Fixing Unreal Engine Physics Asset Misalignment in Custom Skeletal Mesh Movement

Troubleshooting Physics Asset Rotation Issues in Unreal Engine

Working with skeletal meshes in Unreal Engine can often result in unexpected behavior, especially when imported from external tools such as Blender. One common issue is when the physics asset appears misaligned or acts as if it has been rotated by 90 degrees. This can be puzzling, especially when the collision settings and asset preview appear to be accurate in the engine.

In one scenario, a developer used the function to move a bespoke skeletal mesh, but encountered collision discrepancies. The physics asset appeared to be colliding with things as if rotated. Debugging revealed that the collision shape was correct, but the behavior did not match the intended consequences.

Even more perplexing, when the physics object was manually turned by 90 degrees, everything functioned flawlessly. This highlighted the question of why Unreal Engine was not handling physics asset rotation correctly in the first place, especially since the skeletal model appeared to be aligned.

The root problem was determined as a rotation of the skeletal mesh's root bone in Blender. However, this revelation raised another question: why wasn't this rotation immediately translated to the physics asset in Unreal Engine?

Command	Example of use
SafeMoveUpdatedComponent	This Unreal Engine function safely moves the component using position and rotation deltas. It detects collisions on the travel path and adjusts the final location accordingly.
FQuat::MakeFromEuler	This function transforms an Euler angle vector to a quaternion, which is commonly used in Unreal Engine for rotation computations. It enables smooth rotations and eliminates gimbal lock concerns.
SlideAlongSurface	This command modifies an object's mobility when it collides with a surface, sliding it along the collision normal. It is essential for absorbing impacts and ensuring genuine physics-based motion.
HandleImpact	This function handles the consequences of a collision. It can cause events or change movement depending on how and where the impact happens. In this case, it is employed when an item collides with a surface.
FbxImportOptions->bForceFrontXAxis	This option is exclusive to importing FBX files into Unreal Engine and forces the asset's front to align with the X-axis, ensuring constant alignment when the mesh is imported from Blender or other tools.
ensure()	A debugging tool for Unreal Engine's C++ code that determines whether a condition is true. If false, it results in a warning or assertion failure. This is used in unit tests to ensure that conditions have been met.
UpdatedComponent->GetComponentQuat	Retrieves the quaternion indicating the component's current rotation. This is required to calculate the new rotation after using the rotation delta in physics-based movements.
CalculateRotationDelta	A proprietary method for calculating the change in rotation over time, which is used to determine how much an object should spin during a frame. It is limited to the context of smooth rotation in Unreal Engine.

Understanding and Solving Unreal Engine Physics Asset Rotation

The custom Unreal Engine script relies heavily on the function's handling of movement and collision. This command moves a component (in this case, a skeletal mesh) according to calculated position and rotation changes. The issue at hand is that the physics asset behaves as if it has been rotated by 90 degrees, resulting in erroneous collision detection. The script compensates for these differences by computing position and rotation deltas with specialized techniques.

Another important aspect of the script is the use of to handle rotation. Quaternions are used here to avoid common rotation problems, such as gimbal lock, which can occur when employing Euler angles for rotation. The script takes the component's current rotation using , multiplies it with a newly calculated quaternion from the rotation delta, and applies it to the component. This guarantees that the mesh is rotated precisely in accordance with its movement in the game environment.

The and commands manage the collision reaction. After detecting a collision using the movement function's hit result, these commands dictate how the object should interact with the surface it collides with. Sliding down the surface is essential for realistic physics in games, especially when working with skeletal models that may interact often with the environment. These commands ensure that the mesh moves smoothly and accurately even after collisions.

In addition to the physical interactions, the script contains a solution for unit testing using the command. This command ensures that certain conditions are met during runtime, which is important for troubleshooting. In this context, it helps to ensure that the rotation and collision behavior work as intended after each frame of movement. These tests are critical for ensuring that the physics asset works properly across a variety of gaming settings. The general goal is to handle the root bone's 90-degree rotation while maintaining consistency between the skeletal mesh and its associated physics asset.

// Approach 1: Correcting Physics Asset Rotation via Root Bone Adjustment
#include "YourCustomMovementComponent.h"
#include "GameFramework/Actor.h"
#include "Components/SkeletalMeshComponent.h"
#include "DrawDebugHelpers.h"

// Calculate position and rotation deltas based on DeltaTime
FVector PositionDelta = CalculatePositionDelta(DeltaTime);
FRotator RotationDelta = CalculateRotationDelta(DeltaTime);

// Correct the rotation based on root bone orientation
FQuat CorrectedRotation = UpdatedComponent->GetComponentQuat() * FQuat(RotationDelta);

// Check for collision and handle impacts
FHitResult Hit(1.0f);
SafeMoveUpdatedComponent(PositionDelta, CorrectedRotation, true, Hit);
if (Hit.IsValidBlockingHit())
{
    HandleImpact(Hit, DeltaTime, PositionDelta);
    SlideAlongSurface(PositionDelta, 1.0f - Hit.Time, Hit.Normal, Hit, true);
}

// Update velocity to account for movement
UpdateComponentVelocity();

// Unit test for verifying correct collision behavior
void TestPhysicsAssetRotation()
{
    FVector TestPositionDelta = FVector(100.0f, 0.0f, 0.0f);
    FQuat TestRotation = FQuat::MakeFromEuler(FVector(0, 90, 0));
    // Simulate movement
    SafeMoveUpdatedComponent(TestPositionDelta, TestRotation, true, Hit);
    ensure(Hit.IsValidBlockingHit());
}

// Approach 2: Adjusting Root Bone and Axis Orientation in Blender
// In Blender, apply transformations to your mesh before exporting
// 1. Select your mesh and press Ctrl + A to apply rotation and scale.
// 2. Ensure that the root bone has no inherent rotation (rotation set to 0).

// Unreal Engine: Use FBX Import Settings
// 1. When importing into Unreal, set the import rotation to ensure
//    that Unreal Engine aligns the asset correctly.
FbxImportOptions->bForceFrontXAxis = true;
FbxImportOptions->ImportRotation = FRotator(0, 0, 0);

// Unit test in Unreal to verify import orientation
void TestImportedPhysicsAssetRotation()
{
    USkeletalMeshComponent* TestMesh = NewObject<USkeletalMeshComponent>();
    FRotator ExpectedRotation = FRotator(0, 90, 0);
    ensure(TestMesh->GetComponentRotation().Equals(ExpectedRotation));
}

Addressing Unreal Engine Physics Asset Alignment Issues

Unreal Engine's physics system distinguishes between a and a physics asset. The skeleton mesh, which specifies the appearance of the character or item, can undergo different transformations (scale, rotation, and translation) than the physics asset, which defines how the mesh interacts with the environment. In many circumstances, adjustments done to the skeletal mesh do not instantly propagate to the physics asset, resulting in issues such as the one mentioned, in which the physics asset seems rotated by 90 degrees.

This issue frequently happens when skeletal meshes are imported from external tools such as Blender. Blender and Unreal Engine use distinct coordinate systems, resulting in orientation problems. When importing, check that the mesh and its are properly aligned and that transformations (such as the 90-degree rotation) have been applied before exporting. This allows Unreal Engine's FBX import system to appropriately understand the data, resulting in accurate alignment of the skeletal model and associated physics asset.

Another factor to examine is the role of Unreal's matrix. This matrix determines how a component is translated in world space. If the root bone's rotation is not properly wiped out or modified during import, it can create errors when Unreal calculates the component's world position and rotation. Correcting this matrix and ensuring the root bone is aligned with the world axes during import can resolve many misalignment issues.

1. Why does my physics asset behave as if it were rotated 90 degrees?
2. This is commonly caused by a mismatch between the skeletal mesh's root bone rotation and the physics asset. Ensure that the mesh's root bone is appropriately oriented in Blender to resolve the issue.
3. How can I resolve the 90-degree rotation issue when importing from Blender?
4. Before exporting the model in Blender, apply all transformations (rotation, scale) by pressing . Check Unreal's FBX import settings and make sure the root bone is not rotated.
5. What is the matrix in Unreal Engine?
6. It's a matrix that maps the component's local position, rotation, and scale to global space. Issues with root bone alignment might lead to miscalculations in this transformation, resulting in the rotated physics asset problem.
7. How can I debug physics asset collisions in Unreal?
8. Use in Unreal to visualize collision limits and ensure that the physics asset is aligned with the mesh.
9. What happens if I rotate the physics asset by 90 degrees manually?
10. Manually rotating the physics asset may temporarily resolve the issue, although it is a workaround. The fundamental cause is usually found in the skeletal mesh's import parameters and root bone alignment.

Finally, mismatches in the skeletal mesh's root bone rotation are the primary cause of the physics asset's improper behavior. Aligning the root bone in Blender before importing the mesh into Unreal Engine is critical for avoiding the 90-degree offset issue. Understanding how Unreal Engine handles movement and collision can help fix this problem.

Using routines like and correctly handling rotation using quaternions ensures seamless physics interactions. Developers should also use Unreal's debugging tools to visualize collisions and verify their solutions thoroughly.