4. Mesh Construction

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.

Box modelling

Box modelling is a very popular method which involves the process of starting with a primitive box and manipulating it though various methods. It's basically the process of turning a very simple shape into a very complex one allowing you to construct these meshes though two simple tools. Firstly there is the subdivide tool which spilts faces and edges into smaller pieces by added new vertices and connecting them. If you wanted to subdivivde a square, for example, you could subevide it which would add one vertex in the center and one on each edge, resulting in four smaller squares. Another method includes using the extrude tool; this tool allows you to effectivly drag out/elongate or invert a form from a face or 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. It's very useful for quickly extending objects. An example of the stages of box moddling is below.

(http://sillord.free.fr/wip/tutotopology/textrusion_fichiers/boxtohead.jpg)

Extrusion modelling

There's another method of 3D modeling usually referred to as extrustion modelling or inflation modeling. Exactly as the name suggests, it involves the process of extending or shortening a polygon from it's origin, whereby a 2D shape is created (often using placed points and connecting them to create a polygon), it is often traced from a photo or drawing. The object is traced from two different angles, in diferent viewports, before the user extrudes the shape into 3D making sure it matched up in both view ports. It's a very popular method of making faces and heads due to the complexity usually involved- it can take a long time, but often produces a much more organic effect. Normally, the artist only models half the object (if it is symettrical) and then duplicates and flips the other half to save time- This is not exclusive to extrusion modelling but is a method often used in this scenario.

(http://my.safaribooksonline.com/book/animation-and-3d/9780470102602/working-with-meshes/extrusion_modeling_and_box_modeling)

Primitive modelling
A more reserved a less common approch to 3D modelling is the primitive modelling method. This involves creating larger objects by simply combining primitives to create new shapes. The method normally isn't a very effective way to produce complex shapes, and thus is usually only applied to very rigid or simple objects. Some of the primitives could be any of the ones included in the package, which could be things like cubes, pyramids, cylinders, spheres, and 2D primitives like squares, triangles, and disks.

Specialised modelling
The modelling techniques such as box modeling and extrusion modeling are alright for general modelling of objects, but if you need something a bit more organic or detailed, you need more specialised modelling. For this kind of modelling, specialised methods of constructing high or low detailed meshes exist. For example there is sketch based modeling available allowed construction of low detail models very quickly in a user friendly interface, and 3D scanners can be used to make very high detail meshes based on real world objects in a very automtic way. However, these kind of devices are very expensive are thus are usually reserved for researches and industry professionals that really need the high level of accuracy and sub-millimetric digital representations of an object.

(http://www.3dscanco.com/assets/images/products/artec_mht.jpg)
(http://blog.3d3solutions.com/Portals/43940/images/3d_face_scan.jpg)

Info:
http://southerngfx.co.uk/tutorials/box-modelling
http://jessgrafton.wordpress.com/3d/mesh-construction/
http://my.safaribooksonline.com/book/animation-and-3d/9780470102602/working-with-meshes/extrusion_modeling_and_box_modeling
http://en.wikipedia.org/wiki/Polygonal_modeling
http://www.education.vic.gov.au/school/teachers/teachingresources/discipline/maths/assessment/pages/primitivemod.aspx
http://www.peachpit.com/articles/article.aspx?p=30594&seqNum=5
http://en.wikipedia.org/wiki/3D_scanner