Bridge Infill Missing when using Multi Material Painting

[arusso]

Version

Version 2.4.0-alpha3+x64

Operating system type + version

macOS 11.6

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

Pruse MK3S/MMU2S Firmware 3.9.2-3524

Behavior

When using multi-material painting, a bridge infill is not generated and thus the internal cavity / infill would be visible through a small opening at the top of the model. When not using multi-material painting this the bridge infill is present and the internal cavity is not visible.

• Expected Results -- we should see a bridge infill around layer 221 that matches the multi-material color, similar to the unmodified stl:

• Actual Results -- we observe the missing bridge infill.

Project File (.3MF) where problem occurs

• 3MF and Original STL

[hejllukas]

Thank you for reporting it. I looked at it, and you are right. Bridge infill should be generated there to ensure that the painted region are not floating in the air. To keep the internal cavity internal not visible, you can turn on Detect thin walls (Print Settings -> Layers and perimeters -> Quality). But I know this doesn't solve the core of this issue. Unfortunately, we have found out that this issue is quite complex. It doesn't just affect multi-material painting, but modifiers/regions in general. Multi-material painting is affected because, in the background, it creates regions with assigned extrudes in a similar way as modifiers do.

For our future reference, I'm attaching two screenshots and 3MFs. An extruder modifier is used on the first screenshot, and on the second screenshot, an extrusion width modifier is used. In both these cases, the bottom part of the modifier is printed in the air because of no bridge infill.

Extruder-Modifier.zip	Extrusion-Width-Modifier.zip