How to Implement Continuous Collision Detection

Continuous collision detection—checking an object's path between frames rather than just its position each frame—prevents fast objects from tunneling through thin obstacles, a common bug with discrete collision detection. Understanding when and how to use it is key to handling fast objects correctly.

Discrete detection lets fast objects tunnel through obstacles

Discrete collision detection checks for collisions at each frame's position, which works for slow objects but fails for fast ones: a fast object can move far enough between frames that it passes entirely through a thin obstacle—at one frame it's before the obstacle, at the next it's past it, with no frame where it overlaps the obstacle, so the collision is missed and the object tunnels through. This tunneling is a common bug with fast objects (fast projectiles, fast-moving characters) and thin obstacles (thin walls, thin platforms), because the discrete per-frame check misses the collision that happened between frames. Recognizing that discrete detection lets fast objects tunnel through obstacles—missing collisions that happen between frames—is the problem continuous collision detection solves, because the discrete per-frame check is insufficient for fast objects, which can pass through obstacles between frames.

Continuous detection checks the path between frames to catch the collision. Continuous collision detection prevents tunneling by checking the object's path between frames, not just its position each frame: instead of checking only where the object is at each frame, it checks the path (the swept volume) the object traveled between its previous and current positions, detecting any collision along that path. This catches collisions that happen between frames—if the object's path between frames crosses an obstacle, the continuous check detects the collision even though no frame's position overlapped the obstacle, preventing the tunneling. Checking the path between frames (the swept motion) rather than just the per-frame positions is what makes continuous detection catch the collisions that discrete detection misses for fast objects. This is more expensive than discrete detection (checking the path is more work than checking a position), so continuous detection is typically used selectively for the fast objects that need it (fast projectiles, fast characters) rather than everything, while discrete detection suffices for slow objects. Continuous detection checking the path between frames is what catches the fast-object collisions that discrete detection tunnels through. Combining the recognition that discrete detection lets fast objects tunnel (the problem) with continuous detection checking the path between frames (the solution) is what makes continuous collision detection prevent tunneling for fast objects. Using continuous detection this way—checking the path for fast objects, where discrete per-frame checks miss collisions—is what prevents the tunneling bug for fast objects and thin obstacles, while using discrete detection for slow objects (where it suffices and is cheaper). Use continuous collision detection for fast objects to prevent tunneling, checking the path between frames to catch the collisions that discrete per-frame detection misses.

Why finishing beats perfecting

The hardest skill in indie development isn't any particular technique — it's finishing. Most games that never ship didn't fail on talent; they failed on scope, polished forever, or chased one more feature. The developers who build a real body of work are almost always the ones who got good at choosing something small enough to complete and then completing it.

That's worth keeping in mind here, because it's easy to let any one part of development expand to fill all your time. Decide what 'good enough to ship' looks like, protect that line, and treat the endless list of possible improvements as a backlog rather than a set of obligations.

Plan for the parts you can't see

Once a game leaves your machine, a lot of what happens to it becomes invisible by default. Players run it on hardware you don't own, hit problems you never reproduced, and most of them never tell you — they simply move on. The gap between 'it works for me' and 'it works for everyone' is where a surprising amount of churn quietly lives.

So plan to see what you otherwise couldn't. Watching real players, capturing the bugs and crashes they hit with the context to fix them, and paying attention to where they drop off all turn invisible problems into ones you can actually act on — which protects the reviews and retention everything else depends on.

Consistency beats intensity

Indie development is a long game, and it rewards steady, sustainable effort more than heroic bursts. A little progress made consistently — on the game, on the marketing, on the community — compounds in a way that last-minute sprints never do. The developers who finish and find an audience are usually the ones who kept showing up, not the ones who worked themselves into the ground for a week and then burned out.

Build a pace you can sustain, and protect it. Momentum is fragile and expensive to rebuild, so steady forward motion is worth more than any single intense push.

Let real players be the judge

It's remarkable how differently real players behave from how you imagine they will. The tutorial you think is obvious confuses them; the feature you agonised over goes unnoticed; the thing you almost cut becomes their favourite. None of that is visible from inside your own head, which is why watching real people play is the single highest-leverage thing most developers under-do.

Watch without intervening, resist the urge to explain, and pay attention to what players do as much as what they say. Their confusion and their choices are data, and acting on that data is what turns a game that works for you into one that works for everyone.

Polish where players actually look

Polish is not evenly valuable. Players form an impression in the first minutes and spend most of their time in the core loop, so effort spent there returns far more than effort spread thin across content few people reach. The opening, the moment-to-moment feel, and the things every player touches are where polish converts directly into how good the game feels.

Be deliberate about it. Make the first impression strong and the core interactions satisfying before widening out, because a great core with less content almost always beats a sprawling game that never feels good to play.

Continuous collision detection checks an object's path between frames, not just its per-frame position, preventing fast objects from tunneling through thin obstacles. Use continuous detection selectively for fast objects, where discrete per-frame checks miss the collisions that happen between frames.