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I'm a blender newbie. I'm trying to build an helmet, and I've got to the point where I have to cut the global object I've built so far to get the visor.

After creating the bulk shape (basically a half sphere on top of a cylinder), I've selected the knife tool and started cutting and everything seemed fine, until I reached this point. enter image description here

Here, the shading is weird:

enter image description here

I thought maybe was just something with the calculation of the new normals so I used Tools > Shading: Smooth and now the weird "box", the most visible problem, is gone. That line on the upper side, however, remains.

enter image description here

How can I fix this? Is there something I can do or is related to something I did/didn't do when I cut the faces to get the visor shape?

Thanks!

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Shading issues most commonly occur because of the presence of non-quad geometry (especially near edges).

As can be seen in your picture, the "quads" surrounding your highlighted section have more than 4 vertices apiece. This is as a result of your use of the knife tool. There are a myriad of methods you can employ to "correct" the geometry (often this involves moving unavoidable n-gons to flat sections to avoid shading issues), but could also involve simply extending the new edges all the way around the mesh - it all depends on how "clean" and "orderly" you need the final mesh to be.

A second reason for shading issues (and what seems to also be the case here) is a case of supporting geometry. Generally, supporting geometry is used to sharpen the appearance of edges,however, it can have the side effect of causing "ugly creases" to appear (especially on curved surfaces) when there is uneven spacing between edges or edge loops. If two edges on a curved surface are too close together, they will highlight the "crease" between two flat faces that are being shaded to appear smooth.

I suggest you spend a bit of time practicing some simple topology exercises, just to get a feel for the "rules" so to speak, as well as what you can and can't get away with without causing shading issues.

Best of luck.

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  • $\begingroup$ oh, the SURROUNDING faces are not quads... I was so focused on those two faces that I forgot to check the surrounding ones. Can you please elaborate on the supported geometry subject or at least point me in some direction where I can learn more? $\endgroup$ – Davide Vitali Jun 6 '20 at 9:40
  • $\begingroup$ Sure, as always, look to the Guru - youtube.com/watch?v=lITV4F_P4E0, $\endgroup$ – Christopher Bennett Jun 6 '20 at 13:13
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As Christopher Bennett explains, you have ngons all around, it creates artifacts when the surface is round. What you should do is use the current topology, here is a solution:

enter image description here

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  • $\begingroup$ Considering that I'm following a background image as a reference, then I should change the topology altogether, right? $\endgroup$ – Davide Vitali Jun 6 '20 at 10:01
  • $\begingroup$ It really depends, actually the topology may evolve because at one moment you'll realize you need more or less edges $\endgroup$ – moonboots Jun 6 '20 at 10:17

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