Perimeters are not aligned to external perimeters.

[fjpanag]

Version

Version 2.3.0+linux-x64-GTK3

Operating system type + version

Fedora 33

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

Custom delta, based on Duet3D.

Behavior

In some cases the external perimeters are not aligned with the internal perimeters.

Typically, there is the Seam position setting which controls the starting point of the external perimeters. This starting point will produce a visible seam.

However, it seems that the internal perimeters do not follow the same rules for their starting point.
This causes the internal perimeters to start from a different point than the external. Thus the printer has to retract, and then unretract on the external perimeter.

This unneeded unretraction on the external perimeter produces a visible artifact, making the "seam" to be much more visible than it would be otherwise.

As an example, see the attached project. I can see the issue in layer 342.
The internal perimeters start from the back of the model. But when it goes for the external perimeter, it retracts, move somewhere to the front, and unretracts on the external perimeter.

The correct behaviour would be to start all perimeters at the same point. So the unretraction takes place only on the innermost perimeter. Exactly as it is seen in the skirt of the attached project.

Is this a new feature request?
No, its a bug report.

Project File (.3MF) where problem occurs

perimeters.zip

[fjpanag]

Unrelated to my actual issue, but possibly another bug, is the fact that in the example project provided above the Seam Position is set to "Rear". But on layer 342 the external perimeter starts-off from an unrelated point in the front.
It does not respect the setting Seam Position.

[bubnikv]

Fixed in PrusaSlicer 2.4.0-beta1.