Confusion over non-planar faces

[t_smith5]

I was just checking through the different options under Cleanup to see if I had any triangles or n-gons I'd missed, and when I selected for it to check non-planar faces, I noticed a number of them aren't selected, meaning they're non-planar...? What exactly does it mean for a face to be non-planar and why are some of these faces not highlighted? They all seem to have four sides/vertices and they're joined to each other from what I can tell.

[mspeer]

Hi!

The selected faces are non-planar, meaning not all vertices of a face are on the same (mathematical) plane.

[t_smith5]

I see. With the faces in my image that aren't selected, would it be best if I delete those and redo them?

[mspeer]

Hi!

Don't delete the faces, it's nearly impossible to prevent the creation of such faces.

If you don't need planar faces you can ignore this.

[zewt · Accepted solution]

Most of the time these aren't a problem and can be ignored, but if you do need to flatten a non-planar face, you can do this by selecting the face's vertices (select the face and press control-F9), selecting scale (r), setting component space in the marking menu, and dragging the X (red) axis towards the face.

[t_smith5]

Thanks for the suggestion. Only thing I'm confused about is setting component space in the marking menu. I can't see that option.

[zewt]

Hold R and LMB for the scale tool marking menu, and select "Component" in the top-right.

[t_smith5]

Thanks. Okay, I get now how to clean them up, but I'm still confused with something. When going to Cleanup, ticking 'Select matching polygons' and ticking 'Non-planar faces', are the faces that are highlighted in orange the ones affected, or the ones that aren't selected? I figured it was the ones that aren't selected, but trying what you've suggested still makes the unselected ones show up the next time I go to Cleanup again. Some don't even show the red X axis on the Scale widget either.

[zewt]

It selects components that it detects problems on.  If it doesn't detect anything, it just deselects everything.  I'm not sure how you wouldn't have scale axes.

The non-planar faces check would be more useful if it had a threshold, so you could specify the minimum angle to detect.  Faces that are severely non-planar can point to topology problems, if you have a lot of folds across quads, and it might cause inconsistencies when exporting to games where the mesh is often triangulated.  But, mesh cleanup has a very low threshold and picks up even very slightly non-planar faces, which just isn't very useful most of the time.