As illustrated in the previous chapters, there are no surfaces in a wireframe model; it consists only of points, lines, and curves that describe the edges of the object. Surface modeling was developed to provide the surface information that is missing in wireframe modeling. Essentially, defining the skin of a design creates a surface model. Although it is possible to create a surface model without using a wireframe model, in most cases it is much easier to create a surface model on top of a wireframe model. In surface modeling, a wireframe model can be used to provide information about the edges and corners so that the desired faces can be easily positioned and placed.
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3D Surface Modeling Although all three commands can be used to create planar surfaces, the resulting surfaces are not equal. In fact, the three commands are developed for specific tasks in mind. The 2D Solid command is mostly used in 2D drawings to create 2D filled area and the Region command is designed so that general 2D shapes can be easily transformed into solid models. The 3D Face command is the only one that is designed specifically for surface modeling and therefore it is the most suitable for such tasks. The use of the 2D Solid and Region commands in 3D surface modeling can be somewhat awkward and at times very difficult. Note that the use of the Region command will be focused on in the solid modeling chapters of this text.
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The AutoCAD surface modeler defines faceted surfaces using a filled polygon. The created faces of surface models are only planar, which means the surface models can only have approximate curved surfaces. It is important to note that the AutoCAD surface modeler does not create true curved surfaces. To differentiate these two types of surfaces, faceted surfaces are called meshes in AutoCAD. Because of the use of faceted approximation on true curved surfaces, the computer requirements of most faceted surface modelers are typically much less than that of solid modelers.
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