Making faces all face outward consistently

Question

I've read several posts here on the Blender StackExchange, but none of the solutions seem to work, unfortunately. I'm very new to Blender, so please forgive me if I use the wrong terminology. My question doesn't relate to a specific model, I face this issue regardless of the model I import. I'm trying to get all faces to face outward, I'm using Blender's setting that highlights faces in blue/orange to designate if they are outward/inward facing, and I can select each face individually and tell it where to face, but I'm trying to do this for all faces without the need to select each one individually.

It seems that in some instances the faces can't be automatically normalized because they've doubled up. For example, deleting one of the faces will then allow it to be flipped correctly. However this isn't always the case, sometimes when I select all of the faces and tell it to flip outward there will be a face that doesn't, and after deleting it nothing takes it place, indicating that it wasn't doubled up.

I've also tried the Clean Up > Merge By Distance trick, this still doesn't seem to make all faces face outwards consistently.

I feel like having a script is the way to go, I've tried some of the examples given in responses here on StackExchange but they are a little old now and don't seem to work for me. No error is produced, but they don't seem to have an effect.

I'm curious why it's difficult to make all faces flip consistently, I had assumed it would be a case of looping through every face and checking whether it's facing outward or not (which it must know in order to colour it blue/orange) and then actioning the flip if it's inward.

I thought I'd submit a new post incase there are any newer solutions that I've not found through my searching on StackExchange, or if there's a new approach that I could be taking to try and resolve this issue.

Answer 1

If you're not getting consistent normals from recalculate normals, the problem is non-manifold geometry.

Here's an example:

This is a mesh where one of the edges connects three different faces. No matter what Blender does, it cannot find a consistent set of normals for these three faces. If it calculates one way, the normals will flip over one transition; if it calculates another way, they'll flip over another.

This kind of geometry can actually be created by Merge operations, and is one of the reasons those have to be done thoughtfully, rather than just merging on a few million vertices without paying any attention.

If you are having trouble with getting consistent normals, you should take frequent advantage of the "select non manifold" operation in Blender. Non-manifold geometry can cause all sorts of problems for rendering, and is something you should probably be fixing anyways. Generally speaking, you can afford to ignore the "boundary" condition of the operation, but not the others:

That operation will draw your attention to problem areas of your mesh.

Answer 2

It sounds like the two features you are looking for are both in Edit mode:

a. recalculate Outside (shift+N)

b. Select All by Trait (Interior Faces)

example: I have set up a cube with an irregular pattern of normals facing the wrong way, and one interior face.

By selecting all the faces and clicking on recalculate outside, it will turn all the faces in the right direction. However, I still have one interior face, which I may not be able to find that I can select using select all by trait, and then dissolve or delete:

Finally, one extra feature that is useful. I find going to overlays and enabling statistics helps me to find doubled faces as well. Then when I select a face I can see if there are more vertices than I expect.