This Client Server article is republished in its entirety from 2003 for reference purposes.
By Dan Abney, Technical Support (Visualization/3D Design Specialist), Bentley Corporate Office 14 November 2003
MicroStation has a very accomplished 3D solid modeling package. However there are times that using the solid tools may not be the best avenue to complete a modeling task. Surface modeling may be the way to go for some models. Some examples include: irregular shaped furniture that an interior designer needs to model for a particular room layout, ergonomically designed computer hardware, or automotive body design.
Figure 1.1: Designing furniture using surfaces
Figure 1.2: Designing landscapes using surfaces
The model in Figure 1.1 was designed by placing B-spline curves to define the edges of the chair, then using Create Surface by Network to create the surfaces of the chair. It was then mirrored, then stitched together to form a solid.
Terrain is another example of good use of surface modeling.
The terrain in Figure 1.2 was designed for an animation illustrating a vehicle driving down a road. The surface of the terrain was created by placing B-spline curves to form "sections" of the terrain. The Place B-spline Curve tool is located in the Create Curves toolbox. Go to Tools>B-splines>Create Curves (See Figure 2.1).
Figure 2.1: Place B-spline curve
Figure 2.2: Create surfaces by section or network
The B-spline curves were then formed into a surface by using the Create Surface By Section or Network tool, which is located in the Create Surfaces Toolbox. Go to Tools>Surface Modeling>Create Surfaces (See Figure 2.2).
There are a few important things to keep in mind and be aware of when creating profiles for surfaces. The two main things are "Element Direction" and "Element Start Point." These two primarily affect closed profiles more than open profiles. This is because the element start point can be anywhere along the closed element and the direction can be pointed in either direction along the element. However, open profiles can only have the start point at one end or the other and the direction has to point inward along the element. The tool to change direction and/or start point is the Change Element Direction tool and is located on the Modify Curves toolbox. Go to Tools>B-splines>Modify Curves (see Figure 3.1).
Figure 3.1: Change element direction If the directions are not the same on each profile used when creating the surface, unwanted "twisting" will occur. As you can see in Figure 3.2, one of the direction arrows is starting from the opposite side of the profile, which will cause a "twisted" surface, as shown in Figure 3.3.
Figure 3.2: Difference in element direction on open profiles
Figure 3.3: Surface generated with undesired "twisted" appearance If Element Start points of closed profiles are not the same, or relatively close, unwanted "twisting" in the resulting surface will occur (See Figure 3.4 and Figure 3.5).
Another thing to be aware of is the amount of points the profiles have and where those points are located along the profile. When a surface is generated from profiles, the control points of the profiles are matched up, starting from element start point. The surface is blended with regard to where the profiles match control points located along the participating profiles (See Figure 4).
Figure 3.4: Difference in start points of closed profiles
Figure 3.5: Surface generated from profiles with different start points
If one profile has more or less points than its neighboring profile, it can cause the generated surface to be deformed. In most cases this appears only as a slight deformation, but it is worth noting. This slight deformation is caused by the difference in points between the surfaces. This mostly affects surfaces that are created from closed profiles, but it can also affect lengthy profiles, for example, profiles that were taken from a survey contour plan.
Figure 4.1: Distorted surfaces due to difference in control point spacing
With the addition of the feature-based modeling tools in MicroStation comes a new surface tool. This tool, "Deform Surface/Face," is used to interactively deform (stretch) a face of a solid, a closed shape, or a surface. This tool can be found by going to Tools>Feature Modeling>Modify Face Feature (See Figure 5.1).
This tool can be used for a variety of deformation tasks. Here, the face of a solid slab feature is deformed to start the beginning design for a computer-input device (See Figure 5.2).
Figure 5.1: Deform surface/face This mouse was designed by starting with a slab feature, then using the Deform Surface/Face tool to deform the top face. In this case, I used two crossing B-spline profiles, then set the Type to "Space Curves." This will cause the selected face to form to the B-spline curve profiles. After that, I applied additional features to get the end result (See Figure 5.3).
Figure 5.2: Deform surface/face example usage
Figure 5.3: Fictional mouse computer input device
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