Multicolor model - Perimeters are not supported by infill bridging if they are differing extruders

[rcmaniac25]

Description of the bug

A print where perimeters start inside the print, does not get supported and prints in mid-air.

There's existing logic where if a top layer needs to be printed

it makes some bridge infill to contact the existing infill

then does the solid infill on top of it

But, if it's a perimeter

it does none of that and prints in mid-air

In some cases

it outright prints in mid-air

The logic for this could also benefit two other issues I've seen with organic supports:

• Organic supports generated in mid air (I know there's a ticket for this somewhere, need to find it again). Basically: if it's known there's nothing under the printed part, add bridging somewhere so it can build on top of that.
• Perimeters that need supports get interface layers generated mid-air (causing a failure of both the supports and the print). I don't remember if this one exists already, but I have a project that I had to work around this issue.

Edit: per comments, this occurs when they are differing extruders. I would've expected that which extruder is being used doesn't change the slicing, but I guess each are treated independently sliced models and then combined. So if one is mid-air, the other doesn't know to support it. Obviously something that is desired, as prints with multiple colors/materials/etc. tend to take longer and bad supports just makes it more likely to fail... not something you want after spending a lot of time on a model.

Project file & How to reproduce

This is from trying to print https://www.printables.com/model/1129607-metikoro-a-3d-printable-strategic-board-game-for-2

metikoro.zip

1. Open project
2. Slice it
3. On the first bed: Check layers 1.20-1.40 and 10.60-10.80 and you'll see mid-air printing of perimeters.

Checklist of files included above

• Project file
• Screenshot

Version of PrusaSlicer

Version 2.9.0

Operating system

Windows 10

Printer model

Prusa MK4S

[neophyl]

Looks like its because of the multi colour nature of the project. If you change those features to match the extruder (2) of the underlying parts then it slices ok.

[rcmaniac25]

Ah, that's a good point @neophyl . Though, given I want the multicolor, let me update title and such to indicate this bug applies to multicolor.