__Alias Overview__Alias is a free-form 3D surface modelling system. It is widely used in Automotive and Industrial Design as well as the film industry. If you want to create high quality 3d models which are organic in nature then you should use a free-form surface modeller such as Alias as opposed to a solid modeller. Alias is NURBS based. You don’t need to understand exactly what NURBS are in order to operate Alias. For now all you need to know is that NURBS is a type of mathematics which defines how curves and surfaces behave. It allows you to create very complex shapes relatively easily and control them using “control vertices” (cvs).

If you speak to experienced Alias modellers you’ll find that there is no one agreed method on how to use Alias. It is always amazing to see the number of ways in which people use the same software. In my mind the only thing that matters is the end result and how efficiently you get there. Exactly

*how*you get there is relatively unimportant. It can depend on what end result is required and the personality of the user. I also think that it is important to stay open-minded as far as other methods are concerned – not just in Alias! Even when you have been using Alias for several years I think you should carry on experimenting with your own and other people’s ideas. In the design industries the end result will, at one end, be a concept model used for visualisation of an idea, or maybe to create a machined clay model and at the other end a fully finished A-class production quality surface model, used for machining tools. At the concept end you will want to create a model as quickly as possible which accurately represents the designer’s wishes. It needs to have a high level of quality but without getting hung up on tolerances or engineering detail. At the A-class end speed is less important. You will want to create the very highest quality surfaces which respect the designer’s intentions while meeting all engineering requirements as well as being made to a tight tolerance which will allow the model to be passed to downstream CAD systems.

We will talk about different methods of producing concept surface models as well as A-class surface models. My own working methods have developed from experience, experimentation, sometimes impossible time-constraints (which always focuses the mind) and from discussion with other users – so I owe much thanks to them.

__Alias Interface__

__Function Layout__The exact function set depends on the WorkFlow selected. The layout of the Palette, Shelf and Control Panel menus is set under Windows. The menus at the top of the screen are all pull-downs. They are global type functions in general. By default on the left hand there is the Palette which is subdivided by Tabs into Selection etc. This is where all the curve and surface creation, modification and analysis functions are. On the right hand side is the Control Panel. At the lower edge is the user definable shelves where you can put your own set of icons.

__Customisation__**One of the great things about Alias is the fact that the menus can be customised. You can create your own shelves where you can place the icons that you use regularly. You can also customise the marking menus. These are menus accessible on-the-fly. By default these are set up for functions that are used most often such as object selection (Pick) and moving objects (Transform). Finally you can specify hot-keys e.g. you could set Ctrl+Alt+h to delete the history of a 3D model.**

Every user uses Alias differently, even if they are trying to achieve the same end result. And so every user will have their own interface set-up. It is possible to work with only hot-keys and marking menus and without a Palette or Shelf. This has the advantage of maximising screen real-estate. The disadvantage will be that you have to remember lots of hot-keys. Most users have a shelf of some sort. Some users choose to have only a single tab – the benefit being that all the user selected functions are always at the top level of the menu system. Others prefer to make use of the tabs to structure the functions as they wish. You might want the tabs to be object-centric e.g. curves, surfaces etc or function-centric e.g. trimming, moving, creating etc or maybe workflow-centric e.g. wheel creation, headlamp functions, surface checking etc. It’s entirely up to you. There is no right and wrong. It will take time to evolve your particular interface setup. You can save your setup onto a memory stick if you need to move onto another machine or into another company.

The other form of customisation is the Preferences-ConstructionOptions. This is where you specify the tolerances that you want to build the model to. When creating a quick concept model you should set this to be quite loose. If you’re creating a production quality surface model then the tolerances should be tight. The exact tolerances are usually set by the client if you’re doing paid work.

__Viewing models__You can set up viewing windows exactly as you want. By default there are 4 windows: 3 orthogonal (i.e. top, right, side) and 1 perspective. Of course you can leave these as they are. Personally I like to work in one perspective window. I switch to an orthogonal view within this window if I need to.

Alternatively you could set up hot keys to quickly switch between the four standard windows where each fills the available screen space.

Windows can be resized and moved as required using the corner controls.

**Alias Curves**In order to create 3D surfaces you will almost always start from one or more NURBS curves. However at the start we are actually going to discuss Bezier curves. Bezier mathematics is simpler than NURBS but, being a mathematical subset of NURBS, shares many of the NURBS features (if you want to get technical, then a Bezier curve is a single-span, non-rational NURBS curve). The shape of a Bezier curve is completely controlled by control vertices (cvs). For Bezier curves the number of control vertices is one more than the degree of the curve.

The lowest practical degree is 1. A degree 1 Bezier curve has (degree+1)=2 control vertices (cv's) and is a straight line drawn between the two cv's (as far as the maths is concerned it actually extends to infinity in both directions). In this case the end points and the control vertices are at the same locations.

Increasing the degree to 2 results in 3 control vertices (i.e. degree+1=2+1=3). As for a degree 1 curve the end points and end cvs coincide. The middle cv however, doesn’t, in general, lie on the curve. Pulling it around will change the shape of the curve. Notice that only convex or concave shapes can be created. You might think that, because there are 3 cv points and that an arc can be defined with 3 points then surely this curve must be an arc? This is not the case. In fact Bezier curves cannot 100% accurately define an arc shape. This is one of the quoted benefits of NURBS which we’ll talk about later.

Lets increase the degree to 3 (so 3+1=4 cvs). The middle cvs now give us more possibilities to change the curve shape. We can still create convex/ concave shapes but also an S-shape. Where the curve crosses over from convex to concave is known as the inflexion (inflection) point. Increasing the degree once more to 4 allows us to create a curve with up to 2 inflexions. You can hopefully see a pattern emerging here where the number of inflexions = degree-2.

In theory the degree can go up to any number. For practical curve design it is best to keep to lower degrees. There is no absolute rule on what the limit should be but I would suggest a maximum of degree 7. Lower degrees will give you better control over the quality of the curve. Too high a degree is more likely to contain unwanted ripples.

Lets look at a degree 7 curve. There are some features of this Bezier curve that we need to understand. Firstly we have seen how the end cvs are the same as the end points. If we look at the first 2 cvs from either end, these define the end tangent angle (slope) of the very end of the curve. The first 3cvs from each end define the end curvature value. Curvature = 1/radius and is a measure of how flat the curve is. The flatter the curve the smaller the curvature value (i.e. the higher the local radius). A curvature value of zero means that the radius is infinite – i.e. the curve is flat. If you position the end 3 cvs to form a straight line then at the very end of the curve the curve will be flat.

Cvs (control vertices ) can be positioned anywhere you like. You can place them at regular distances apart or you can bunch them. Some level of bunching is acceptable. For example if you wanted to create a hockey stick type shape where the curve accelerates from flat to tight then bunching the cvs towards the area of tightness (higher curvature) will help to achieve the shape while allowing the lowest possible degree to be used. Extreme bunching can be done but is usually not recommended. This will become clearer when we look at surfaces. Of course, ever rule is there to be broken and they can be big benefits in heavy cv bunching for rapid concept work. We’ll consider this later on.

How do we decide what degree to use? The answer is by experience and trial and error. Start with low degree and then increase if absolutely necessary. If you know that the shape needs 1 inflexion then, if we’re using a single Bezier curve, the minimum degree has to be 3 (remember that the number of inflexions = degree-2). Increase the degree above this as required to get the shape you need. If you want to know whether your degree is too high then test it by reducing the degree and checking the deviation to the original. E.g. if you were creating a model of an aircraft, say, and you had a degree 6 curve defining the upper contour of the fuselage if you dropped the degree to 5 and the curve only changes by 1mm then your degree was too high for the shape you wanted. The acceptable deviation is going to depend on the length of the curve. Only you can judge what is acceptable, but always remember that the lower the degree the better. Your model will be easier to change and control and your database (wire file) will be smaller. All good!

So that was Bezier curves in a nutshell. Why do we need anything more? In fact some systems such as ICEMSUrf create only Bezier curves and surfaces and do a perfectly good job. ICEMSurf still dominates (in terms of the number of seats) in the A-class surfacing field with Alias close behind. The “limitations” of Bezier are (1) a single curve cannot achieve complex shapes, (2) every cv affects the whole curve not just part of it and (3) arcs and circles cannot be accurately represented by Bezier curves (in order to create the most reliable CAD software, mathematicians ideally want to use the same mathematics behind all types of object as this reduces the software complexity and the potential bug count). In practice none of these issues is a real problem for the user. Complex shapes can be represented by Bezier curves simply by joining several together. If a shape is described using a number of joined Bezier curves then the cvs of each will only affect that curve, unless you change the end tangency (the last 2 cvs) or the end curvature (last 3 cvs). Arcs and circles can be approximated very closely by splitting them up and choosing a suitable degree. Most of the time, if a circle is split into 4 90 degree arcs then degree 7 curves will be a good approximation. In Alias you can create arcs anyway, but Bezier approximation becomes important when you want to create circular surfaces.

NURBS was invented to try and address the perceived limitations of Bezier geometry. Let’s look at the characteristics of NURBS curves. They use cvs in a similar way to Bezier curves. They also have degree. The first difference is that they also have internal points known as edit points in Alias or knots by mathematicians. The segments in between the edit points are called spans (in Alias) or segments (by mathematicians). What spans give you is the ability to create very complex shapes using a single curve. Whether this is always a good idea will depend on the situation.

In Alias the addition of spans affects the relationship of the number of cvs to the degree:

(number of cvs)=degree+(number of spans)

E.g. spans = 1 (i.e. Bezier), degree=3 gives (3+1) = 4 cvs as before.

Add an edit point to create 2 spans and you get 5 cvs.

Moving a cv will only affect a portion of a multi-span NURBS curve. The extent of the change is highlighted in Alias.

As for Bezier curves the end cv defines the end position of the curve, the end 2 cvs define the end tangency and the end 3 cvs determine the end curvature.

For a degree 1 multi-span NURBS curve the edit points coincide with the cvs. They behave as hinges between the spans i.e. they are joined together at these points (this is known as position or G0 continuity) but are free to rotate. Interesting though this is, there is not much practical need for degree 1 multi-span NURBS curves in Alias.

If we up the degree to 2 then the internal cvs no longer sit at the edit points. Now the curve at the edit points is not only G0 continuous (hinged) but is also tangent continuous (G1 continuity). i.e. the slope of the curve at the edit points is maintained across the edit point but higher levels of continuity are in general not present.

For degree 3 the minimum continuity at the edit points goes up to G2 (curvature) continuity, while degree 4 has G3 continuity (rate of change or slope of curvature).

The other characteristic that NURBS curves have is the ability to be able to exactly match the shape of a circle or arc (in fact, any conic – i.e. any flat section cut through a cone). It can do this with very few cvs. This, potentially, means a more accurate model and a smaller database. The ability to exactly represent conics is the “rational” part of NURBS (non-uniform rational B-Splines). You can turn rational geometry off or on in Preferences-ConstructionOptions. In practice, most users avoid using rational geometry as it is not supported in all downstream systems. It’s always safer to approximate conics using single-span NURBS (i.e. Bezier curves) as described above.

There are 2 extremes in terms of the way in which curves can be used – and lots of flavours in between:

**“Concept curves”**– use degree 3 multispan curves to create complex shapes. Sticking with degree 3 when you create these makes it easier to anticipate the shapes you will get. Imagine you wanted a shape consisting of 3 “straights” connected with two "elbows". The minimum number of spans you would need would be 3 for the “straights” and 2 for the elbows i.e. 5 spans. Remember that the (number of cvs) = degree+spans = 3+5 = 8. You should position the 1st 3 cvs near where the first “straight” is required, the 4th and 5th cvs along the 2nd “straight” and the last 3 cvs along the 3rd span. With degree 3 curves this is always a good starting point – end spans “defined” by 3 cvs, intermediate spans by 2cvs. If this doesn’t give you enough control then either add edit points in the middle of spans or increase the degree. You can use “concept curves” to develop large, complex but easy to control surfaces.

**“A-class curves”**– break up complex shapes into blocks and fillets/blends. This makes for a very controlled model and is the digital equivalent of how clay modellers work. Some people will tell you that only single span (Bezier) curves and surfaces should be used. This is the approach that ICEMSurf takes. Even though I would do this in ICEMSurf, I personally think that we should make use of the fact that we can create multispan curves and surfaces in Alias, the benefit being that there will be fewer curves and surfaces and therefore less alignment or continuity issues to resolve. Of course it is important to keep the number of spans to an absolute minimum. We don't want 100s of spans when 2 will do! If you do need to subsequently have single spans then you can detach the multispan curves at the edit points later.

__Alias Surfaces__NURBS surfaces are an extension of the same principles behind NURBS curves. But now instead of one direction we have two. These are known as the u and v parametric directions. The directions are shown on the first cv in each direction.

As with curves we can define a degree and number of spans. These can be different in each direction. If you think of the edges of the surface as being the same as a curve then the same rule applies as above:

i.e. (number of cvs in u degree) = (u degree)+(number of spans in u)

(number of cvs in v degree) = (v degree)+(number of spans in v)

With surfaces of revolution we can use rational NURBS if we want although as we discussed earlier this is not necessarily desirable. It’s always safer to avoid rational geometry because of issues with passing the models to other CAD software. It is interesting to see what rational geometry looks like. Create a curve on the XZ plane as the contour for your surface. The pivot point will by default be at 0,0,0. This will be the position of the central axis of rotation. Use RevolveSurface to create a 360deg surface. Notice the pattern of cvs. Check the weights of the cvs (pick cvs, then use the InformationWindow) and you will find that not all the weights are 1 – which is a characteristic of rational geometry.

Instead we need to very closely approximate conics (arcs, circles, ellipses etc) with non-rational geometry. So, firstly we need to switch off rational geometry creation using Preferences-ConstructionOptions. Let’s now create a non-rational 360 deg surface of revolution. Note the positioning of the cvs. You can see straight away that this is no where near being accurately circular. If you can’t, then look at the curvature map with min/max radii on. Now lets create another Revolve but only 90 degree. Set degree 3, 30 sections. Check the curvature and you should find the values of min and max radii are virtually the same – i.e. very accurate. For complete peace of mind you can also check the deviation between this result and an arc of the same radius. But we do have a high number of spans. Simply dropping the number of spans to 1 gives a very poor result. Instead, firstly increase the degree to 8. There will be no change to the shape. Now drop the the number of spans to 1 and you’ll get a good result. Oddly enough it is sometimes BETTER than the original 30 span curve! I don’t know why this works but it does, certainly for automotive wheel type sized objects. If you’re modelling a space shuttle full size and you need to be accurate to 0.01mm then you will doubtless need more spans!

__Alias Concept Modelling__Concept modelling is the next stage from the concept sketch or render, where we want to move the design from 2D into 3D. It may be that the surface model that you create is then used for 3D visualisation of the design or it might be machined as a quarter scale or full-size clay model. Sometimes the concept model may be sent for 3D printing (SLA etc). Usually concept models have to be created as quickly as possible – a full exterior of a car for example in 3 days. It needs to be to the shape that the designer wants. Just because there are 2D sketches it doesn’t mean that the shapes will work first time once it is in 3D. There will lots of changes to the surfaces required. If the model is to be machined or 3D printed then it has to be to a reasonably tight tolerance. Even with 3D visualisations or animations the model should be to the best quality possible in the time.

So we need techniques which will allow changes - both large and subtle - to be made quickly and in such a way that the designer can always see and understand the model clearly. Designers do not want to see a screen full of intersecting surfaces. They want to see their product evolving on the screen. Part of the trick to this is to have two Alias windows open all the time. One will have your last clean model in, the other will be your very latest work-in-progress. Just switch between models when the designer wants to see the model. Two separate Alias windows are better than having two Stages just in case Alias crashes at the wrong moment.

As I said before, there is no one way of doing anything in Alias. I rarely do things in the same way twice if I feel that it’s worth experimenting with another technique to improve quality or efficiency.