Knife tool randomly effective snapping and trashing faces

Support Modeling
1

Hello,

This is a recurrent issues since 2.8 version, so it’s been years. I had never understood how this tool is supposed to work and why it is so bad, be it related to the clip start distance for snap correctly to vertices, or cutting backface faces

Is the knife tool complete trash or else?

Case 1: Working in perspective view, I can’t cut ngons or any face, it is impossible to snap. The cut result is useless, creating new vertices and edges. One way I found was to locally triangulate faces, then keep the edges I need if I get lucky. Anyway ngons, quads or tri, snap is still ineffective sometimes and I don’t understand why. makes me trully mad about Blender.

case 2: working in orthographic view. snapping is better but cutting is trashing faces behind… ruining my topology.

trashing faces
trashing faces1920×1200 498 KB

what are these black faces? how to fix them keeping UVs?

2

Hi, it looks like flipped normal.

3

hello, normals are similar to neighbouring faces, but displayed with stronger colors. maybe double faces.

My question is more: how to fix those fucked up faces by knife tool? and ideally how to get this tool work properly…

4

Can you show us an example which cut did not work?
Where you want to snap?

Some cuts need unwanted edges, they are created by the knife tool if needed. For example if you cut a hole in a plane a edge is needed to connect the hole to the surounding geometry.

5

Hmm…

…in perspective snapping on vertices and faces works without using any other snapping settings… also n-gons are no problem… at the end i “cut” through already established edges…

Maybe you are using it wrong ?? IDK…

6

This is the geometry you see in the picture I have posted. Nothing fancy, nothing special. K can’t snap at all on it in perspective view, only in orthographic. No clean cut is possible in pers. (while ortho is displaying backface wireframes, makes it hard to see)

I had this issue so many times and I don’t understand why sometimes not. What I know for sure is I regret choosing Blender as my my software years ago. Losing my energy and time with this half-made hobbyist toy

7

Sure, the usual and useless answer. Nobody needs “proof”… :roll_eyes:

Sure, if I do the same on a plane it works. Only I need it to work also on the actual geometry.
at some point it was related to the near clipping value.

8

Please post a scene that shows the problem so we can study it.
A .blend file, not a screenshot.

9

Well… for one" a plane is also geometry… and two:

so… to make your “request” a bit more understandable you maybe have to elaborate this to get a more suited answer. Because at least i do see no problem and so am not able to help you :person_shrugging:

10

Exemple.blend (1.1 MB)

There it is. version is 3.6.4

11

Helping is reading the question, thinking about it and if one happens to have an insight about it, sharing it. This is what helping means.

posting videos of default usage to prove the question wrong is not helping. This is useless and a waste of time.

12

No more problems if you increase View Clip Start to 0.1

13

I changed the value for test but this value of 0.1 is not working either

14

I’ve just tested your scene on 3.6.13.
I can see your problem but as I said, it’s gone when I increase View Clip Start to 0.1
That’s all I have :man_shrugging:

1 Like
15

Well… you have some overlapping edges here… so blender gets puzzled about where to snap…

It seems to be you aren’t using this supdivision on the outer geometry… or you think you dod not need it on the inner… ??

Anyway i suggest to make them conform liek for exampel so…

conform
conform800×549 34 KB

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