Symmetry on subdivision models

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For a symmetrical design,

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we can either build one subdivision across the centerline like this roof

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or build one half and then use layer symmetry either up

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here in the layer bar or here in the Object Lister.

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Building the whole shape will generally give us a smoother limit surface across the center.

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Building one half is generally more efficient,

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but we have to work harder to get a good result on this centerline join.

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There are tools that can help us with both methods,

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but they all rely on the symmetry direction set in each layer.

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And this yellow plane defaults to a centerline along

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the X direction which works for this car,

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but that can be changed, so you need to be aware of the direction

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as it's this layer symmetry, which is key to using the symmetry tools.

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So I'm going to start with this roof that we've built as one half.

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And if we wanted now to make that the whole roof,

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then I've got two ways to get the other half.

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I can use Edit > Duplicate > Mirror.

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And the default setting of XZ gives us

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the same result as the default layer direction.

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And so I get the mirrored side.

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Or if I delete that

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alternatively, I can turn on the layer symmetry

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and then say Create Symmetric Geometry.

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And the end result is exactly the same.

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And I've still got two separate subdivisions

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so I can join them into one using the Weld tool.

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And I'm gonna use the To Tolerance setting

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and just drag over these coincident center CVs to merge them.

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So you can see that that's been welded into a single subdivision.

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But over here, the CVs weren't snapped accurately to the center.

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So I can use Weld again with two midpoints

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to join those together on the centerline.

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And that then gives me a reasonable smoothness across the middle.

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But of course,

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the difficulty is that it's very easy to make a change and lose the symmetry.

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So we have a tool up here on the subdiv shelf

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or here on the palette in the Object Edit tab

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called Symmetric Modeling.

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And if I pick the subdiv,

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it reads the symmetry direction from the layer.

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And when I apply it,

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it forces the geometry to be symmetrical in that layer direction.

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And then down here, I can choose which side to use as the master.

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And you notice that the subdiv is now green, which means construction history.

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So if I pick and move CVs on one side,

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then the same actions are applied to the other side.

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But also the subdivision tools work symmetrically.

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So if I do an Insert,

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that's applied on both sides.

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Or if I pick this edge loop and do some extrusions,

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then I get the new faces built on both sides

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and it's the same with the Crease tool

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and most of the other subdivision tools.

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And if I ever want to get rid of that forced symmetry,

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I just need to use Delete Construction History

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and then the sides are independent again

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until I reapply symmetric modeling.

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So the other method is to stick with one half like on this hood.

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And you can see that I have deliberately made

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this very badly lined up on the centerline.

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I am pretty zig-zaggy here.

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So of course, I could fix that manually with moving and snapping vertices.

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But I've got a tool on the Object Edit > Align pull-down here

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called Symmetry Plane Align.

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So if I apply it here on this edge, both the edge CVs are snapped to the centerline

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and these second CVs are moved so that they're

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And these blue lines show where it's been applied.

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And because it does the two CVs I can skip this edge and then do this one

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and this one

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to get a tidied up center edge.

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And so that gives me a reasonably smooth result across the center.

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And again, the wireframe is now green, so

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if I pick this vertex and move it,

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then construction history causes this second one to

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move and maintain that 90 degree relationship.

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And if I try to move the second one,

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it will always ping back level to the centerline CV.

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So both ways of working are equally useful,

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and particularly when you use the two symmetry tools to help.