Real-Time 3D Game Engines

Engine responsibilities and frame budgets

A game engine coordinates simulation, input, rendering, assets, audio, and scene lifetime. A frame budget turns an intended presentation rate into a concrete resource limit: 60 frames per second allows about 16.67 ms per frame. CPU and GPU work can overlap, so summing separately measured times without understanding synchronization can mislead. Profile a representative scene and identify the bottleneck. A faster subsystem does not improve the frame rate when another subsystem remains on the critical path.

References: Godot official physics introduction — collision objects and simulation updates

Simulation time and interpolation

A fixed simulation step makes numerical behavior easier to reason about than using arbitrary render intervals. Accumulate elapsed time and run enough fixed updates to catch up, with a defined policy when the machine falls behind. Rendering can interpolate between simulation states to smooth presentation. Fixed stepping alone does not prove determinism: floating-point behavior, iteration order, external inputs, and physics implementation still matter. Avoid tying movement directly to frame count, because faster rendering would then change gameplay speed.

References: Godot official physics introduction — collision objects and simulation updates

Scene organization and articulated models

A scene graph expresses parent-child transforms; an articulated character combines joint transforms with mesh deformation. Keep object identity and lifetime separate from a transient pointer or array position. Spatial organization supports visibility and collision queries, but the best partition depends on motion and density. Terrain and vegetation often need different levels of detail as distance changes. A portal can represent visibility or passage between spaces, while a trigger is an event region rather than necessarily a solid collision surface.

References: Godot official physics introduction — collision objects and simulation updates

Broad phase, narrow phase, and response

The broad phase cheaply identifies potential collision pairs using bounds or spatial partitions. The narrow phase performs more exact shape tests. Collision response then changes motion or reports events according to gameplay and physics rules. Detection and response are separate problems. Fast-moving objects can pass through thin geometry between discrete samples, motivating swept tests or continuous collision methods. A simplified collision shape may be more stable and efficient than using every triangle of a visually detailed mesh.

References: Godot official physics introduction — collision objects and simulation updates

Worked fixed-step timing example

Assume a simulation step of 10 ms and a render interval of 25 ms. Starting with an empty accumulator, the engine performs two simulation steps and retains 5 ms. Interpolating the previous and current simulation states uses alpha=5/10=0.5 under the usual one-step-behind rendering scheme. On the next 15 ms interval, the accumulator reaches 20 ms and again permits two steps. Define a catch-up limit to prevent unlimited simulation work from starving rendering after a long pause.

References: Godot official physics introduction — collision objects and simulation updates

Effects, resource management, and testing

Particle emitters create many short-lived objects with properties such as position, velocity, and lifetime. Pooling can reduce allocation churn, but reused objects need a complete reset to avoid stale state. Shadows, mirrors, and environmental effects have distinct visibility and render-pass costs. Keep gameplay behavior testable outside a fully rendered scene when possible. Build deterministic toy scenarios for transforms and collision rules, then add visual inspection and profiling for artifacts that numerical tests alone cannot explain.

References: Godot official physics introduction — collision objects and simulation updates

Practice & worked answers

Original exercises for this guide.

Public notes & references

Course sources establish the scope above. Reference materials support the explanations; linked lecture notes and books remain with their authors.

• Carleton 2026–27 calendar — COMP 4002 ↗Course scope · 2026–27 calendar
• Godot official physics introduction — collision objects and simulation updates ↗Official engine documentation
• Glenn Fiedler — Fix Your Timestep ↗Author technical article
• Physically Based Rendering — geometry and acceleration structures ↗Open author textbook