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I have a plane with subsurf modifier, And a circle to cut(knife project) with. The hole is cut, but it leaves mesh(artifacts) inside of it. artifacts

Creasing (Ctrl E) works on the outer edges only, leaving the artifacts. with crease

Why is this happening?

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    $\begingroup$ The knife tool is creating ngons, which don't work very well with subserf. See blender.stackexchange.com/q/89/599 $\endgroup$ – gandalf3 Sep 20 '14 at 23:30
  • $\begingroup$ As @gandalf3 pointed out, catmull clark subsurf prefers quads (the generated mesh will always be all quads). Subsurf also doesn't like sharp corners and concarve faces (the mesh will sometimes fold over itself). Also n-gons tesselation is somewhat unpredictable from a users perspective. That being said, why not use subsurf to make a square hole round like this: http://www.pasteall.org/pic/77423? :o) $\endgroup$ – user2859 Sep 21 '14 at 0:06
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    $\begingroup$ @user2859 Just a small detail, which probably isn't that important from an artistic perspective, but subserfing a square does not tend towards a circle. $\endgroup$ – gandalf3 Sep 21 '14 at 0:16
  • $\begingroup$ @gandalf3 I know. That is why I wrote "round" ;). It's just a suggestion for the example above and "usually" close enough. If it is not, then subsurf might not be the best way to do things in the first place. $\endgroup$ – user2859 Sep 21 '14 at 0:30
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    $\begingroup$ You could also do something like this: pasteall.org/pic/77427 $\endgroup$ – gandalf3 Sep 21 '14 at 0:36
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The knife tool is creating ngons, which are faces with more than 4 vertices.

Ngons and tris do not work very well with the subdivision surface modifier. See When should N-Gons be used, and when shouldn't they?

When working with subserf, you pretty much always want to use quads (faces with four vertices) exclusively.
Instead of knife project, you could make a rough circle shape out of quads and then use the subserf to round it out:

enter image description here

What I did in the gif:

  1. Inset the plane (I).

  2. Then in edge select mode (⎈ Ctrl↹ Tab), ⎈ Ctrl⎇ Alt⇧ ShiftRMB RMB on the edge ring created by the inset to deselect it.

  3. Subdivide (W).

  4. Switch to vertex select mode (⎈ Ctrl↹ Tab) and delete the center vertex.

  5. Make the inner loop circular with the To sphere operator (⎇ Alt⇧ ShiftS), or the circle operator of the bundled loop tools addon.

  6. Extrude the shape to make it 3D (if you want), and add a constraining loop cut to make the subserf stay closer to the shape of the base geometry (⎈ CtrlR+E to make the new cut aligned to the existing edge loops, and F to pick between the edge loops you want to align to).

  7. Make the outer edges of the cube/plane sharp with edge crease (⇧ ShiftE).

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