AliasStudio Concepts > Construction history
Object properties
Explains the properties common to NURBS objects.Degree
Degree is a mathematical property of a curve or of a surface dimension that controls how many CVs are available for modeling.
The number of CVs for each curve span is controlled by the degree of the curve. The default curve type in Studio is degree 3, which has four CVs for the first curve span.
You can choose to have fewer CVs per span, or, if you have an advanced version of Studio, you can create curves with more than four CVs per span.
- Degree 1 creates curves or surfaces with straight lines.
- Degree 2 curves or surfaces do not automatically have smooth transitions between spans or patches.
- Degree 3 is the default degree for new curves and surfaces.
- Degree 5 and degree 7 curves are generally used in automotive design. They are slower, but give you smoother curves, better internal continuity, and more control.
The degree of your curves can affect data transfer to CAD packages. Some other packages cannot accept curves with degree higher than 3.
Surfaces can have different degrees across their width and length. So, for example, a surface could be degree 3 along its width, and degree 5 along its length.
Parameters and parameterization
Parameters are the unique numeric values (like a coordinate) of points on a curve or surface.What are parameters?
You can think of a curve as made up of an infinite number of points. Each of these that make up a curve has a number, called its parameter. Parameters let you refer to specific points along the length of a curve. The higher the parameter, the further is the point along the curve.
Just as points in space have three dimensions, called X, Y, and Z, the parameters of a point are measured along the one internal dimension (length) of the curve. We call this dimension U.
Since surfaces have two internal dimensions (length and width), we need another parameter (in addition to U) to specify a point on a surface. We call this parameter V.
Just as every point along the length of a curve has a U parameter, every point across a surface has a pair of U and V parameters.
What is parameterization?
The method Studio uses to number the points along a curve is called the curve's parameterization. Studio has two parameterization methods: uniform and chord-length.
Each method has advantages and disadvantages depending on how the curve will be used. You can choose which parameterization method to use when you create a new curve, and you can rebuild existing curves to use a specific parameterization.
> Uniform
Uniform parameterization assigns integral parameter values to the edit points, and evenly distributes parameters along the spans between edit points. So the first edit point is always parameter 0.0, the second edit point is always 1.0, the third is always 2.0, and so on.
A bonus feature of uniform parameterization is that the parameter value of the last edit point is the also the number of spans in the curve. However, unlike chord-length parameterization, the parameters of a uniform curve have nothing to do with the actual length of the curve.
> Chord-length
Chord-length parameterization assigns parameter 0.0 to the start of the curve, then increases the parameter value proportionally to the chord length, or the shortest linear distance, between the surrounding edit points.
Unlike uniform parameterization, the parameters of a chord-length curve are irregularly spaced between the edit points, and the edit points do not have integral parameters.
> Comparison
Each parameterization method has advantages and disadvantages, depending on how you will use the curve or surface.
Just as with degree, surfaces can have different parameterization methods for their U and V dimensions. For example, the U isoparms of a surface can be degree 3 with uniform parameterization, while the V isoparms are degree 1 with chord-length parameterization.
Normals
Normals are imaginary lines perpendicular to each point on a curve or surface.
The direction of U and V isoparms on a surface determines the direction of the surface's normals.
Normals are a mathematical side-effect of NURBS.They are often used as a way of specifying which side of a surface points "inside" or "outside" (for example, when creating shells).
Normals are also an indirect indicator of the shape of a curve or surface. Since they are always perpendicular to the curve or surface, the way normal lines point toward or away from each other can reveal subtle curvature.
Starting with AliasStudio 13.5, we treat surface sidedness differently from in the past. Previously, sidedness has been a geometric concept based on the so-called "right hand rule", and has been dictated by the U and V directions of surfaces and the triangle vertex ordering of meshes. This has had unintended consequences for users, in that the "front" and "back" of surfaces were subject to the way U and V directions happened to be, and operations like negative scaling and mirroring would tend to turn surfaces inside out.
Starting in 13.5, surface orientation is controlled not only by handedness and transformation, but also by the Opposite flag that has, until now, only been part of the rendering workflow, and is shown in the Render Stats window. So the Orient Normals tool leaves handedness and transformations alone, and sets the Opposite flag appropriately. The Opposite flag is also used and set by other operations, such as Zero Transform.
This allows orientation to be controlled independently of handness and transformation. Flipping orientation no longer involves transposing UVs, and so it is possible to preserve history. Also, surfaces can now be reoriented without flipping texture maps, something that would happen if you transposed UVs. Having better control of orientation means that orientation sensitive operations-a long list including ambient occlusion, surface offset, mass properties, and STL output-are now more reliable.
You can see for yourself how this works by creating some simple geometry (a plane primitive in what follows) and turning on the Multi Color diagnostic shade (the blue icon), along with Show Reversed Normals. Open Windows > Information > Information window
and Render > Editors > Render stats
. The top of the plane should be blue, the underside yellow. If you flip the Opposite flag in the Render Stats Window, the color should change. If you add a negative sign to the X component of Scale in the Information Window Transform Info, the color will not change. Leave the scale negative, then choose Transform > Zero transforms
. The color still does not change. But note that the Opposite flag has changed.
If you perform this experiment in AliasStudio 13, you will get quite different behavior. In AliasStudio 13 the Opposite flag is ignored, except for rendering. And a negative scale affects orientation.
Also examine the Reverse Direction option box, which has a new default option, Reverse Normal Direction.
If you use this, the UVs are unchanged, but the Opposite flag toggles on or off. In fact, there's no difference between choosing Reverse Normal direction and toggling the Opposite flag.
What is the result? AliasStudio 13.0 and 13.5 are not compatible with respect to orientation. The exportBakedOrientation plug-in can be used to export oriented geometry so that it has the same orientation in 13.0 and earlier. However, the exported file will have zeroed transforms, and no history.
Changes have been made to some data translators. The main change is that orientation will be baked into geometry, by swapping UVs and reversing mesh winding (the baking occurs in the exported file, not the Studio geometry).
Pivot points
The pivot point is the point around which an object rotates and scales, and which represents the point location of the object when it moves.When you pick objects in the view windows, you can see a small blue-green dot associated with every object. This is the pivot point of the object.
Pivot points allow you to control how objects rotate and scale, and also represent the exact locations of objects in space.
All transformations to an object are relative to the pivot point:
There are actually two separate pivot points: one for rotation and scale, and one for movement. They can be separated by using Transform > Local > Set Pivot
. Placing the two pivots at different locations can be useful for creating animations, where you may want the movement of an object to follow a path while it rotates or scales about another point.
Construction history
Construction history is the saved information about how an object was created. When you edit the construction history the object will automatically update.For almost every tool, AliasStudio gives you the option of saving the history of how an object was constructed. This means you can edit the curves, surfaces, manipulators, tool options, and so on that were used to create an object, and the object will automatically update.
For example, when you use the Revolve tool to create an object with construction history, you can:
...and the surface(s) will update automatically.
To create construction history when working with tools, turn on the Create History option in the option window. This option is on by default in all tools.
Objects that have construction history are drawn in green in the default color scheme.
If a surface or curve has been built with construction history, it cannot be moved, scaled, or rotated even if its constructor objects are transformed along with it.
|
Autodesk www.autodesk.com aliasstudio.docs@mail.autodesk.com |
