Unreal Engine 5 Tutorial - Detect if Inside a Spline Loop
Why It Matters
This lightweight inside‑spline test lets developers create dynamic, physics‑free zones for stealth, AI, or gameplay triggers, accelerating prototyping and reducing runtime overhead.
Key Takeaways
- •Create a closed-loop spline actor to define custom area.
- •Use Tick or timer to call detection function each frame.
- •Find closest spline point, get direction vectors, then dot product.
- •Positive dot product indicates character inside the spline loop.
- •Enable debug drawing via console to visualize spline during testing.
Summary
The video walks through building a UE5 Blueprint that determines whether any actor—typically the player character—is located inside a closed‑loop spline. It starts by creating a new actor, adding a spline component, and enabling the ‘closed loop’ option to form a polygon‑like area.
A detection function is called each tick (or via a timer) and receives an actor reference. The Blueprint extracts the actor’s X‑Y position, aligns it to the spline’s Z height, and uses “Find Location Closest to World Location” to get the nearest point on the spline. It then subtracts the two vectors, normalizes the result to obtain a direction, and retrieves the spline’s right‑hand vector at that point. A dot product between these two vectors reveals whether the actor lies inside (negative) or outside (positive) the loop.
The presenter demonstrates the logic by printing the boolean result and inverting it to match intuition. Debug visualization is toggled with the console command “show splines” (or “show flex splines”) so developers can see the spline overlay while testing. He notes that the method works for any actor—enemies, projectiles, or AI—making it a versatile tool.
By converting geometric containment into a cheap per‑frame check, developers can implement stealth zones, dynamic grass hiding, or custom trigger areas without heavy collision meshes. The technique expands UE5’s procedural content toolbox and can be adapted to vertical splines by including the Z axis.
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