Tips and Techniques

Tips and Techniques

What kind of surface do I need?Nathan Haller, MSC.Software CorporationWith the wide variety of surfacing tools available in CATIA V5, how do you know which tool to use and when?

Surface geometry can be generated in a variety of different ways. Some methods are unique to the program generating the surfaces while other methods have been used and proven over the years. When creating a surface, you should consider not only the capability of your software tools but also how and where the surface is going to be used. CATIA V5R14 has seven primary shape design products. Each product is different in its functionally and intended use.

Generative Shape Design 2 (GSD) is one of the more widely used products. It helps to design advanced wireframe1 and surface1 geometry with fully associative specifications. GSD is a feature-based design environment for the capture and reuse of analytical1 design methodologies and specifications. See example in Figure 1.GSD is commonly used as an alternative or complement to solid modeling tools, resulting in a hybrid1 modeling scheme. Laws and knowledgeware built into GSD provide productivity enhancing functionality to create complex surfaces even faster. GSD has tools that allow the user to create free-form1 spline1 curves and conic curves. It also has the ability to control point, tangency, and curvature continuity1 between curves and surfaces.

Generative Shape Optimizer (GSO) is added on top of GSD and gives the designer access to smart tools that allow for faster shape deformation. GSO also adds productivity gains suitable for automotive design with the addition of variable offsets on a complex shell.

Figure 1: Example of GSD in use.Freestyle Shaper (FSS) is used to generate aesthetic1 non-associative (datum) 3D curves and surfaces from scratch, and to dynamically deform and analyze all produced elements. See example in Figure 2. FSS uses control points1 to control the continuity1 between elements and to adjust curve and surface curvature. The quality of the curves and surfaces can be checked using industry-oriented diagnostic tools.

In addition to FSS, Freestyle Optimizer (FSO) adds global deformation of multiple surfaces and the ability to fit geometry to physical mock-up scan data.

Figure 2: Example of FSS in use.Automotive Class A (ACA) is an industry specific product used to create Class A surfaces that meet or exceed strict tolerance and smoothness requirements. Similar in methodology to Freestyle Shaper, ACA surfaces can obtain G3 continuity matching, or the rate of change of curvature continuity. ACA has some associativity when creating and modifying elements, but the end results are usually non-associative. See example in Figure 3.

In addition to ACA, Automotive Class A Optimizer (ACO) can be added for global deformation capabilities on multiple surfaces.

Figure 3: Example of ACA in use.Imagine and Shape 2 (IMA) is used to capture and make tangible the conceptual phase of aesthetic1 product development. See example in Figure 4. Complemented by Freestyle Sketch Tracer (FSK), IMA is based on mathematical subdivision1 technology: an algorithmic technique to generate smooth surfaces as a sequence of successively refined polyhedral meshes. This will allow you to form your concept from a sphere, a flat surface, or a curve. IMA is quite possibly the future of CAD modeling.

Figure 4: Example of IMA in use.Freestyle Sketch Tracer (FSK) is used to convert 2D images into a 3D scene in order to create 3D geometry from your hand drawings or other sources such as pictures. Once added, the images are able to move normal to the positioning plane. The designer can then use products like Generative Shape Design, Freestyle Shaper, or Automotive Class A to generate the geometry. FSK works with JPG, BMP, TIFF and RGB files. See example in Figure 5.

Figure 5: Example of FSK in use.Digitized Shape Editor (DSE) is not used to create geometry, rather it is used to import and process digitized point clouds1. The point cloud can be tessellated1 for quick visualization of the imported polygon mesh1. DSE is a vital component in the reverse engineering process. Once the cloud is imported and adjusted to suit your need, Generative Shape Design, Freestyle Shaper, Quick Surface Reconstruction, or Automotive Class A can be used to generate the geometry. See example in Figure 6.

Figure 6: Example of DSE in use.Quick Surface Reconstruction (QSR) allows you to create prototype quality surfaces from digitized data that has been cleaned up and tessellated using Digitized Shape Editor (DSE). Essentially a shrink wrapper is produced based on user defined tolerances1. In addition to organic shapes, QSR will detect and create mechanical shapes (plane, cylinder, sphere, and cone). The resulting surfaces are datum (non-associative). See example in Figure 7.

Figure 7: Example of QSR in use.In summary, for surfaces in a hybrid environment that are fully associative, Generative Shape Design is the product you should choose. If you need more flexibility in aesthetics, then Freestyle Shaper is your choice. For flexibility in aesthetics with high tolerances for Class A surfaces, then the Automotive Class A product should be your choice. To quickly construct surfaces from point cloud data, youll want to invest in Quick Surface Reconstruction.

"Surfacing" LexiconAesthetic:Pleasing in appearance. Used in context to describe styled (Organic) products.

Analytical:Separating something into component parts or constituent elements. Used in context to describe products that are primarily functional (Mechanical).

B-rep:Boundary Representation. The description of a 3D objects shape, resulting from the assemblage of topological entities. See Topology.

Bezier:A type of spline curve named after its inventor, Pierre Bzier. Popular within the CAD/CAM and computer graphics industries because of their ease of manipulation. See Spline.

Points:Sometimes referred to as Control Vertices (CVs). Suspended above or below the span of a curve or surface, control points establish a support lattice (Control Polygon or Hull) that defines and influences the curve or surface shape.

Cell:A single element (surface or curve) in CATIA V5 that has no visible discontinuities. For a curve, this implies no internal vertices; for a surface, no internal edges. See Continuity.

Conic:Mathematically, a curve that results from the intersection of a plane and a cone. In practice, conics are used to define smooth shapes that require analytical precision. Conics can be fully defined from only five pieces of information: beginning/end position, beginning/end tangency, and a parameter ranging between 0 and 1. Entities belonging to the conic family are: circle, ellipse, parabola, and hyperbola.

Continuity:The connection and/or smoothness between curve or surface segments. In mathematical form, continuity is the derivative of a curve(s) defining function, expressed as Cx. Beginning with C0 as positional connectivity, C1 implies tangent, C2 curvature, C3 rate of curvature, C4 rate of rate of curvature.

Foreign:In the context of CATIA V5, refers to geometry, e.g. IGES data, created in another CAD system. Foreign curves and surfaces have no associative (parametric) history.

Free form:A term used to describe geometry with little or no prismatic characteristics (straight lines, planar faces, etc.).

Hybrid:Refers to a mixed environment in which surface and solid modeling technique can be utilized to create geometry. In this context, CATIA V5 is a true hybrid modeling program.

Isoparm:Short for Isoparametric Curve. A line that exists on a surface, having a constant position along the latitude or longitude direction (U and V) of the surface. See U,V,W.

Loft:An ambiguous industry term used to describe (1) a surface consisting of multiple cross sections, (2) the act of creating free form shapes (lofting) or (3) the individual or department responsible for such activity (loftsman). The term is derived from the pre-computing age in which curvilinear shapes were manually drafted, often 1:1 scale. In the case of shipbuilding, these curves were too large for a drafting table; therefore this activity was often performed in the loft area of a large building. See Spline.

Manifold:A mathematical condition in which a sum of individual pieces of geometry can be defined in 3D space without ambiguity. It is typically used as a check for valid geometry in CATIA V5. A simple example would be a cross-section in the shape of an X (two intersecting lines). Such a shape cannot be offset or thickened as a whole because it is non-manifold.

Normal:In context, refers to a vector that is perpendicular to a curve or surface; much like the Z axis of an XY plane. Sometimes referred to as the Porcupine.

NURBS:Non Uniform Rational B-Spline. A robust mathematical form used to describe curves and surfaces. Almost any shape can be described with NURBS, including exact circles. Bezier curves are a special type of NURBS curves. See Bezier.

NUPBS:A term unique to CATIA, stands for Non Uniform Polynomial B-Spline. Also known as NUPS, the mathematics behind this type of curve is slightly different from NURBS in that it is non-rational. This means that the control points of a NUPBS curve all have the same amount of influence (weight) on the overall curve. In other words, modifying a single control point changes the entire curve vs. just having a local effect in the case of NURBS. Most curves in CATIA V5 are of this type. See NURBS, Control Points.

Patch:In context, a surface method of filling an area that has defined boundaries.

Point Cloud:A collection of XYZ points, usually collected with a scanning laser or light beam, that describes a 3D object. Often used in the process of reverse engineering a physical model. A supporting industry file format is ASC (ascii text).

Polygon Mesh:A set of polygonal faces (triangles) which represent the surface(s) of a 3D model. In industry practice, this type of data is commonly used for Stereolithography, or rapid prototyping. A supporting industry file format is STL.

Raster:A graphical image consisting of pixels or dots. Common industry file formats are BMP, JPG, and TIFF.

Segmentation:Splitting up a heterogeneous data set into subsets based on homogeneous characteristics. For curves and surfaces, this is typically based on curvature continuity. See Continuity.

Spine:The characteristic backbone of certain surface types, e.g. multi-section loft. A spine is used to define the position of a series of planes along its length, upon which curves are calculated that describe the surface.

Spline:Formerly a thin metal or wooden strip used by a draftsman that was held in place by weights positioned along its span. With the advent of computers, splines are now represented digitally, but serve the same purpose of defining curvilinear profiles. Beginning with Cubic Polynomial Splines, then B-splines (B stands for Basis), and then NURBS, we now have the mathematical equivalent of the former draftsman's tool. See NURBS.

Subdivision:A relatively new technology used to represent 3D models. Widely used in the computer graphics industry, e.g. Pixar Animation, a subdivision surface is actually a polygon mesh that is refined via smoothing algorithms to closely approximate a NURBS surface. Some believe this technology will replace NURBS modeling in the future. See NURBS, Polygon Mesh.

Surface:In context, refers to a 2D shape that defines the boundary of an object in three dimensions.

Tension:A force or stress causing stretching. In context, the amount of influence put upon a curve or surface by a tangency or curvature constraint.

Tessellate:The process of decomposing a curve or surface into a polygon mesh. See Polygon Mesh

Tolerance:The acceptable deviation from specification. A consequence of attempting to digitally model real world objects, tolerance is a persistent concern when using surfacing tools and technique. Often times, a design is iterated many times before it satisfies tolerance, continuity and segmentation requirements. See Continuity, Segmentation.

Topology:The physical anatomy of a 3D model. Primary topological elements are vertices, edges and faces.

Trim:In context, a relimitation of a surface or curve. Almost all surfaces intrinsically have four sides and do not allow holes. Irregular shapes and holes are created via trimming, sometimes automatically by the system if provided sufficient input parameters.

U,V,W:The coordinate system of a surface, expressed as a ratio of overall length ranging from 0 to 1 along each direction. In the case of a curve, only U exists. Akin to Cartesian XYZ space, U corresponds to the X direction, V to the Y direction, and W (usually referred to as the normal) to the Z direction.

Wireframe:A general term used in CATIA V5 to represent zero and one-dimensional geometry (points, planes, curves, etc.). A significant portion of time spent during the surfacing process is actually creating and modifying wireframe entities.

Vector:An entity that has magnitude and direction. In graphics form, a vector image is described by curves and algorithms. Supporting industry file formats are HPGL or DXF.