Incorrect plane slice with non uniform scale

If the object is not scaled in a uniform way (e.g scaleX, scaleY, and scaleZ are different) then the slicing is not correct.

How to reproduce:

1. Open SliceDebugScene from https://github.com/DavidArayan/EzySlice-Example-Scenes
2. Change scale of ObjectToSlice in the editor from (10, 10, 10) to (10, 50, 10) for example
3. Set the SlicerPlane field and click Cut Object
4. Inspect the resulting objects (Lower_Hull and Upper_Hull)

[DavidArayan]

In order to solve this issue the cut will need to happen in world space. This means transforming all vertices into world space, performing the cut operation and transforming the resulting points back into object space. This results in a fairly heavy computation cost so instead of “fixing” the current method, i’d probably add in an option to slice non uniform scaled objects separately at the cost of additional computation/time.

My apologies for not communicating this sooner. At the moment i’m slowly investigating moving the slicer into Unity Job System that would allow slices to be performing drastically faster. The new redesign would allow me to fix lingering issues such as these much easier.

[DavidArayan]

Hello,

As of the latest commit b98da2a this issue should be resolved and can finally be closed. I’ve modified the SlicerExtensions.cs code so the Plane direction vector is properly transformed into the object local space before slicing.

Unfortunately this also means that the old method of using Transform.InverseTransformDirection or Transform.InverseTransformVector are incorrect. Unity should be using the Transpose Inverse Matrix to transform direction vectors for correct results. Maybe this is something that can be brought up with the Unity3D engineering team directly as a bug report. As a work around, i’m transposing and inverting the matrix manually for the correct results.

I will now close this issue. Thanks everyone!

Edit: See correct results after cutting a non-uniformly scaled object from following screenshot.