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[Old Codger]

I'm building a space freighter. Big sucker. Currently working on the crew cab. It needed windows so I'm working away with the topo knife. I back out and look at my work and something odd is going on on the left side. I have attached screen caps in both rounded and flat shading.

I swear I did everything the same on both sides. They look the same flat. When I am in point mode and select (a) it only shows as a single face. But there are two things going on. #1 the face I cut - only on the left side - shows up flat. #2 the face below the one I cut is showing a crease.

Help! Anybody know what is going on? More importantly, anybody know how to FIX it? I still have the original, uncut shape if necessary. I can also rebuild it. But I'd rather not HAVE to do that. It looks like there's something wonky with the normals.

[selden]

Sorry: can't help with this one. I vaguely recall something similar happening to me, but I don't even recall if I managed to fix it or gave up in disgust

[chuft-captain]

I think I have seen something similar when selecting and detaching groups of faces  ... if I then re-combine the detached parts into a single mesh (Join Solids) it appears that different parts of the resulting mesh are "lit" differently ... almost as if the tangents have been somehow messed-up/changed in the process.

Not sure if this is related to your issue, but seems to exhibit somewhat similar symptoms.

[NickE]

It is easy to check the normals of the faces by selecting all the faces.  The ones highlighted in yellow are positive normals and the blue ones are are negative.

[Steve]

This could be caused by several things.
1) Two adjacent faces may have one more more duplicate vertices.  Try moving the vertices in point-edit more to see if they separate or not. If so you can join them with the Merge Points command.
2) The smooth angle may be too small. Try increasing it in the Mesh Editor.
3) As mentioned before, the face normals might be opposite. If you turn on the Draw As Clay mode in the top toolbad. Orange faces have normals pointing out, blue ones have them pointing in. You can usually align the faces normals by a) Selecting the back facing face and using the Flip Normals command, or 2) selecting the mesh and applying the Fix Normals command.

[Old Codger]

Thanks for the suggestions, all. Eventually I solved the issue empirically good old cut-and-try. I tried different ways/orders to cut the faces and edges until I got something useful. The odd thing is that the face that developed the apparent crease WASN'T the face I used the topo knife on. Totally wierd!

Stephen, I have noticed that whenever I do a lathe the resulting faces are always inverted. I've learned immediately after doing a lathe to mark all the faces and flip them.

You can see what I'm building on my Deviant Art page: https://www.deviantart.com/bmullins3

[ENSONIQ5]

Re faces being flipped after lathing, this depends on how you prepare the spline that is rotated/lathed. A lot depends on whether the spline is created to the left or right of the centre line (assuming the lathe will be around the Y-axis) and whether the resulting object will include internal or external angles.  In the attachment below, the upper two objects were created from splines drawn to the left of the Y-axis centre line, the bottom two from splines drawn to the right of Y-axis centre line.  The normals of the upper object with external angles are 'correct', the next one down with internal angles are reversed.  The reverse situation is evident in the lower two objects where the splines were drawn to the right of the Y-axis centre line.  I've got into the habit of checking the normals of lathed objects as I can never remember which way they will go

Also, re edge creasing, the Edge Property Editor (accessible by double-clicking an edge in point-edit mode) allows individual edges to be creased or smoothed irrespective of the mesh's Smooth Angle set in the Mesh Editor, though you may need to open the Mesh Editor (double-click the mesh) after changing an edge's properties to see the changes in Smooth view.  Importantly, changing the Smooth Angle for the mesh doesn't over-ride any individual edge creases you may have set for the object which is good but means there's no simple 'reset' to clear any individually-tweaked edge creases.

[Old Codger]

@ENSONIQ5, the problem is that it was a face that suddenly developed a diagonal crease down it. I never did figure that one out but was able to get a usable result anyhow.