# Topology Diagnostic Skills - How do we tell if a mesh is right?

So I went to make a half round pole yesterday and I found myself humbled. If I am to learn subdivision surface modeling I am going to have to learn what is different about these meshes.
These Two meshes look the same to me but something is very different about them

I am able to make loop cuts all the way around this half pole and I am able to inset the the top and bottom faces but below shows what happens when I put a sub-dmodifier on them :

It looks PROLAPSED ! so I was wondering if it was due to normals

above if you notice this is the one with the bad topology
Below are normals for the good

So here is the file I am working with
v01-TopologyStudyVer281a-feb26-2020.blend (946.6 KB)
below is a screenshot of this file:

So ya if I am going to be any good at modeling I am going to need to figure some things out. I did some study on poles stars and quads and tris plus ngons. In this example Are we looking at all the mesh or is there something that I need to do to make more things visible ?
Stan-pancakes showed me how to make the half pole the right way HERE

But anyway I am going to need to figure out some things about how to know if a mesh is right.
After making this post I have the idea that I will find info on youtube with the keywords diagnosing mesh problems

One piece of advice i can give you, donâ€™t focus so much on right and wrong. You only really need good enough for something. If it looks right and it doesnâ€™t cause problems down the line then it is good enough.

You should rather focus on how the subdivision surface modifier works and how it reacts to changes in the mesh. Some people waste hours on perfect topology where an Ngon with support loops would suffice, while others use the bare minimum geometry on a curved surface with some shapes cut out and then wonder why they have visual artifacts.

Basically it boils down to caring about topology and the amount of geometry where it matters and then using quick fixes in other areas where it isnâ€™t as important (like a flat surface or part of a hidden piece)

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The first thing you should do to the object with bad topology is make sure the normals are facing the correct direction. An easy way to do this is to enable backface culling in the viewport shading menu since inside-facing normals will become transparent. Alternatively, you can enable face orientation in the viewport overlays menuâ€“blue means facing outside and red means facing inside. Most of the normals are facing inside right now, which will cause subsurf to bug out. In edit mode, select all and use the top menu > mesh > normals > recalculate outside.

Next, you seem to have a duplicate of the half-cylinder inside of it thatâ€™s connected in weird areas. Making objects manifold is a good way to ensure they play well with the subsurf modifier. You can enable the 3D Print Toolbox addon to check your objects for problems and its make manifold command can help fix them to a certain extent. Relying on it to fix meshes wonâ€™t let you learn much. Instead of making it a crutch, I suggest using it mostly to check for problems and identify what you mightâ€™ve done wrong in order to improve yourself.

It helps a lot if you can find good modeling tutorials and I havenâ€™t found much for 2.8x yet. CG Mastersâ€™ combat knife tutorial teaches some great fundamentals, but you have to be willing to dig through the 2.8x UI, manual, or google in order to find the operations used in 2.7x. If you donâ€™t mind paying money, their hard surface modeling course seems to be pretty comprehensive, but again itâ€™s for 2.7x.

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Thank you Nominous - That link to manifold is a good one.
I watched this video on duplicate faces and found this little bit of python to duplicate faces

And then I watched this video below but it confused me a lot:

As far as the hotkeys I have been having good luck going into the preferences and searching the keyboard shortcuts there. I am still using the 2.79 shortcuts for 2.80 .
I have realized that the more I know about this subjects the less problems I will face down the road. I need to know I am creating mesh that will not have to be cleaned up at a latter time.

so one thing I have learned is the Boolean does not always do what its supposed to do. That when I edit a mesh I need to use ctrl x to dissolve edges so things tie back together somehow.
I am going through the Combat knife tutorial and learning some pretty cool stuff there. I still do not know how to know why this one mesh is messed up and the other one is not. But it looks like the subdivision modifier is a good test to see if the mesh is right.

I selected non-manifold and this face was selected. I have read about manifold and I get its when stuff is inside a solid but I am not fully understanding it

I could not find Make Manifold in the spacebarcsearch.https://sinestesia.co/blog/tutorials/non-manifold-meshes-and-how-to-fix-them/