Hollowing previously hollowed part generates body inside hollowed out part [SLA]

[soswow]

Version

2.3.0-alpha0

Operating system type + version

MacOS

Behavior

Steps to reproduce:

1. New project. Right click. Add Shape. Cube
2. Hollow with 1mm wall, 0.5 accuracy, 2mm closing dinstance
3. Export that as STL
4. Import that STL in
5. Hollow that part with 2mm wall.

Expected behaviour: nothing would happened.
Actual: You get this:

Fun fact. if you choose 3mm, you will get this:

Notice how normals a flipping between two screenshots.

• Describe the problem
• Steps needed to reproduce the problem
  • If this is a command-line slicing issue, include the options used
• Expected Results
• Actual Results
  • Screenshots from PrusaSlicer preview are preferred

Is this a new feature request?

Project File (.3MF) where problem occurs

No need I described how to get example with simple shapes.

Can it be related to #4677 ?

[bubnikv]

This is quite a complex scenario and in general case it is not well defined what result should our poor slicer produce.

For example, imagine your favorite babushka. What shall we do if presented with a single STL of multiple babushkas, one inside the other? One solution would be to discard all the surfaces but the outer most and then to create a new inner wall. We may consider implementing such an algorithm if there is demand for it, however it may not work reliably if presented with broken meshes (meshes with cracks, meshes made of multiple disconnected open surfaces).

Frankly, I am not sure why are you pushing our poor Slicer to such limits. What is the purpose?