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I am trying to cut two different meshes A and B with the same polyline P so that I can join A and B afterwards with no holes or anomalies. I am using the knife project tool and this works quite well.

The problem: the knife project seems to have vertex snapping enabled. Whenever P is too close to any vertex of A or B the new inserted vertex snaps to those vertices. However, the vertices of A and B do not lie at the same positions. Thus, it often happens that-using the same polyline for the projection-the A and B do not fit perfectly due to holes in the joint mesh.

Is there any (hidden) parameter or other possibility to disable this snapping behavior in the knife projection? Otherwise, I need to implement the same functionality by myself just because of the snapping... and I do not want to do that :-D

Edit: it seems there is not only a vertex snapping but a general inaccuracy. I tried to illustrate the problem with the following images.

The polyline (vertices in orange) and one of the meshs: enter image description here

After applying the knife project tool (polyline in orange): enter image description here

Hiding the polyline: enter image description here

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  • $\begingroup$ I'm having some difficulty reproducing this .. can you share an example file on pasteall.org/blend ? Possibilities.. projection, automerge.. floating-point error... or I'm wrong.. $\endgroup$
    – Robin Betts ♦
    Commented Jul 21, 2021 at 11:19
  • $\begingroup$ I am going to share an example in a minute. But... is the accuracy of the knife project tool depending on the current zoom level? I got different results when calling the tool in different zoom levels... $\endgroup$
    – trippeljojo
    Commented Jul 22, 2021 at 8:34
  • $\begingroup$ Could be.. floating point? .. Stupid question, but have to ask, sorry .. you are in an orthographic view? $\endgroup$
    – Robin Betts ♦
    Commented Jul 22, 2021 at 8:41
  • $\begingroup$ Yes, I'm in orthographic top view. I already thought about the floating point problem since you mentioned it in your previous comment. However, if I zoom in until the background grid is in cm then the knife projection is pretty accurate. So, I guess, the zoom dependency is built-in... $\endgroup$
    – trippeljojo
    Commented Jul 22, 2021 at 8:47
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    $\begingroup$ In fact, my initial questions leads into a wrong direction. As we found out in the meantime, there is no snapping which could be deactivated but there is another fact that results in different calculations on different zoom settings. $\endgroup$
    – trippeljojo
    Commented Jul 23, 2021 at 11:34

1 Answer 1

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For those that don't read through the folded comments, knife project doesn't specifically snap to vertices. Nor does it have a numeric threshold or precision you can type in to control it.

If the viewport camera is zoomed in close when Knife Project is invoked, the result will be precise, with many new verts/edges/faces created. In the following, the orange line is the edge of the curve that was projected:
good projection

If the viewport camera is zoomed far out when Knife Project is invoked, the result will be imprecise, re-using existing vertices and faces after the projection:
poor projection

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