Join Veejay Gahir for an in-depth discussion in this video Extending curves and surfaces, part of Alias Essential Training.
- Let's take a look at how we can extend or extrapolate entities in Alias. So in this case here, I've got some very simple entities. I've got a single Bézier curve here. I've got a NURBS curve here. And see the indicator here, that's telling you that it's five span. And I've got another entity down here, which is a problematic entity. And we're gonna take a look at why this would be causing problems during the extrapolation process. So let's go to Object > Edit > Extend and I'm gonna do a reset on this. So we're going to use Extrapolate.
We're gonna merge and we're gonna go by distance. So let's go with the first entity. So with Extrapolate and Merge, you notice that it actually redistributes the control points along the extrapolation. And as I release, it's actually merged that extrapolation back into the basic curve. Now if I do this on the NURBS curve, you'll notice that the extrapolation only occurs on the last segment. In other words, it goes back to this edit point, to the end and merges back in. Now let's zoom into this entity here. Now I'm gonna try to pick the end here and you can see that's causing a bit of an issue.
It's not allowing me to pick on the end. Now when it eventually does pick, you'll see that I'm getting some very strange results here. And that's because the control points for the end of this occupy the same space. So there's two control points very close, or almost on the same location. And that's a really bad situation for Extrapolate and surfacing in general. And you can see here we have a bad distribution of control points. And again if I try to do an extrapolate, I get some very strange results. So if you're doing an extrapolation and you see some very strange twisting of the control points or the edge of a surface, chances are that you're control points are too close together or they're off just by a fraction and it's causing the surface or curve to twist.
So let's go back to Extend. If we switch off Extrapolate and just go with Linear, what we can do here now is essentially we're creating a tangent line from the end of this curve and when I release, it merges it back in. Now if I select this curve, now you'll notice it's gone to four spans. Let's do the same for this one here, which is the NURBS. And if we extend, select, now we've gone up to eight spans. So the linear extrapolation is essentially a tangent extrapolation.
And on this one here, we've already seen that we're gonna have some problems anyway. So let's go back, Control Z those. Open up our Options again. And in this case we're gonna go to percentage. So with distance we can just either specify distance, let's say 20 millimeters and it will extend 20 millimeters. Or we can actually go with a percentage. So if we go with percentage, let's say 20%, we'll say go to that. We can extend each one of these entities by 20%. Now you'll notice a difference between the Bézier and the NURBS.
It takes 20% of the last segment. On the Bézier it takes 20% of the overall curve length. And finally if we don't have Merge on, what we can do is we can create these entities and have them as separate entities. So we'll go back to distance, and now if I pick this end, we've got linear. So what I've created here now is a linear line. And on this one here, a linear line. And you'll be using these quite a lot during your modeling process because it's a great way to create some entities as tangent entities to your existing geometry.
So let's go into Surfaces and let's switch off Curves. Let's shade this up to see what we have. So we're gonna start with this blue surface here. In fact, I'm just going to go to regular shading. I'm gonna go to the top view. And I've switched off my grid for the moment. So let's go back to Extend, let's do a reset. So with Extrapolate> Merge, we can select this edge and I can drag freely. I can also hold down my Control and snap to these control points on this curve, like so.
I can hold my Control and Alt and I can snap to a point on that curve like this. And you'll notice that on the point down here we have an intersection point and it allows me to snap to that intersection point. If I switch my grid back on, I can hold my Alt down and snap to a grid location, like this. Let's go ahead switch grids off. Okay, let's go back to the Extend command. And in this case, I'm gonna go with linear and I'm going to say Merge and we'll say go to that. And this time, we're gonna pick this edge and holding the Shift down, I can pick this edge as well and now it allows me to extend both ends like this.
And this would work with extrapolate as well. Control Z and this function also works with this line. So if I click this end here, Shift, click this end here and I can then increase both ends at the same time. Let's go to Wire Frame here. And we're gonna look at this entity here. Now this entity is a trimmed entity. So if I light up the control points, you can see that it was originally it was a plainer entity and it's been trimmed back to this curve. Okay, let's go to Extend and let's do a reset on that.
I'm going to do Extrapolate with Merge and let's say go. I wanna pick this edge here and you'll notice that Alias is not the allowing me to extend this edge here and if I pick this entity here, again it's not allowing me to pick this trimmed this edge either. So you can't pick a trimmed edge with Merge on. So let's take Merge off, and we'll say go to that. And now it's allowing me to extend this like that. Let's try this end here. And again it failed. So a raw trimmed edge, you cannot extend in any mode.
Let's move over to this entity here. And with this surface, let's go back to Extend, and we're gonna do a reset, Extrapolate with Merge. Now it follows the curvature of that surface, the natural curvature. When I release, it will merge everything into one. Let's do an extension again. We'll go Linear. Now with Linear what it's gonna do is basically create a tangent surface. It's gonna be a single span in this direction here like so. But we have Merge on, so it'll merge the two surfaces together.
If we take Merge off now we have a surface that we can use to tangent back into when we're creating some modifications to this particular surface. So in other words, I can do to Object > Edit > Align. Let's to a very simple align here. Tangent G1 and we're gonna pick this edge and we're gonna align it to that edge there. So let's move on, we're gonna take this surface and delete that. Okay let's go back to Extend. Now the other part that we can do with extend is that we can actually extend an internal isopalm on the surface.
So let's do a reset and if I use Merge on and let's say go. Click this isopalm here, you'll notice that it jumps to the very edge. Well let's think about what we're asking Alias to do. We're asking Alias to create an extension from an inside portion of a surface and merge it back into that surface. Well quite frankly, that's impossible to do. So let's open this back up. Switch off Merge and let's say go. And now I can pick this isopalm, I can drag this surface like so. And then at the same time I can just go ahead and move it and I've extracted a portion of that surface like that.
If I go back to this option, let's go to Linear. I can pick this edge and I can extend a linear tangent surface from that as well, like so. So again, a very useful option to create construction geometry. Finally, let's take a look at this scenario at the end here. And I'm going to go ahead and shade this up. So you can see that we have, one, two, three, four, five separate surfaces. And let's go ahead and do an extension on these.
So double click Extension, let's do a reset, then do Extrapolate with Merge. And this time I'm gonna select "Chain Select". Let's say go and pick this edge here. Now Alias doesn't allow me to do this or on this edge here at all. The reason is that we have trim edges and we've got Merge on. So with trimmed edges and Merge, we can't do this particular operation. So let's go ahead and take Merge off and let's say go. So now Alias picks with Chain Select all the way to the very end here and stops on this last surface.
And I'll take a look at why that's happening. Let's go ahead and do an extension. We can do the same for this side here. Well the reason why Alias stopped at this point here is because these two surfaces have a tangency error and if I rotate in this view, you can see that there is a crease between those two boundaries. Let's measure the tangency error, let's see what it is. We can go to Evaluate > Continuity > Surface Continuity and let's check for Tangency and we'll say go to that.
We'll pick that edge there and at this point here I'm going to go to Wire Frame and let's got to Control five, which is our information window. The information window gives us some information about the error itself. So it's telling us that we're checking for Continuity Evaluation, it's Per Span. We're checking for ten points. Gap Tolerance that we're using is 001. The normal angle of tolerance is 05. And we can switch on our edge labels and we can switch on Show Max Labels and we can also show a Comb.
Now at this point here we can also change the continuity from tangent to positional. And we can also change it to curvature. So that's how you extend entities in Alias.
- Manipulating views and entities
- Working with layers
- Creating curves
- Sweeping, extruding, revolving, offsetting, and blending surfaces
- Modifying geometry
- Moving, scaling, flipping, and rotating objects
- Trimming curves and surfaces
- Creating copies of objects
- Aligning, combining, and splitting objects
- Analyzing geometry
- Shading models