5.3 SURFACE PATCHES
• Surfaces are typically defined with a variety of techniques. These include,
- fixed primitives
- swept surfaces
- rotated surfaces
- splines/free form
5.3.1 Fixed Primitives
• These are basic geometric shapes defined either explicitly, or parametrically. These often include,
• Normal analytical geometry is used for these shapes.
5.3.2 Swept Surfaces
• Basically we start with a line in space, and sweep it through space to define a surface.
• A swept surface is normally represented parametrically,
• The displacement vector T could also be a curved line. When the line is straight we often call the surface extruded.
5.3.3 Rotated Surfaces
• This approach is much like the swept, except there is a central axis that we rotate about.
• Basically to rotate a section we define an axis of rotation, and rotate the line profile.
5.3.4 Free Form Splined Surfaces
• Basically, we can look at a splined surface as composed of normal spline curves.
• Procedure for using splines in 3D,
1. Take a surface divided over an area, and break it into regions.
2. For each region find points needed for the spline. Here there are 16 shown in a 4 by 4 grid.
3. Use the measured x,y,z-points to calculate spline parameters in the u direction for each line of points, and then in the v direction.
4. To find new z-points use (x, y) to find the appropriate patch. Convert (x, y) to (u, v). Substitute (u, v) into splines to find estimated z-value. The calculation shown below uses the blending functions.
• Surface normals can be calculated using partial derivatives. (Note: these are very important for many functions, such as rendering)