Collision broad phase and narrow phase: candidates versus contacts

Why split collision work?

A collision pipeline has two different jobs: discover which shapes might interact, then determine what their geometry actually does. Confusing these jobs produces misleading debug views and misplaced optimization. An overlapping bounding box is a reason to investigate, not sufficient evidence that two gameplay objects touched.

The candidate contract

A broad phase uses conservative bounds to avoid testing every shape pair. Box2D's dynamic tree organizes axis-aligned bounds in a binary hierarchy. [1] Narrow-phase routines operate on the shapes themselves and can produce contact manifolds containing contact points and a normal. Collision filtering and the later constraint solver have distinct responsibilities. [2]

Worked example

Imagine 100 moving crates. Testing every unordered pair would require 100 × 99 / 2 = 4950 pair tests. Suppose an illustrative spatial query produces 180 candidates after layer filtering, and geometric tests find 24 touching pairs. Those invented counts explain pipeline stages; they are not a performance result.

A long crate rotated diagonally can have a large axis-aligned box with empty corners. Another crate entering one of those corners should survive the broad phase but fail the shape test. Labeling every one of the 180 candidates as a collision would create false damage events. Conversely, a bound that fails to contain its shape can discard a real contact before the narrow phase sees it.

Practical workflow

Define who updates spatial proxies after movement, resizing, activation, or destruction. Use stable pair identities so repeated discovery does not create duplicate gameplay events. Decide whether filters apply before candidate emission, before shape testing, or at both points; document any rules that can change during play.

Instrument candidate count, actual contact count, tree update cost, and geometric test cost separately. In a diagnostic scene, visualize both the true shape and its bounds. Include rotated thin objects, dense stacks, and a large moving object among small ones. Review missed contacts before tuning candidate reduction.

Tradeoffs and limits

Generous bounds may reduce spatial-index maintenance while admitting more candidates. Tight bounds can reverse that balance. A grid, a tree, or sweep-and-prune must be assessed against object sizes, motion, and density in the intended workload.

Dense scenes can still produce many valid pairs; a spatial structure cannot make unavoidable interactions disappear. Keep broad-phase overlap, narrow-phase contact, solver response, and gameplay hit policy separately observable. Continuous collision detection also needs a time-aware query policy; a static end-position overlap alone does not cover a movement interval.

Sources

[1] Dynamic tree — Erin Catto and Box2D contributors.

[2] Collision — Erin Catto and Box2D contributors.