Polygonal Modeling - Construction of Polygonal Meshes

Construction of Polygonal Meshes

Although it is possible to construct a mesh by manually specifying vertices and faces, it is much more common to build meshes using a variety of tools. A wide variety of 3d graphics software packages are available for use in constructing polygon meshes.

One of the more popular methods of constructing meshes is box modeling, which uses two simple tools:

• The subdivide tool splits faces and edges into smaller pieces by adding new vertices. For example, a square would be subdivided by adding one vertex in the center and one on each edge, creating four smaller squares.
• The extrude tool is applied to a face or a group of faces. It creates a new face of the same size and shape which is connected to each of the existing edges by a face. Thus, performing the extrude operation on a square face would create a cube connected to the surface at the location of the face.

A second common modeling method is sometimes referred to as inflation modeling or extrusion modeling. In this method, the user creates a 2d shape which traces the outline of an object from a photograph or a drawing. The user then uses a second image of the subject from a different angle and extrudes the 2d shape into 3d, again following the shape’s outline. This method is especially common for creating faces and heads. In general, the artist will model half of the head and then duplicate the vertices, invert their location relative to some plane, and connect the two pieces together. This ensures that the model will be symmetrical.

Another common method of creating a polygonal mesh is by connecting together various primitives, which are predefined polygonal meshes created by the modeling environment. Common primitives include:

• Cubes
• Pyramids
• Cylinders
• 2D primitives, such as squares, triangles, and disks
• Specialized or esoteric primitives, such as the Utah Teapot or Suzanne, Blender's monkey mascot.
• Spheres - Spheres are commonly represented in one of two ways:
  • Icospheres are icosahedrons which possess a sufficient number of triangles to resemble a sphere.
  • UV Spheres are composed of quads, and resemble the grid seen on some globes - quads are larger near the "equator" of the sphere and smaller near the "poles," eventually terminating in a single vertex.

Finally, some specialized methods of constructing high or low detail meshes exist. Sketch based modeling is a user-friendly interface for constructing low-detail models quickly, while 3d scanners can be used to create high detail meshes based on existing real-world objects in almost automatic way. These devices are very expensive, and are generally only used by researchers and industry professionals but can generate high accuracy sub-millimetric digital representations.