The Geometry Collection fragment is stretched when play the Chaos cache

Development World Creation
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8

Hi, I was also experiencing this issue, and I can confirm that setting p.Chaos.Cache.UseInterpolation 0 works, but since it disables interpolation it can result in teary simulation, especially in slow motion. I found a workaround, but it requires a custom engine build:

In the ChaosCache.cpp FParticleTransformTrack::Evaluate you need to replace the

Result.Blend(TransformA, TransformB, Alpha);

with:

const FTransform NA = TransformA;
FTransform NB = TransformB;
	
if ((NB.GetRotation() | NA.GetRotation()) < 0.0f)
{
	NB.SetRotation(NB.GetRotation() * -1.0);
}
Result = (FDualQuat(NA)*(1 - Alpha) + FDualQuat(NB)*Alpha).Normalized().AsFTransform(FMath::Lerp(NA.GetScale3D(), NB.GetScale3D(), Alpha));

and include on top of the file.

#include "Math/DualQuat.h"

(You can also remove const from TransformB local variable from the function and use TransformA and TransformB variables instead of copying values into NA and NB, but I’ve wanted to keep the changes to the engine code minimal and in one place.)

This results in buttery smooth caches without any artifacts. I’m honestly not sure why the normal blend function is causing troubles, but using dual quaternion interpolation seems to fix the issue. Btw I’ve copied the code for it from UKismetMathLibrary::TLerp, so don’t even ask me how it works :DD

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