Fix: Construct 3 Tilemap Collision Overlap Flicker

Player walks along a flat row of tiles. Sprite jiggles up and down a single pixel each step. Or jump-clings to a wall and the player snaps free for a frame at every tile boundary. The Solid behavior is doing its job; the tilemap is feeding it bad polygons.

The Symptom

Player flickers vertically by 1–2 pixels while walking. Or x-velocity stutters at tile boundaries. Or hitbox-vs-tilemap overlap test toggles between true and false within a frame.

What Causes This

Each tile in a Tilemap has its own collision polygon. By default, Construct generates one based on the visible pixels. If your tile art doesn’t fill the full grid (e.g. a grass tile with a slight bevel) the polygon shrinks inward, and adjacent tiles don’t share edges. The Solid behavior pushes the player out of one tile, into the gap, then immediately overlaps the next, push back, repeat.

The Fix

Step 1: Edit tile collision polygons. Project → Tilemap object → Properties → Edit collision polygons. The polygon editor opens.

Step 2: Align corners to tile bounds. For a fully solid tile, set the four corners to exactly (0,0), (size,0), (size,size), (0,size). Ctrl-click a vertex and type the coordinates if the visual editor is finicky.

Step 3: Apply across all solid tiles. Each tile has its own polygon. The polygon editor lets you copy & paste between tiles. Use this to ensure all your solid tiles use the identical full-tile polygon.

Step 4: Half-solid tiles. For a half-height platform, the polygon is (0, size/2)–(size, size). For diagonal slope, three vertices.

Solid floor tile (32x32):    (0,0) (32,0) (32,32) (0,32)
Half-tile platform:           (0,16) (32,16) (32,32) (0,32)
Slope-up-right:               (0,32) (32,0) (32,32)

Solid Behavior Settings

The Tilemap object should carry the Solid behavior (or Jumpthru, for one-way). Don’t put both Solid and Jumpthru on the same tilemap and rely on per-tile polygons to differentiate — behaviors are object-wide. For one-way platforms, use a separate tilemap with the Jumpthru behavior.

Player Hitbox

Open the player Sprite’s Edit collision polygon. A capsule (or beveled rectangle) hitbox slides past tile seams more smoothly than a square. Even with perfect polygons, a square-on-square contact can feel sticky.

Verifying

Project → Properties → Layout → Show collision polygons (debug overlay). Run the layout. Tile polygons should appear as flush rectangles. Walk the player; the player’s polygon shouldn’t bounce. If it does, the polygon is the wrong size.

Understanding the issue

Tilemaps are dense data structures. A single tile change touches several other systems: rendering, collision, possibly navigation. Bugs at the intersection often look like 'I changed one tile, why did three other things break'.

The specific bug described above is the kind that surfaces during integration rather than unit testing. It depends on a combination of factors: the asset configuration, the runtime state, the platform's specific behavior. In isolation, each piece looks correct; in combination, the bug emerges. This is why thorough integration testing - playing the actual game in realistic conditions - catches things that automated tests miss.

Why this happens

Bugs of this class are particularly easy to ship past internal QA because they often depend on specific runtime conditions - hardware combinations, network states, or asset configurations that QA didn't reproduce. Players hit them in the wild, file reports that are hard to repro, and the bug accumulates negative reviews while engineering tries to recreate the failure mode.

At the engine level, the behavior comes from a deliberate design decision in Construct 3. The engine team chose a particular trade-off - usually performance versus convenience, or generality versus specificity - and that trade-off has consequences when you push against it. Understanding the trade-off is what turns 'this bug is mysterious' into 'this bug is the expected consequence of this design'.

Verifying the fix

After applying the fix, the verification step has three parts: confirm the original repro is resolved, confirm no obvious regressions in adjacent functionality, and (for shipping titles) deploy to a small player cohort first and watch the crash and report rates. Each step catches something the others miss.

Reproducibility is the prerequisite for verification. If you can't reliably reproduce the bug pre-fix, you can't reliably verify it post-fix. Spend time getting a clean reproduction before you write any fix code. The fix is fast once you understand the reproduction; the reproduction is the slow part.

Variations to watch for

Related bug classes often share the same root cause. If you find yourself fixing this issue, look for cousins: similar symptoms in adjacent systems, the same data flow but a different value, or the same fix pattern in another module. The catalog of 'we've seen this before' becomes valuable institutional knowledge.

Adjacent bugs often share a root cause. After fixing the case you've found, spend an hour searching the codebase for similar patterns. What's the same call with different arguments? The same data flow with a different entity type? The same lifecycle issue in a sibling system? Each match is a candidate for the same fix, or a related fix that prevents future bugs of the same class.

In production

In shipping builds, this issue may interact with other production-only behavior. Stripping, encryption, asset bundling, and platform-specific code paths can each modify the symptoms. When players report a related issue, capture build SHA, platform, and any feature flags - those three fields cover most of the production-only variations.

When triaging a similar issue in production, prioritize gathering data over hypothesizing causes. A player report describes a symptom; what you need is a build SHA, a session timestamp, and ideally a screen recording or session replay. With those, the bug becomes tractable. Without them, you're guessing at hypothetical reproductions that may not match what the player actually hit.

Performance considerations

Performance implications matter when this bug class scales with player count or asset count. A bug that fires once per session is annoying; a bug that fires once per frame compounds. After fixing, profile the affected code path under realistic load. The fix that's correct for one entity may be too slow for ten thousand.

Diagnostic approach

Before applying any fix, gather enough context to be confident you're addressing the actual cause and not a similar-looking symptom. The cheapest diagnostic step is reproducing the bug deterministically - if you can't get the same failure twice in a row, your fix attempts will be hard to evaluate. Lock down the reproduction first.

For Construct 3-specific diagnostics, the editor's profiler is the canonical starting point. Capture a representative frame with the symptom present; compare against a frame without the symptom; the diff often points directly at the cause. If the symptom is non-deterministic, capture multiple frames and look for the pattern - the cause is usually a state transition or a specific input value rather than a continuous effect.

Tooling and ecosystem

Modern engine versions ship better tooling for this kind of issue than older versions. If you're on an older release, the diagnostic step may take significantly longer because the tools you'd want don't exist yet. Sometimes the right answer is upgrading rather than fighting through limited tooling.

Within Construct 3, the relevant diagnostic surfaces include the standard frame debugger, memory profiler, and engine-specific debug overlays. Each one shows a different facet of what's happening. The frame debugger reveals draw call ordering and state transitions; the memory profiler shows allocation patterns; the debug overlay reveals per-system state. Bugs that resist one tool usually surrender to another - the trick is knowing which tool to reach for first.

Edge cases and pitfalls

Edge cases for this class of issue often involve specific timing: the first frame after a state change, the last frame before a transition, frames where multiple subsystems update simultaneously. Reproducing these reliably is part of what makes the bug class hard to test.

When writing a regression test for this fix, focus on the boundary conditions that surfaced the original bug. Tests that exercise the happy path catch obvious regressions; tests that exercise the boundary catch the subtler regressions that look like new bugs but are really the original returning. The latter are the tests that earn their keep over the long life of the project.

Team communication

Document the fix and its rationale in the commit message or attached engineering doc. Future engineers will encounter related issues; the rationale tells them whether your fix is reusable or specific to the case at hand. Without rationale, the fix gets reverted or copied incorrectly.

If this fix touches a system several engineers work in, a short writeup in the team's engineering channel helps. Not a full design doc - a paragraph explaining what was wrong, what's fixed, and what to watch for. Future engineers encountering similar symptoms will search for the fix; making it findable is a small investment that pays back later.

“Polygons flush to tile bounds. Adjacent tiles share edges. Solid behavior glides.”

Related Issues

For Construct collision detection misses, see fast object collision. For platform jitter, see platform jitter.

Polygons full-tile. Edges shared. Player walks straight.