Avoiding Bugs: When To Use Time-Based Vs Collision-Based Hit Detection

The Core Problem: Tradeoffs Between Performance and Accuracy

Hit detection is a crucial gameplay mechanic that determines if and when two entities, such as a player avatar and enemy, make contact. More sophisticated hit detection methods typically provide more accurate results, but have a greater performance cost. Understanding these tradeoffs allows developers to strategically choose the right hit detection approach for their specific needs.

Time-based hit detection relies on periodically checking for collisions after set time intervals. This approach is fast and has a low computational load, but can miss collisions in between update checks, leading to bugs like tunnelling where objects pass through each other. Collision-based methods use detailed volume and shape data to accurately detect hits, but require more intensive calculations that may reduce gameplay frame rates.

By considering factors like the game’s core mechanics, number of collidable objects, target hardware specifications and acceptable accuracy thresholds, developers can analyze the performance vs precision tradeoff to select an optimal hit detection strategy.

Implementing Efficient Time-Based Hit Detection

For less accuracy-critical games, time-based hit detection offers a lightweight yet functional approach. The basic method involves defining hit test points on collidable objects and checking for overlapping test points after set time intervals.

To improve accuracy, time thresholds can be combined with spatial interpolation to estimate object positions between updates. Care must be taken to avoid tunnelling issues where colliding objects initially overlap but then fully pass through each other before the next update.

Potential solutions include sweeping collidable geometry through the interpolated positions to catch tunnelling cases. However, these additional checks can impact performance. An optimization is to cache the results of the more intensive sweeps and reuse them for subsequent frames whenever objects have not moved significantly.

Engines like Unity and Unreal Engine provide built-in components to implement various time-based hit detection strategies which can serve as good starting points for most games. Developers can also customize and extend these implementations to handle specific edge cases.

Getting Pixel-Perfect Results with Collision Volumes

For genres like platformers requiring precise, pixel-perfect hit detection, a collision volume approach is more robust. This calculates hits by detecting overlaps between the actual bounding shape representations of objects like boxes, spheres and capsules.

With detailed volume data, the engine can accurately register hits even for fast moving objects with complex trajectories. Special case edge collisions can also be handled gracefully instead of relying on time sample thresholds.

The tradeoff is that the continuous collision checks and computations over many framed polygons or primitive shapes has a higher performance cost. Careful optimization is needed to minimize impact, especially on mobile hardware.

Typical optimizations include simplifying volumes to lower poly counts, disabling collision checks for distant objects, and prioritizing checks for actively colliding object pairs. Code examples for implementing performant box, sphere and capsule collider solutions are available in many game development guides and engine resources.

Hybrid Approaches for Special Cases

For certain games and mechanics, neither a pure time-based nor collision volume approach may provide the right balance of accuracy and performance. Hybrid solutions attempt to get the best of both worlds for these special cases.

One example is shooting games with complex ballistic systems. An initial fast time-based hit check first registers that a shot has potentially hit a target. Then, precise volumetric checks between the bullet and target model can run to determine an accurate collision point for spawning the impact VFX and applying damage.

Fighting games can benefit from using collide volumes for player controlled characters to enable pixel-perfect hits, while simplified bounds checks handle the numerous dynamic background objects. The right split between time and collision systems depends on the specific context.

Regardless of approach, accurately simulating advanced scenarios like angular collisions can require extensive tuning and field testing. Multi-step verification, customizable collision reactions and conditional hit logic may be necessary to make special mechanics look and feel right.

Optimizing Your Implementation

All hit detection systems have optimization opportunities regardless of initial approach. Profiling box colliders versus capsule checks may reveal that fewer axis-aligned bounds checks can improve frame times with minimal accuracy loss. Benchmarking may demonstrate that interpolation for time-based methods provides diminishing returns past a threshold.

Priority queues can optimize update order to check active collisions first, avoiding wasting cycles on distant objects unlikely to hit. Object pools can reuse collision handler components to reduce spawning overhead for effects. And data-oriented design can batch similar checks to utilize cache coherence.

As the number of simultaneously colliding entities increases, spatial partitioning schemes like quad trees may be necessary to scale up efficiently. They subdivide worlds into sectors to cull distant objects from checks altogether.

When To Reevaluate Your Hit Detection Solution

Even with a solid initial implementation, at some point nearly every growing game will need to revisit hit detection strategies. As new mechanics and abilities introduce complex targeting scenarios, edge case bugs or exploitable collisions may emerge.

Performance bottlenecks can reveal a need to upgrade to a hybrid model or shift workload between the game thread and physics engine. And extending to multiplayer brings aspects like latency compensation and prediction to consider.

Developers aiming for more realistic physics and destructibility may require custom collision geometries, volumetric hit calculations or structural integrity systems instead of basic bounds checking.

By continuously evaluating hit detection needs against implementation costs, incrementally upgrading systems and learning from other games, developers can anticipate problems before they impede gameplay experiences.