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Knife makes two vertices instead of one when cutting an edge
Closed, ResolvedPublic


System Information
Kubuntu Linux 13.04 64bit, AMD Radeon HD 4670 graphics

Blender Version
Broken: 2.70 official release (2014-03-19, hash 19e627c)
Worked: 2.68a (2.68.0 r58536)

Short description of error
Knife tool in some situations (not rare) makes not just one vertex on edge, but two in same location with edge between them.
"Same location" - that is "remove doubles" fixes this.

Bug is clearly seen when "Subdivision Surface" is in use.

Exact steps for others to reproduce the error
Steps (dublicated in *.blend textblock):

  1. Select plane oblect and switch to EditMode (if not already).
  2. Make knife cut like at image below - hitting every edge, not just top and bottom.
  3. Hit Enter. Now you should have image like one in the bottom.

Event Timeline

Aleksander Tarkhanov (stark) raised the priority of this task from to Needs Triage by Developer.
Aleksander Tarkhanov (stark) updated the task description. (Show Details)

same here on Blender 2.70 official release

I would add that the problem only arises when "apply subsurf to editing cage" is on. If that option is off then everything works fine here.


  • create a sphere and add a subsurf modifier
  • turn on "apply subsurf to editing cage"
  • use knife to cut geometry

Oh! Fantastic! It's really the case!
In 2.68 all was fine and I was wondering since 2.69 - why people don't see such a salient problem!
With that knowledge this "singularity" will be much less annoying to me!

Thanks for your reply, Alessandro!

you're welcome,

may be the reason why people don't notice it too much is because most Blender users use the detail-out approach to model, thus knife is rarely used. While in true box modeling knife is essential to define topology.

Hope they can fix it .. bye :)

Thanks for the bug report. Knife isn't well tested with subdivision and I guess I'm not too surprised it is broken. Will dive into this to debug.

thank you very much Mr. Howard :)

Thomas Dinges (dingto) lowered the priority of this task from Needs Triage by Developer to Normal.Apr 8 2014, 8:49 AM