UE4 16.3 Potential bug in CableComponent.CPP and solution

Development Programming & Scripting C++
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1

Bug Symptom:

When a cable attached to 2 actors with both ends, and both 2 actors move long distance by setting location, the cable component will sometimes disappear. (This bug seems to occur more often when a cable has more segments, ex: segments > 12)

Bug Cause: When this bug occurs, printing out the location of each particle of that cable, I get this:

LogGamePlay: C++ CABLE_DEBUG_TEST() particle index: 0 x: -284895.406250 y: 120371.539063 z: 111.272942

LogGamePlay: C++ CABLE_DEBUG_TEST() particle index: 1 x: -nan(ind) y: -nan(ind) z: -nan(ind)

LogGamePlay: C++ CABLE_DEBUG_TEST() particle index: 2 x: -nan(ind) y: -nan(ind) z: -nan(ind)

LogGamePlay: C++ CABLE_DEBUG_TEST() particle index: 3 x: -nan(ind) y: -nan(ind) z: -nan(ind)

LogGamePlay: C++ CABLE_DEBUG_TEST() particle index: 4 x: -nan(ind) y: -nan(ind) z: -nan(ind)

LogGamePlay: C++ CABLE_DEBUG_TEST() particle index: 5 x: -nan(ind) y: -nan(ind) z: -nan(ind)

LogGamePlay: C++ CABLE_DEBUG_TEST() particle index: 6 x: -nan(ind) y: -nan(ind) z: -nan(ind)

LogGamePlay: C++ CABLE_DEBUG_TEST() particle index: 7 x: -nan(ind) y: -nan(ind) z: -nan(ind)

LogGamePlay: C++ CABLE_DEBUG_TEST() particle index: 8 x: -nan(ind) y: -nan(ind) z: -nan(ind)

LogGamePlay: C++ CABLE_DEBUG_TEST() particle index: 9 x: -nan(ind) y: -nan(ind) z: -nan(ind)

LogGamePlay: C++ CABLE_DEBUG_TEST() particle index: 10 x: -nan(ind) y: -nan(ind) z: -nan(ind)

LogGamePlay: C++ CABLE_DEBUG_TEST() particle index: 11 x: -nan(ind) y: -nan(ind) z: -nan(ind)

LogGamePlay: C++ CABLE_DEBUG_TEST() particle index: 12 x: -283573.937500 y: 120196.406250 z: 266.018738

Except for the start and end of the cable, all the particle positions are all undefined. Which leads me to suspect
a potential bug in this function:

static void SolveDistanceConstraint(FCableParticle& ParticleA, FCableParticle& ParticleB, float DesiredDistance)

	// Find current vector between particles
	FVector Delta = ParticleB.Position - ParticleA.Position;
	// 
	float CurrentDistance = Delta.Size();
	float ErrorFactor = (CurrentDistance - DesiredDistance)/CurrentDistance;

If for some reason ParticleB.Position = ParticleA.Position, then ErrorFactor will be undefined, and cause all the particles to have invalid values, except for !bFree particles (the end points of the cable)

Bug solution:

I simply add a small number to the CurrentDistance variable before dividing it:

	// MYW modify bgn
	//float CurrentDistance = Delta.Size();
	float CurrentDistance = Delta.Size() + 0.01f;
	// MYW modify end

This is only my suspicion, I will be glad if an Epic staff can comment on this problem and solution.

Thank you!

2

Yup, for me the cable never seems to appear in the first place. Using version 4.25.4

If I manage to find a fix I’ll be sure to post it here

3

For anyone else finding this, yes this is still an issue in 2021.

4

Do you mean you still experience cable disappearing in later versions of UE4? (ex: UE4.2x ??)

Since no one reply to my original post for a long time, I always thought I am the only one who encounters this bug.

5

thanks in advance!! :stuck_out_tongue:

6

Asked about this on UDN, got this response;
“Thanks for reporting this. A fix has been submitted and should be available in the next release.”

7

Hey!
Thank you for taking the action to notify Epic of this bug! :smiley: