Modifier mesh leave artifact, possible structural weakness

[inner-ari]

Version

(note: applies to previous versions as well, this is not specific to the latest version)
Version: 2.3.0-beta3+win64
Build: PrusaSlicer-2.3.0-beta3+win64-202012121809

Operating system type + version

Windows 10 (build 19041), 64-bit edition

3D printer brand / version + firmware version (if known)

Original Prusa i3 MK3S, standard config (but this is not a printer issue).

Behavior

• Describe the problem
  When creating a modifier mesh to change perimeters, the slicer puts visible artefacts on the surface of the object. It looks similar to ghosting but is stronger and is visible in the slicer simulation too (see below). These artifacts are seen in the printed object as well (see below) and could potentially weaken the structure due to missing material.

• Steps needed to reproduce the problem
  The simple example shown here is to add a cylindrical modifier to reinforce a screw-hole. In the images below, the left hole is "reinforced" with a cylindrical modifier that specifies 3 perimeters instead of the default 2.

• Expected Results
  Whether or not the slicer treats the modified region as a separate object, it should recognize the continuity and lay down a single continuous line of filament rather than printing the two regions as if they were unconnected objects with external walls each.

• Actual Results
  The slicer appears to treat the region as a separate body, even though it is fully contiguous. It "refuses" to lay a line of filament across this fictitious border, resulting in a "ring" of missing material on the surface, around the modifier.
  (Incidentally, another problem with this fictitious new body is that more perimeters are added than requested. In the example here, 3 perimeters were specified for the modified cylinder, but in fact 5 perimeters are laid down because the modified region is treated as a new object, which causes two sets "external perimeters" to be laid out inside the object body. )

  • Screenshots from PrusaSlicer preview
    The left hole has a modifier cylinder; the right hole does not, for comparison.
    Note: Around the left hole, a ring of missing material is visible on the surface. Also, "external" perimeters (orange lines) are laid down inside the object. Both of these correspond to the extent of the modifier cylinder.
    

  • Photo of actual printed object
    

*Is this a new feature request?
No. The issue produces structure/weakness/blemishes that aren't in the model.

*_Related Issues
This issue may be relevant to issues #1314 and #3787 as well.

Project File (.3MF) where problem occurs

Block with two holes.zip

Thank you for all your amazing work!

[rtyr]

Duplicate of #3495, #3370.

There is also some discussion about it here #2982.

Closing.