Cura: Tree supports sometimes don't touch the thing they're supposed to support

[lukehutch]

Application version
4.7.1

Platform
Linux Fedora 32

Printer
AnyCubic i3 Mega

Reproduction steps
Download files from here:
https://www.thingiverse.com/thing:196038

Try slicing file "20150903 WingedVictory Full Scale Decimated to 1MTris.stl", with tree support enabled

Use fine settings (0.1mm), everything else default

Screenshot(s)

Actual results
Tree support on back does not touch the part of the dress that is flaring out behind the Winged Victory statue (three angles shown above so you can see it doesn't touch).

Expected results
The support should either touch, or is useless.

Project file
Linked above

[fieldOfView]

When inspecting supports, it is much easier to see what is going on if you change the color scheme from 'material color' to 'line type'.

[Ghostkeeper]

I'm assuming you rotated the model to be upright, and scaled it so that it's not actually 3 metres tall, as with the original STL? What scale did you scale this to? A project file would still help to reproduce your issue.

I'm slicing the model now at 6% scale. It's going to take some time though, as it's going to have almost 2000 layers and tree support scales badly with tall and highly detailed models like this.

[lukehutch]

Yes, correct -- it's significantly shrunk, and rotated upright.

Here's the last screenshot again, with line types colored: (this is at the back of the model)

Notice how the tree support contacts the surface at the bottom of the image, but does not contact the surface higher up.

There are other places where the supports do not meet the model, such as under the left wing:

Under the right armpit:

Right hip -- most of the support material is superfluous, and is doing nothing as it is too far away from the model:

Top of right leg -- support material suddenly appears out of nowhere (is not supported below) and has quite a big gap to the thing it appears to support

Sometimes the supports are not even underneath the things they're supposed to be supporting, e.g. top side of left leg:

etc. -- I'll let you examine the rest of the model for support issues.

I printed this anyway, and found that the supports failed frequently under the bottom edges of the wings, which left those edges "furry"-looking. I think that's because the gap between the supports and the model are often large -- and the size of the gap is very inconsistent.

Here's the project file:

https://gofile.io/d/1jNXd8

[Ghostkeeper]

I can reproduce it. The problem seems to be significantly reduced if the Support Roof is disabled.

Note also that there are still other known bugs with the Z distance.

[Ghostkeeper]

The existing issue with the tree support Z distance (CURA-7624, for Ultimaker developers) is currently in the 25th position of priority on our backlog, which may mean that we'll get to it in a few sprints as long as there is no significant influx of important new bugs. I've added this result to that as a related issue that should be fixed at the same time as #5759. I think the two issues are related and possibly fixed with the same fixes.