Unexpectedly brake when AI agents use RequestDirectMove to move

Development Programming & Scripting
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1

In our project using UE5.6.1, AI characters sometimes brake unexpectedly. After one hour debugging, the cause of problem has been found.

If we invoke RequestDirectMove with a speed that is a little bit smaller than current speed, for example MaxSpeed=500 RequestSpeed=497, the code will run in the following way:

  1. UCharacterMovementComponent::ApplyRequestedMove normally consume this speed
  2. UCharacterMovementComponent::ShouldComputeAccelerationToReachRequestedVelocity will return true because 500 < 497 * 1.01
  3. UCharacterMovementComponent::CalcVelocity (CharacterMovementComponent.cpp : 3837) will invoke IsExceedingMaxSpeed, which will return true because 500^2 > 497 ^ 2 * 1.01
  4. UCharacterMovementComponent::CalcVelocity (CharacterMovementComponent.cpp : 3845) will invoke ApplyVelocityBraking, which will add a significant speed reduction.

This bug is really ​a coincidence. I think a reasonable way to fix it is change the check logic in ShouldComputeAccelerationToReachRequested :

bool UCharacterMovementComponent::ShouldComputeAccelerationToReachRequestedVelocity(const float RequestedSpeed) const
{
    return bRequestedMoveUseAcceleration && Velocity.SizeSquared() < FMath::Square(RequestedSpeed * 0.99f);
}
2

重现步骤

  1. add a AI Character to level, with charactermovementcomponent MaxWalkSpeed=500
  2. set level blueprint the same as LevelBlueprint.png. In brief, we just request direct move at constant max speed every tick except a little bit smaller speed at one frame.
  3. start game
  4. check out the log, you will find a wired brake happens.
3

Hey Zhou Yang,

Thanks for the bug report and steps to reproduce the issue! I’ve reproduced the issue locally and made a Jira to track it. In the meantime, your solution with changing logic in ShouldComputeAccelerationToReachRequested does fix the issue you mentioned so it is definitely a decent workaround but that could have other unintended effects. My initial thought is to just avoid braking entirely for this specific scenario (RequestedSpeed < CurrentSpeed and within 1%) in UCharacterMovementComponent::CalcVelocity, but I’ll have to do some thorough testing to make sure there aren’t any adverse effects of this.

Thanks,

Nate

4

Thanks for your rely. Please let me know the CL if you fix it.