Join Veejay Gahir for an in-depth discussion in this video Rebuilding curves, part of Alias Essential Training.
- Let's take a look at how we can rebuild data. Now, during the concept phase of modeling, we are creating very fast, very quick entities just to get our ideas down in 3D space. There comes a time when we have to smooth this data out or simplify it to create better quality services. And if we take an example here like this curve here, now this could be imported from another CAD system, it could be imported, and during the translation period of the import, it can produce some errors. So in this case here, we're going to use this as an example to try to smooth this out and get a better result.
So the command we're going to use is Rebuild Curve. And if we go to Reset, we're going to use the very first option which is Reduce spans. So let's go ahead and let's select this curve. And I want to press the spacebar to say go, but before I do that, let's click on these options here, spacebar to say go. So what we're doing here is, with Reduce spans, we're asking Alias to try smooth another curve all the way through here within a tolerance of one millimeter. Now, just bear in mind that each one of these grids is 100 millimeters so this curve is approximately two meters in length.
So one millimeter over two meters actually is quite a tight tolerance. So Alias does its best and says, "Well that's the best I can do," and if we look at the result, it's 9 spans by 4 degree. Now, if we increase these tolerances to let's say 25 or 20, now you'll see that we've dropped down to 8 Spans and we get an indicator of where the maximum deviation is, what the mean deviation is and what the minimum is. So again, this one has a very marked effect depending on what type of tolerance you set here.
But at the same time, when I move these tolerances around, Alias is trying to recalculate. But with Smoothing on, I do have the option, with my left mouse button, just to move left and right to just fine tune this curve. And what Smoothing does ideally, if we could smooth to the nth degree, we would end up with a flat line, in other words, we are going to take those control points and just relax them to the point where they are almost flat. But there's a conflict here because Alias is still trying to get to this curve and we're trying to push it flat so again, with Smoothing, just use it very carefully, you don't want to go too extreme on the smoothing factor.
So you can just fine tune with smoothing like this. So that's one of the options we can use. Let's just Undo All. Let's go to the next option here which is Delete multi-knots. Now with Delete multi-knots, as we've mentioned before, a multi-knot is a curve or a surface that has two edit points that occupy the same space. Now, in this case here, we don't have any multi-knots, so we're going to bypass this one and we're going to go to Uniform knots. Now again, I like this one here because I have the control of the number of CVs and I have control over the degree as well.
So, I always like to start off with a very simple curve, initially, and we're going to click on this, spacebar to accept, so the simplest curve we can have just goes from point to point. We have this graphical representation of where the deviation is or how far this curve is away from our target. So let's increase the Degree to 2, and we'll go to 3, and you can see now that Alias is trying to capture the overall shape. Now, this is really the essence of surfacing.
This is a balance between a smooth shape of trying to get back to the reference data as accurate as possible. Now, if the reference data has some flaws in it, like we clearly do here right at the top here, we have to make a decision, we're the designer to say, "Well okay, we can bypass this "or allow a greater deviation in this region "for the sake of getting a smoother surface." So when we get to 3 or maybe even 4, you can see that 4 starts to create a very strange kind of distribution so I'm going to go back to 3, and I can up the Spans to 2.
Let's just go to 2 here. So again, this is a case of just playing between the spans and the degree to get the best result. But in this case, if I go back to 1 and leave it at Degree 3, you can see that this is not a bad result, and over the length of two meters, it's pretty acceptable for this point. So let's accept this, we'll come out of the command, select Nothing, and I'm going to press L to delete the locators. Now you'll notice if I press Q for Query Edit and I pick this curve, I can't re-invoke that command, it's actually dead now. But what we can do here is do a quick check.
Let's select both of those entities and we'll switch off the CVs and we're going to go to Locators, Curve Curvature and let's open this up, that's just to reset we're going to use Curvature, we'll go with the default values and let's check the curvature on the original curve. Now, left mouse button changes amplitude. Middle mouse button changes the density. What we're looking for here is just a nice smooth line that goes right the way through this curve and if it has to fit to this side, it's just a smooth transition.
And you can tell just by looking at this, that we have an issue right here. So press L to delete that. Let's go back to Locator, Curve Curvature and let's pick our new curve. So now, if I use the middle mouse button to increase the amplitude, you can see that that's a really nice smooth curve. Even though we're quite far away here, we can still go back into this now, at this point, we can put on our CV Hulls and we could increase to 4, like this and now we can go into a manual mode of manipulation and then we can just fine tune this curve like that, try to see how close we could get.
And that's kind of one of the methodologies that most people will use, start off very simple or get something down there that's quite good and then fine tune it accordingly. Go ahead and delete this. Let's go back to our Curve Curvature, Rebuild Curve. So, we'll do a reset and the next option we're going to look at here is Match knots. Now, with Match knots, let's click these on again, let's select this curve and I'm going to press Go because we have the option down here. I've selected the curve, spacebar.
Now if we look at the prompt it's saying, "Well okay, you have to match the knots, "you're asking to match the knots, "where's your reference curve "that you want to match the knots to?" Now, if we look at this curve at the top here, you can see that we have three edit points bunched up in the left hand side here. Now this is purely for an example. And I'm going to simply just pick this curve here and the resulting curve here, if I say move now, it's just moving out the way like that, and let's just press L to delete those locators. You'll notice if I take these control points off that we have three edit points that are bunched up in this top left corner, trying to mimic the edit points on this reference curve here.
So Match knots will allow you to try to match the DNA of a reference curve. Let's go back to that, we'll pick that, spacebar, Match knots and again, we can use our left mouse button to interactively smooth that particular piece of data. Let's Undo All that and let's go to the last one which is Curvature. Now with Curvature, what this tries to do, if I select this piece of data, it tries to accomodate with more control points in areas of higher curvature.
Now again, we've set a very high tolerance here so let's first of all, open up the Tolerance to let's say 25 and also, I'm going to change the maximum span types to Absolute. Relative is a multiple factor of the current number of spans or control points in your curve. It's a very confusing way of looking at it so if we had five spans in this curve, we're telling Alias you can go by five by five, 25 maximum spans. Quite frankly, I don't want to be doing this calculation so I'm going to go straight to Absolute and I know now that I'm only going to be allowed a maximum of 3 spans, it's a lot easier way to work.
So with 3 spans, tolerance of 25, and curvature, you can see that we have a pretty reasonable result but if I do tighten up this tolerance here, I'm going to have to increase the spans maybe to 6. Let's open this back up again, there we go. It's a balance now between the tolerances and the number of spans, and if I say Next and let's take this curve and let's move it up. And let's press L to delete those locators. You'll see that Alias tries to cluster more information in the areas of higher curvature, for example, like here, and also down at the bottom here.
So again, this is not a great result but nevertheless, it is an option that you can use. So to recap here, my preference would be to go to Uniform knots because I have full control over the CVs and the Degree. Let's take the grids off here and let's take a look at this example. So what we have here is we have a surface, a very simple surface. And we've projected a curve onto that surface so we've generated a curve on surface.
Now what happens if we try to rebuild this curve on surface to extract a curve? Well, let's go to the Rebuild command and we'll use my favorite one which is the Uniform knots. I'm going to go back to single Span, Degree 3 and let's just pick this piece of data and let's say go to that. Look's pretty good, but if I go to move now and try to pull this away from the surface, you'll notice that it just slides along the surface. Now straight away, that's a clue as to what it's actually generating. What is generated here is another curve on surface.
So if I say Nothing, let's delete those locators and I go to pick Object. When I say pick Object and it's a curve on surface, it picks the actual curve. So in this case here, it'll only create another curve on surface, which is this one here. If I want to extract a curve on surface, what I have to do is go to the Duplicate Curve command which is this one here. Let's do a Reset on that and we'll pick the entity, spacebar, and if I say move now, you'll notice that that is a physical curve, not a curve on surface.
So let's take this information, we'll press I to make that invisible. And let's go over to this piece of data here. Now this again is a mesh, you can see they're all tries and into that, with a mesh, it's quite hard to see like this so if I do shade this up, again, it's not easy to see because the mesh kind of interferes with the texture. What you can do here is you can go into Transparency here, but that's only going to make the shading factor transparent. But in Control Panels, we have a option to change transparency for multiple entities.
So what we can do here is we can change the transparency of a mesh right down, so I like to see just a faint outline of a mesh like this. It just makes it a lot more clearer when we're modeling. So again, let's go back to Rebuild Curve, we're going to use Uniform and I'm going to pick this mesh curve. Now this is not a curve on surface because this is not a surface. This is what we call a mesh curve. So if I pick this mesh curve, press spacebar and let's say move now, you'll notice with a mesh curve I actually do get a physical curve.
So again, it depends on what entity you're trying to extract from, to what you would use. Whether you would use duplicate or whether you'd use rebuild.
- 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