Trim and untrim limit surfaces

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Adding cutout details or holes using only the control cage

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means that you've got to add a lot of geometry

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which adds complexity to the subdivision.

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The alternative is to keep the control

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cage simple

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and then create the detail on the limit surfaces using the NURBS trimming tools.

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So I'll go back to my untrimmed subdivision

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and I've created some curves here in this view.

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And I'll start by using the Project tool to create some curves-on-surface.

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And this defaults to the view direction which

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would work as I am in this orthographic view

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or if you prefer to work in perspective, then you can choose the axis.

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So in this case, I'll project along the X direction.

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But the most important option is this Subdiv Select

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because when it's asking me to select the NURBS surfaces,

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I can just click once on the subdivision

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and all the limit surfaces get selected.

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So I'll say Go

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and then it says "Select the curves" and I've got three here.

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So I can box select those and say Project

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and that creates a curve-on-surface which is drawn on to the limit surfaces.

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But also the projection has gone all the way through.

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And I've got one here at the front

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and a bit here too.

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So I can use this Limit Projection option here to reduce that.

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And I don't need to be too accurate. Just make sure that it only covers the area I need.

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So these five limit surfaces have curves-on-surface

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and so the boundaries are shown dotted for these.

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But I've also got these two spare ones,

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which is why the Trim tool also has this Subdiv Select option.

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So again, I can click on the subdivision to activate all the surfaces

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and then select here.

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And I don't want to keep that I'll do a Discard instead

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and that then hides those two spare surfaces and trims these five

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and changes the outline color to yellow.

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And it's got construction history, so if I change the filet radius, for example,

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the trimmed region will update

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and I'll get a more controllable and precise cutout

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shape than I could with just the subdivision vertices,

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and any changes that I make will always update the trim.

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Now with this circle for the badge area, I'm not gonna want to project that in X or Z.

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So in project I can choose the Normal vector option.

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So if I pick the subdiv it's gonna go 90 degrees to the surface. So I'll do a Go

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and pick the curve and do a Project.

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So I'll just hide the curve layer. So you can see the curve-on-surface here.

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And then I can just do the trim,

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pick the subdiv and Discard.

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So I've added detail without adding complexity to the subdivision cage.

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You can also Intersect to create curves-on-surface.

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So let's say I've done a quick exploration of this roof wing

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and I've kept the face layout simple and I haven't yet

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worked out how to combine the two subdivisions along this join.

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So I can intersect the limit surfaces instead.

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And again, it's got the Subdivision Select options.

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So I can just pick one whole subdiv

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and then Go

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and then the other.

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And if I take the shading off, you can see that the curve-on-surface has been created.

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Now for trimming the curve-on-surface has got to go all

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the way to the end and here this one's a bit short,

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but there's an option on Trim called Extend

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and I'll use Selected which displays an arrow here where there's an issue.

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So if I click on that,

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it will delete the construction history, but that's OK.

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So I'm just going to select here and say Keep

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and it will be the same for this one.

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This end is OK.

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But I can see an arrow here as it's a bit short. So I can click on that to extend

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and then pick this region and Keep.

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So that's done a quick tidy up which might be

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useful for a design review or a screen grab.

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Now there isn't an undo with trimming, but we have Untrim instead

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and this Last option will undo step by step.

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So if I click here, it does

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that trim from the intersection

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but not these ones.

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And then if I click again, it'll untrim in sequence.

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Or if I do an Untrim All, then everything is untrimmed here

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and here.

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And it's good practice to delete any curves-on-surface you don't need for trimming.

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And there's a specific Pick tool here

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and I use that a lot, so I've added it to my marking menu as well.

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So I can just box select and delete the curves-on-surface.

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And finally, if you use these tools a lot, it can be useful to add them to your shelf,

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particularly the Project tools in the different directions

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and the Trim and Untrim.