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Self-intersection knife ignores some self-intersections
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System Information
Operating system: Windows-10-10.0.17763 64 Bits
Graphics card: GeForce GTX 970/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 419.35

Blender Version
Broken: version: 2.80 (sub 57), branch: blender2.7, commit date: 2019-04-17 19:26, hash: rBb46245470f79
(also broken in 2.79)

Short description of error
For a particular narrow kind of self-intersection, the Face > Intersect (Knife) > Self Intersect operation finds most of the self-intersections, but leaves some narrow ones on the edges.

Exact steps for others to reproduce the error

  1. Open this file: (or make a plane, then rotate one of its faces 180 degrees and pull it down and stretch it out until it makes a 'bubble' when subdivided, as was done to create that file)
  2. Go into edit mode on the object
  3. Select all
  4. Face > Intersect (Knife) > select "self-intersect" for source, and "all" for separate mode

The bug has now happened. To observe it:

  1. Edge > Edge Split
  2. Select a point on one side or the other
  3. Press 'L'.

The entire shape becomes highlighted, instead of just the half that you clicked on.
To see why, zoom in on one of the faces right at the pinchy edge of the 'bubble':

It still connects to the other side, despite it clearly intersecting.



Event Timeline

It looks like these edges is not intersected anywhere, so it makes sense, that they stay connected.

I don't understand—what is this if not an intersection?

The orange face is one continuous face, that they grey face visually interrupts by passing through it. What makes this not considered an intersection?

You're right, the faces are intersecting. But the edges in my picture don't intersect with a face, which probably results in the behavior you noticed.
Not sure if there is a way to "fix" this.

Jacques Lucke (JacquesLucke) claimed this task.

Still think that this is not a bug. On my picture you can clearly see that there is an edge that does not intersect with any face.

Yes, and on my picture you can clearly see two faces that *do* intersect :)

I guess if you're saying that this feature was never intended to find those, then you can consider it a feature request rather than a "bug"? But we're talking about a feature that describes itself as finding self-intersections, and I've found a type of self-intersection that it can't find. I understand if there's no obvious solution, but I think it's worth keeping the issue around as a reminder that it would be helpful if someone found a way to address this.