. If the bounding box of a node doesn't overlap with the bounding box of another object (or node), then none of the objects within that node can collide with the target object, and that entire branch of the tree is pruned.

Optimizations in the Broadphase:

AABB Optimization: AABBs are computationally cheap to intersect.
Tree Traversal: Efficient algorithms for traversing the BVH minimize the number of intersection tests.
Dynamic Updates: Mechanisms exist to update the BVH as objects move. Small movements might only require updating the leaf nodes, while large movements might necessitate re-inserting the object into the tree.
Multithreading: Broadphase calculations can be parallelized across multiple cores.
Spatial Hashing (Less Common in Modern UE): Older versions of UE or for specific use cases might have utilized spatial hashing techniques. However, BVH is the dominant approach in modern UE.

Which Data is Checked in the Broadphase:

AABB of each FBodyInstance: The primary data used for intersection tests.
World transform of the FBodyInstance: To position the AABB in world space.

Narrowphase Collision Detection

The narrowphase takes the pairs of potentially colliding objects identified by the broadphase and performs precise geometric intersection tests.

Algorithms Used:

The specific algorithm used depends on the types of FCollisionShape involved in the pair:

Box vs. Box: Uses the Separating Axis Theorem (SAT). SAT checks for a separating axis between the two boxes. If such an axis exists, the boxes do not intersect.
Sphere vs. Sphere: A simple distance check between the sphere centers.
Sphere vs. Box: Checks if the sphere's center is within the extended faces of the box.
Capsule vs. Anything: Often treated as a swept sphere or by decomposing it into a cylinder and two hemispheres.
Convex vs. Convex: Also heavily relies on SAT.
Triangle Mesh vs. Anything: Involves iterating over the triangles of the mesh and performing intersection tests against the other shape (e.g., ray-triangle intersection, sphere-triangle intersection). This is the most computationally expensive narrowphase check.

Data Used in the Narrowphase:

Detailed Geometry of the FCollisionShape: Vertices, edges, faces, radii, half-heights, etc.
World Transforms of the FBodyInstance: To position and orient the shapes in world space.
Collision Margins: A small skin around the collision geometry used for early contact detection and preventing objects from getting stuck.

Optimizations in the Narrowphase:

Specialized Algorithms: Using the most efficient algorithm for each pair of shapes.
Early Exit Strategies: Optimizations within the algorithms to quickly determine if a collision cannot occur.
SIMD (Single Instruction, Multiple Data): Vectorized instructions can be used to speed up calculations for multiple vertices or edges simultaneously.
Contact Manifold Generation: Instead of just detecting a collision, the narrowphase often generates a contact manifold, which describes the contact points, normals, and penetration depth. This information is crucial for collision resolution.

Actual Numbers for Positions

Positions and other spatial data are typically stored as single-precision floating-point numbers (float) in UE5.

Why float? Performance. Single-precision floats require half the memory and computational effort compared to doubles. This is a crucial consideration in real-time simulations with potentially thousands of objects.
Trade-offs: Single-precision floats have less precision than doubles, which can lead to issues like:
- Floating-point inaccuracies: Small errors in calculations can accumulate over time.
- Z-fighting: When objects are very far from the origin, the precision of floats can become insufficient, leading to visual artifacts.
Mitigation Strategies:
- World Origin Rebasing: UE5 uses techniques like World Partition and origin rebasing to shift the effective origin of the world closer to the player, maintaining precision.
- Tolerance Values: Collision detection algorithms often incorporate small tolerance values to account for floating-point imprecision.

Other Spatial Data Types:

Rotations: Represented using Quaternions (FQuat) for smooth interpolation and avoiding gimbal lock.
Vectors: Represented by FVector (typically containing three float components).

Collision Response and Resolution

Once a collision is detected and detailed information is available in the FHitResult, the engine handles the response. This involves:

Triggering Events: OnComponentBeginOverlap, OnComponentEndOverlap, OnComponentHit events are fired on the involved components.
Applying Forces (for physics simulations): The physics engine (PhysX by default) uses the contact information (point, normal, penetration depth) to calculate and apply forces to resolve the collision and prevent interpenetration.
Blocking Movement: If the collision response is set to "Block," the engine will stop the movement of the colliding object.

Querying the Collision World

You can actively query the collision world using functions like:

UWorld::LineTraceSingleByChannel / Multi: Performs a raycast against the collision world.
UWorld::SweepSingleByChannel / Multi: Sweeps a shape through the world and detects collisions.
UWorld::OverlapBlockingTestByChannel / Multi: Checks if a given shape overlaps with any blocking geometry at a specific location.

These queries utilize the same underlying broadphase and narrowphase mechanisms for efficiency.

Optimizations Across the System

Beyond the specific optimizations within the broadphase and narrowphase, there are broader optimization techniques:

Collision Channels and Object Types: Allow for fine-grained control over which objects can collide with each other, drastically reducing the number of pairs that need to be checked.
Collision Presets: Predefined sets of collision channel responses for common object types.
Sleeping Objects: Objects that are at rest are often "put to sleep" by the physics engine and are excluded from collision calculations until they are awakened by an external force or collision.
Hierarchical LOD (HLOD) for Collision: For distant objects, you can use simpler collision representations to reduce the complexity of collision checks.
Geometry Simplification: Reducing the polygon count of complex meshes used for collision.

Practical Implications and Developer Interaction

As a backend programmer working with UE5 collision, you'll interact with:

Setting up Collision in the Editor: Configuring collision shapes, channels, and responses on components.
Blueprint Scripting: Using collision events to trigger gameplay logic.
C++ Programming: Working directly with FBodyInstance, FCollisionShape, and performing custom collision queries.
Understanding Performance Profiling: Analyzing the performance impact of collision calculations.

Further Exploration

To delve even deeper, you can explore:

PhysX Documentation: UE5's physics engine is based on PhysX. Understanding PhysX internals will give you more insight into the low-level collision algorithms.
UE Source Code: The best way to understand the nitty-gritty details is to dive into the engine source code, particularly the PhysicsCore and Engine modules. Look for files related to Collision, Broadphase, Narrowphase, and BodyInstance.
Advanced Collision Techniques: Research topics like continuous collision detection (CCD) for fast-moving objects, and custom collision solvers.

This detailed overview should give you a solid foundation in how the UE5 collision system works. You'll find that the separation of concerns, data-driven design, and focus on optimization make it a powerful and flexible system for building complex interactive worlds. Let me know if you have any more specific questions!

grizwako/centaur-ue55-collisions.md