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Skin modifier can produce undesirable geometry
Open, Confirmed, MediumPublic

Description

Blender Version
Broken: blender-2.74-65328fa-win32
Worked: never as the skin modifier always had problems like that

Short description of error
The skin modifier sometime fail {F177511}and generate very bad geometry

Exact steps for others to reproduce the error
load the attached blend, and see the problem as shown in the screenshot

(the underlying vertices are all symmetrical, as that base was done with the mirror modifier)
Test with the various x/y/z options in the skin modifier and see how that buggy geometry can move in other place of the generated mesh

Only way to fix is to try to either move around the vertices or to rescale them (CTRL+A with the skin modifier) , not real solutions if you need all the branches to be +/- the same diameter and at exact locations.

Details

Type
Bug

Event Timeline

Sanc Tuary (sanctuary) set Type to Bug.
Sanc Tuary (sanctuary) created this task.
Sanc Tuary (sanctuary) raised the priority of this task from to Needs Triage by Developer.

These bugs make the skin modifier virtually useless, as you'll get these errors in most common cases. Would be fantastic to get this fixed!

Sergey Sharybin (sergey) triaged this task as Needs Information from User priority.

It is always nice to have .blend file which demonstrates the issue.

@Sergey Sharybin (sergey), the blend is actually attached to the report, see the "F177511" in my report, it's the blend.

Sergey Sharybin (sergey) raised the priority of this task from Needs Information from User to Normal.May 19 2015, 10:34 PM

Ah, for some reason didn't notice it.

Campbell Barton (campbellbarton) raised the priority of this task from Normal to Confirmed, Medium.May 27 2015, 6:53 AM

Investigated this today and think I can see how to fix the problem. However the changes are extensive, not sure its good to make them right before release.


Developer details:

  • This is caused by logic for removing interior geometry. see: build_hull, /* Mark interior frames */
  • Currently the convex hull will output triangles (which get removed).
  • When theres a 4-sided shape meeting a 5 sided shape, its random chance that two of the triangles on the 5 sided shape meet 2 triangles on the 4 sided shape, diagonals dont matter (if they do, a connection is made, if not, the 4 sided frame is removed).

Solution can be to make convex hull return ngons, then split the ngons by existing edges, so we can isolate interior geometry and remove it without making holes.

I've had these problems with the skin modifier for awhile, glad to see that they're being fixed!

@Campbell Barton (campbellbarton) so will you commit fix soon?

@Computer Guy (Terrance8d), well if you had these problems for a while, why didn't you report them? That's a crucial step to get them fixed. ;)

I only deactivated the [ x ] in the skin Mod. panel and it fix, for this kind of model always use the mirror modifier is more flexible and non destructive but if the skin Mod get fixed it will be nice

Here the same blend file