T-Splines Tip – Two Ways to Extrude

One of the first things you learn when transitioning to T-Splines is that you can take a face, and extrude it out or in.  You simply select a face:

And then hold down the alt button, and drag it in or out:

Simple, easy and effective.  But, did you know there is a second way to extrude?  Let me show you how – first let’s start with a 4X4 T-Splines surface:

Now, instead of grabbing a face, let’s take two of the middle edges, and use tsUnweld to open up a slit in our surface, and then move the verts in the center away from each other:

Select the edges around this opening, and then alt click to extrude a new surface:

Now, you can close off this extrusion just by using the tsFillHole command.  Run tsMakeUniform (because we changed our topology) and you get:

This certainly looks similar to our standard face extrusion, but notice that there are only two star points around the base of the extrusion, instead of four:

Okay, so that’s a nifty parlor trick, but what’s it mean to you?  Well, take for example this conceptual model I’m working on these days:

I’m going for that V-12 engine feel on this one, so I need to have bump outs on the top of the cowling for the valve covers.  In the screen shot above, you can see that I’ve used a classic face extrusion to make the bump outs.  This was ok, but I felt that it was a bit heavier than it needed to be, and I was spending bit too long point editing to get the shape I wanted.  A big reason for this is due to the fact that the top of the bump outs are defined by 15 control points.  Do I really need 15 control points to define a nice smooth bump out?  No way.  So, I went back and tried extruding the bump outs using the slit extrude method and got this:

Notice that where we had 15 control points around the top of the bump out, we now have 9!  We’ve also decreased our star points on each side of the model by 2, which is always nice.  Also notice that since the star points around the base of the bump out are aligned along the leading and trailing edge of the feature, it sort of naturally pulls the extrusion into a nice streamlined shape, which is exactly what we are looking for.   For this particular case, this slit type of extrusion is far superior, in my opinion.

For those who have watched my T-Splines webcast on maintaining accuracy in T-Splines, there might be something vaguely familiar about this concept.  Essentially, the way that I join wings to fuselages in that webinar is a hybrid of the face extrusion and the slit extrusion.  You start with a slit extrusion:

But then you add some edges along the back:

Unweld the edges I have highlighted here, and then weld the “free” verts to the end of the edges you inserted:

Now, grab all the edges around the opening, and extrude up:

Notice now you have two star points on one end of the extrusion, and one star point on the other.  If you want to close off the top you can do it like this:

Notice that the face in the “front” looks like it’s a triangle, but it’s actually a quad, since there is a point in the middle.  In smooth mode, it’s a perfectly well behaved surface, that looks (not surprisingly) like a hybrid of the two methods described above.

So, to review, we’ve got two base methods of extruding – a classic “face extrude” and what I’m calling a “slit extrude”, and you can hybridize these topologies as well, depending on the particular needs of what you’re trying to achieve.  Enjoy!

2 Responses to “T-Splines Tip – Two Ways to Extrude”
  1. Volodymyr Kalandyrets says:

    Thanks a lot for lesson.

    Vlad, Toronto, Canada

  2. Corin McCrae says:

    Great info!! Thanks for sharing your knowledge with us.

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