Modeling
With "modeling" I intend all the techniques to create "models" or "objects".
A model is an entity that has a "form", a "shape". Think about you in a dark room examining some unknown objects only by manipulating and touching them. The only information you can obtain this way is about the shape of the objects.
You can point out that this way you can also evaluate weight, temperature and toughness, but shape is the only property that can be obtained also with looking at the object, that is the only thing you finally do with your 3D image.
There are several ways to model objects as there are several model types.
Let's see some common (and less common) techniques to create a model:
Extrusion
Bevel
Lathe
Freeform editing
Skin
Sweep along path
Boolean operations
Cross section editing
Deformations
Displacement mapping
Scanning
The extrusion operation is sweeping a 2D section either open or closed or a group of sections along a linear path for a certain length.
It's very similar to "real" extrusion where a certain plastic matter is pushed across a mask that has various shaped holes and the matter takes the shape while it passes throughout the mask.
Similar to extrusion but the profile changes his size as he get across the extruding segment. This technique is useful for creating several kind of objects but his typical use is to create 3D Logos and titles for animations.
In this case the extrusion section is a group of characters of a certain font.
This is a classic tool to create objects that are so called surfaces of revolution. In this case the section is rotated around an axis for an angle of a certain amount alpha. the maximum angle is 360 degrees.
With this name i refer to the modeling tools that allows to change the shape of an existing object as it is made of clay and you are allowed to manipulate it by hands. There are several ways to achieve this. In general this operation is performed on a object that has been yet created with another tool and need a further refinement.
Skin is a technique to model a surface by drawing a series of cross sections of the desired object and then "skin" these sections to obtain a surface.
With this technique is possible to model very complex objects but a good practice in 2D drawing is required to achieve interesting results.
Similar to extrusion but the profile is swept along a curve instep of a straight line. The curve can be a 2D curve (lying on a plane) or even a 3D curve. Note that only a few 3D programs allows to sweep along a 3D curve and some of them still has problems in doing this. I haven't found yet a program that combine the Bevel and the Sweep along path. Amapi does an interesting thing with his "double sweep", but it's not easy to model a trumpet this way.
Boolean operations are a series of logical operations that can be performed between two objects to obtain a third object.
The most common boolean operations are:
Union
Intersection
Difference
The union of two objects is the whole of two.
The intersection of two objects exist only if the two objects partially share a part of the same space. If so the intersection is the object that contain all and also the points that belongs to both initial objects.
The difference of object A and object B is the object that belongs to object A but not to object B. If A and B doesn't share any portion of space the difference is equal to object A.
This is an uncommon technique I found only in two apps: Imagine and Swivel 3D.
It consists in drawing three orthogonal cross section, one for each axis, like in a orthogonal projection. It's an interesting tool and it can create some kind of objects easy, but in general is limited and I think this is the cause it's so uncommon.
It is possible to obtain complex object by applying deformations to existing object. The most common deformations are: Bend, Twist, Magnet, Taper, Shear, Cut...
I wrote a program to perform such (and more) deformations. Take a look at the D-form Home Page.
This powerful tool is pretty rare to find in 3D apps. It can appear similar to bump mapping, but it's different. A grayscale bitmap is used to alter the shape of an object buy pushing out his surface for an amount dependent to the gray tone on the map.
This technique consist in scanning a real object with a special device that can be a mechanical arm with a pen, or very expensive laser scanner. Many complex model used for movie production are modeled this way (for example the head of Woody and the entire Scud pitbull dog of Toy Story.
The extremely famous Utah Teapot is also a scanned model.
Usually a complex model is created by one or more of the steps listed above. The skill to choose the right techniques to obtain a certain result came with the experience. Sometimes there are several path to achieve a similar result and it's better to choose the one that allows the fastest creation, or the creation of the smaller number of polygons.
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