Fix: Construct 3 Tilemap Collision Offset After Scroll

The player runs through your level and the first few screens feel fine. By the middle of the level, the character is falling through solid tiles, walking into invisible walls two tiles to the left, and getting stuck on empty air. The further the camera scrolls from the origin, the worse it gets. The bug is one property on one layer that should never have been changed.

What Parallax Does to Collisions

Parallax in Construct 3 is a rendering effect: it changes how fast a layer scrolls visually relative to the camera. A parallax of 50×50 means the layer scrolls at half the speed of the camera, creating a depth effect for backgrounds. A parallax of 100×100 means the layer scrolls at the same speed as the camera — no depth effect.

Crucially, parallax only affects the visual position of objects on the layer. The physics engine always uses the world position of collision shapes. When parallax moves the visual tiles but not their collision shapes, the two drift apart. At scroll position 0, they match. At scroll position 1000, a 50% parallax layer has a 500-pixel offset between visuals and collisions.

The Fix

Step 1: Set gameplay layers to 100×100.

Click the layer containing your tilemap in the Layers panel. In the Properties panel, set Parallax X to 100 and Parallax Y to 100. Save and preview. The collision shapes now scroll with the tiles.

Only decorative layers (far background, clouds, close foreground) should have parallax values other than 100. Any layer that contains objects the player collides with, including tilemaps, platforms, walls, and triggers, must be at 100×100.

Step 2: Check the tilemap origin.

Select the tilemap object in the layout. Its origin point should be at (0, 0) — the top-left corner. If the origin was accidentally moved (by editing the collision polygon in the Animations editor), every tile’s collision shape is shifted by that offset. Reset it to (0, 0).

Step 3: Check per-tile collision polygons.

Open the tilemap’s tile editor. Click on a tile and check its collision polygon. If the polygon is offset from the tile image (e.g. a few pixels to the right), every instance of that tile in the level will have an offset collision. Realign the polygon to match the tile’s visual bounds.

Debugging with Collision Overlay

In the preview, open the browser’s dev console and type:

// Show collision polygons (Construct 3 debug mode)
// Enable via Project Properties → Debug → Show collision polygons
// Or in event sheet:
Browser: Log "Debug: collision overlay enabled"

With the overlay active, scroll through the level. Collision polygons should exactly overlay their visual tiles at every scroll position. If they drift, parallax is the cause. If they are offset everywhere (not just after scrolling), the tilemap origin or per-tile polygon is the cause.

Multiple Tilemap Layers

Some projects use multiple tilemap layers: one for the ground, one for platforms, one for decorative foreground tiles. Only the gameplay tilemaps need 100×100 parallax. The decorative foreground can use 110×110 for a subtle depth effect without affecting collisions — as long as it has no collision polygons enabled.

Be especially careful with “near-foreground” layers at parallax 105×105. Developers sometimes put decorative tiles on these layers and forget that Construct still checks their collision shapes. Either disable collisions on the decorative tilemap or keep it at 100×100.

Verifying the Fix

Scroll the camera 2000 pixels to the right and check if collisions match tiles. If they do, the fix is working. Repeat vertically. The test must happen at a large scroll distance because small scroll distances produce small offsets that are hard to notice.

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

This bug class disproportionately affects late-stage development. The work to surface it is interactive testing in realistic conditions, which only really happens after the gameplay is in place and assets are populated. Catching it early requires deliberate testing of conditions that look unimportant.

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

For shipping games, the safest verification is a staged rollout. Apply the fix to 1% of players for 24 hours; watch the affected metric; expand if green. Skipping the staged rollout means the verification is the entire player base, which is too high a stakes for most fixes.

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

There's almost always a less obvious case where the same problem applies. The reported case is the one a player hit; the related cases hide because they're rarer or affect fewer players. After fixing the reported case, search the codebase for the pattern - one fix often unlocks several.

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

For shipping titles with a long support window, watch for this issue resurfacing after dependency updates. Engine upgrades, driver updates, OS releases - each one can resurface a bug class you thought you'd fixed because the underlying behavior changed slightly. Regression tests catch the obvious ones; player reports catch the rest.

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

If this issue manifests under high load (many actors, many particles, many network connections), profile the post-fix code path with realistic counts. The original cost was a bug; the new cost is real work, and real work has a budget.

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.

“Parallax is a visual effect. Physics does not know it exists. Any layer where the player touches things must have parallax at 100. This is the single most common Construct 3 tilemap bug.”

Related Issues

For broader tilemap collision problems, see Construct 3 tilemap collision not working. For collision detection between objects, see Construct 3 collision not detected between objects.

Before adding parallax to any layer, ask: does this layer have collision shapes? If yes, do not add parallax. Ever.