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Maniphest T45669

Skin modifier root exploit
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Description

System Information
Windows 7 64, GTX 980

Blender Version
Broken: c27589e + 00857be

Short description of error
There is an exploit that allows to create more than 1 root on a connected surface with the skin modifier.

Wiki says for skin modifiers root:

Each set of connected vertices should have one root node. Mark Root enforces the one-root per set rule, so it is not necessary to manually unmark roots.

Since an exploit is sort of a bug, I think its not wrong to report it.
Its behaviour is definitely not intended - one-root per set rule is broken.
But I also want to point out that this "bug" is quite friendly and useful for modeling.
Please take a look in the file.

Exact steps for others to reproduce the error

SkinModRootExploit.blend1 MBDownload

Related Objects

Event Timeline

Karja Krähwald (karja) created this task.Aug 4 2015, 1:00 AM
Karja Krähwald (karja) raised the priority of this task from to 90.
Karja Krähwald (karja) updated the task description. (Show Details)
Karja Krähwald (karja) added projects: BF Blender, Modifiers.
Karja Krähwald (karja) edited a custom field.
Karja Krähwald (karja) added a subscriber: Karja Krähwald (karja).
Sergey Sharybin (sergey) lowered the priority of this task from 90 to Normal.Aug 4 2015, 11:40 AM
Sergey Sharybin (sergey) added subscribers: Bastien Montagne (mont29), Campbell Barton (campbellbarton), Sergey Sharybin (sergey).

@Campbell Barton (campbellbarton), @Bastien Montagne (mont29) any ideas here?

Bastien Montagne (mont29) added a comment.Aug 5 2015, 2:51 PM

So basically, we should be able to have several roots per mesh, as long as we only have one root per mesh 'island'… Does make sense to me, but will let @Campbell Barton (campbellbarton) decide here, afaik he maintains that modifier.

Bastien Montagne (mont29) added a comment.Aug 5 2015, 2:57 PM

Actually, to be precise, user here want to be able to define roots for 'fake' islands generated by hidden geometry… So on second thought, not sure we want to support that. Which also means we should check when merging different islands of a same mesh (or different meshes), that we only keep one root… Meh, this is serious corner-case. :|

Karja Krähwald (karja) added a comment.Aug 5 2015, 9:28 PM

I was not sure where to put this. I guess it looks way more like a feature request, but I found it very surprising or contradictary that the modifier is still working properly with multiple roots, although it deletes all previous with the root button.

Campbell Barton (campbellbarton) changed the task status from Unknown Status to Unknown Status.Aug 29 2015, 6:44 AM
Campbell Barton (campbellbarton) claimed this task.

Hi @Karja Krähwald (karja), I see you put quite some effort into this report,

On checking the use-case, its really quite obscure that you would choose to hide some parts of geometry - just so you can cause an island - in what is not initially an island. (using the mask modifier).
While there would undoubtedly be some use case for this. It seems rather contrived.

We could drop the requirement and allow multiple roots on a single mesh island... but this is going to cause problems (odd behavior at least) as well.

So I'm going to close this bug, currently this is working as intended.