Fixing Collision in UE5: Simple Collision and Complex Collision

You meant to land in a hollow in the rock, but your feet float slightly above it. A gap you should fit through stops you with an invisible wall. When that happens, display the "shape that collides" on top of the rock's appearance.

In UE, the shape drawn on screen and the shape tested for collisions are handled separately. Sometimes every bump on a rock is used for collision; sometimes a box roughly wraps it. Understanding that difference makes invisible walls much easier to locate.

Our subject is the Static Mesh , the model type used for rocks and props that are not deformed. We compare Simple and Complex shapes and fix the collision on a duplicated rock. The goal is not just to memorize setting names, but to be able to judge which shape your own game needs.

What You'll Learn

• How to investigate a mismatch between appearance and collision
• The difference between Simple, Complex, and convex hulls
• The roles of Collision Complexity and Trace Complex
• How to build shapes with Auto Convex and adjust where things touch

Compare the visible shape with the colliding shape

Collision is the test for whether things hit or overlap. Where a character approaching a rock stops is decided by the shape used for that test, not by the colors or patterns on screen.

Wrap a rock in one box and that box can cover the hollow as well. The character stops on top of the box, so it looks like they are standing in mid-air. Conversely, if the colliding shape sits inside the surface, part of the body appears buried in the rock.

Start by displaying the shapes.

1. Double-click the Static Mesh asset in the Content Browser
2. In the Static Mesh editor's viewport, open "Show"
3. Turn on "Simple Collision"
4. Then turn Simple off, turn on "Complex Collision", and compare

Some versions also toggle the display from the toolbar's Collision button. Identify them by the property names Simple and Complex , not by display color. The "Collision" menu that adds and removes shapes is separate from turning the display on and off.

If turning Simple on shows no outline, the Simple shapes may not exist yet. Do not conclude "nothing visible, nothing wrong" — check the shape differences in the next section.

Simple Collision: combining basic shapes

Simple Collision is built from boxes, spheres, capsules, and convex hulls. A crate takes a box, a ball takes a sphere: a few shapes cover what you need.

A convex hull is a shape with no dents. Stretch a rubber band around the outline seen from above and it connects the outer bumps without reaching into the hollow. Think of that wrapping made three-dimensional.

That does not mean Simple can never form a hollow. Split the left bump, the right bump, and the bottom into separate shapes and you can leave space between them. For a building with a doorway, splitting the wall into left, right, and top keeps the entrance from being blocked by a box.

What matters is matching the shape to where the player touches, not to how detailed the model looks . You do not need to reproduce every scratch in the collision, but you must not block a doorway players are meant to walk through.

Complex Collision: testing the surface with triangles

Complex Collision uses the mesh's triangles. A mesh represents a solid's surface by joining small triangles, so contact points can be found along that surface.

For example, if you aim at a rock and place a mark where you hit, you want the mark on the rock's surface, not on a large box's surface. That is the kind of test where the surface shape affects the result.

A trace here is the process of extending a line or a defined shape and finding "what is along the way". The Line Trace article builds an example that fires a line from your crosshair.

Turning on a Line Trace's "Trace Complex" requests Complex; leaving it off requests Simple. The shape actually used still depends on the mesh-side setting below. A trace does not automatically become Complex.

Triangle-based tests can use more memory and computation than a few simple shapes. Then again, Simple has a shape count too, and the number and range of tests differ. Do not assume "Complex is heavy because of the name" — choose from the accuracy you actually need.

The four Collision Complexity settings

Search for "Collision Complexity" in the Static Mesh editor's Details panel. It decides which shape gets handed to a request for Simple and to a request for Complex .

Setting	When Simple is requested	When Complex is requested
Project Default	Follows project settings	Follows project settings
Simple And Complex	Simple	Complex
Use Simple Collision As Complex	Simple	Simple
Use Complex Collision As Simple	Complex	Complex

With Simple And Complex, toggling a Line Trace's Trace Complex switches between box-like shapes and triangle shapes. With Use Simple Collision As Complex, turning it on still uses Simple.

Project Default inherits Default Shape Complexity from the project's "Physics" settings. Projects sometimes change it, so check there when you want to know what the default actually is.

Choose Complex based on the shape and how it moves

For complex platforms that never move, Use Complex Collision As Simple is an option. Consider it when standing along the surface matters and simple shapes are hard to fit.

But it cannot be used for dropping or rolling that mesh itself with Simulate Physics. A different physics object with its own Simple Collision can still hit that platform. Keep "moving the platform" and "things hitting the platform" separate.

The hands-on below tries fixing the Simple shapes first. Getting the shape right often resolves invisible walls and floating feet without switching to Complex.

Hands-On: fixing a rock's collision

Use a practice level with a walkable character and a floor. BP_InputPractice from the Enhanced Input introduction works, and so does the Third Person template.

Our subject is a rock Static Mesh. If you have Starter Content, search the Content Browser for SM_Rock . If not, use any bumpy rock you already have. We build one box on the duplicate so the comparison starts the same way regardless of the original collision.

1. Duplicate the asset for practice

Duplicate the rock Static Mesh in the Content Browser and name it SM_RockCollisionPractice . Place that duplicate in the level.

Duplicating the rock in the level keeps using the same Static Mesh asset. Changing the asset's collision would affect every other rock using it, so we duplicate on the Content Browser side.

Set the placed rock's Mobility to Static, Simulate Physics off, and Collision Presets to BlockAll. Sink the bottom into the floor and place it where you can approach from all sides. Use the same value for Scale X, Y, and Z, and size it against your character.

2. Observe the mismatch with one box

Open SM_RockCollisionPractice and set Collision Complexity to Simple And Complex . This standardizes the conditions for using Simple shapes regardless of your project default.

1. "Collision" → "Remove Collision" to strip the existing Simple shapes
2. "Collision" → "Add Box Simplified Collision" to add one box
3. "Show" → "Simple Collision" to display the box outline
4. Save the asset

Remove Collision removes the Simple shapes. It does not remove the drawn rock itself.

Rotate the view and compare the box against the rock, and find one place where the box sticks out further . That is where a character is likely to stop before reaching the visible surface.

Play, approach the rock's side, and see where you stop. If it is big enough to stand on, jump up and look at your feet. Rocks that closely match a box may not show a big difference. Rather than hunting for a floating symptom, first line the outline up with the contact positions.

3. Split the rock into several shapes with Auto Convex

Stop Play and return to the same asset. Strip the comparison box with Remove Collision, then open "Collision" → "Auto Convex Collision".

Auto Convex is a tool that splits a mesh into several convex shapes and builds Simple Collision from them. Space that one hull would have blocked can survive depending on how it splits.

Property	Value to try here	What it adjusts
Hull Count	8	Upper limit on generated convex hulls
Max Hull Verts	16	Upper limit on vertices per convex hull
Hull Precision	50000	How finely the original shape is analyzed

A vertex is a corner point of a shape. Raising Max Hull Verts represents one shape more finely, but start with the values above and look at the result. These are a starting point, not correct values for every rock.

Press "Apply" and wait for generation to finish. Display Simple Collision and look at the place the box stuck out from, at the same angle. Even with Hull Count at 8, you will not necessarily get 8.

What you compare here is not the shape count but whether the outline moved closer to the places you want to touch . Confirm the change, save, and Play again to test the same side and platform.

4. Fix protrusions and gaps separately

If the automatic result fits well enough, you can stop adjusting there. If part of it is still off, fix that spot.

Clicking a Simple Collision outline shows manipulation handles. W switches to move, E to rotate, and R to scale.

If a shape sticks out , select that convex hull and move or scale it, or remove it with Delete Selected Collision if needed. Adding another box on top of an oversized shape does not remove the protrusion.

If a shape is missing where you need one , add it with Add Box Simplified Collision or similar. A newly added shape wraps the whole rock, so shrink, move, and rotate the selected shape into place.

Make sure you are editing the collision outline, not the rock's appearance. At entrances and hollows, support the left, right, and bottom with separate shapes so you do not block the space meant to be passable.

5. Confirm again from the same place

Save the asset and approach the rock from the same direction as at the start.

• Approaching the side, you do not stop unnaturally early
• Standing on a face you want as a platform, there is no obvious floating or sinking
• A gap meant to be passable is not blocked by collision
• You do not fall through places needed as a floor or wall

You do not need a shape identical to the visuals. Success is when the places touched during play behave without getting in the way of the game.

Checks when it does not line up

Symptom	Where to look next
Nothing appears when Simple is displayed	Whether you added shapes, and whether you opened a different Static Mesh
An invisible wall remains	Whether a large box or hull is left over. Did you shrink extra shapes, not just add ones
The hollow is still blocked after Auto Convex	Raise Hull Count slightly and compare, or split that area with hand-made shapes
You fall straight through the rock	Check the placed rock's Collision Presets, which asset it uses, and whether Simple exists
The rock fits but you cannot pass a narrow spot	Check the width and height of the character's CapsuleComponent
You reach the surface but cannot climb a steep slope	Beyond shape mismatch, check Character Movement's walkable angle
Fixing Simple does not change where you touch	Whether Complexity is set to Use Complex Collision As Simple
Other rocks changed too	Whether you duplicated only the level placement and edited the same Static Mesh

Character movement normally tests against a capsule shape called CapsuleComponent. Even with the rock side correct, you cannot pass a corridor narrower than that capsule. The Character Movement article covers the moving side: speed, jumping, slopes.

Bonus: when weighing accuracy against cost

Keep the shape question separate from the "who collides" question

What we fixed in this article is the shape used for testing. Per-target responses like "stop the player" and "let bullets through" are decided by Collision Presets and channels.

If fixing the shape does not produce the response you expect from a given target, move on to collision channels and profiles. Checking shape and response in order lets you find the cause without needlessly raising accuracy.

Keep collision LOD separate from rendering LOD

LOD is the mechanism that prepares the same model at different detail levels. For a Static Mesh's Complex Collision, "LOD For Collision" specifies which LOD to use. It is not always only LOD 0.

Still, rendering switching to a coarse LOD at a distance does not mean collision switches the same way with distance. Check what gets drawn and what gets used for collision separately.

With Nanite, a separate simplified shape called the Fallback Mesh is used for Complex. When visible detail and collision disagree, check which shape Complex is actually using.

Landscape is a terrain feature separate from Static Meshes, with its own collision settings. Terrain creation is covered in the Landscape article.

To make it lighter, compare in an actual scene

Even with Simple, adding too many convex hulls increases the shapes tested. Complex cost, meanwhile, depends not only on triangle count but on what gets tested how many times.

If cost concerns you, compare settings in the same level, along the same route, with the same number of tests. Do not assume collision is why the frame is heavy — investigate processing time with tools like Unreal Insights.

Distance alone does not mean every triangle is brute-forced each time, and being off screen does not mean collision disappears. Start by sorting out the shape where it is needed and which targets need testing at all.

Summary

When collision confuses you, first overlay the outline on the visuals and see where they disagree. Simple is a combination of basic shapes; Complex tests triangles. Which one applies comes from the mesh-side setting together with what the querying side asks for.

For a rock, try checking the one-box shape, splitting it with Auto Convex, and hand-fixing only where needed. Once you know what the invisible wall really is, it gets much easier to decide what to fix in your own game.

Reference: Simple versus Complex, Static Mesh Editor, Auto Convex Collision, Nanite Technical Details.