3D Animation – Research Essay
Dean Barrett
Due Date: 7 March 2011
Topic: Discuss the different kinds of modelling techniques used in a 3D programme (Polygons, NURBS and Subdivisions) as well as the strengths and weaknesses of these methods.
“Three-dimensional modelling can be compared to sculpting”
- Peter Ratner, 2004, pg 4.
Modelling in a 3D program presents the user with many different techniques and approaches to creating a model. This essay shall explore the properties and applications of three of these techniques or approaches: using and manipulating polygons, NURBS, and subdivision surfaces. It shall further discuss both the advantages and disadvantages that each of these techniques presents the user.
Let us firstly define and discuss the polygon. As Ratner defines it, “a polygon is a portion of a plane bounded by three or more lines or segments” (Ratner, P. 2004, pg 4). A polygon therefore contains a collection of points to which lines (or edges) connect in order to form faces (or sides). Faces and points on polygons can then be manipulated in a number of ways to form the shape of what it is the user wishes to create. For example, the face of a polygon can be extruded (or even pulled back) to increase the length of the polygon or to add a certain detail. Points on the polygon edges can be moved, welded together and lengthened amongst many other possibilities. Due to their straight edges, polygons are useful in creating geometric shapes/figures which incorporate angular parts e.g. a table.
The advantages of using polygons are that they are generally very flexible and easy to manipulate. One can rotate them, join/split them, intersect them amongst many other possibilities which allows the user to create complex meshes from single primitives. The main disadvantage of polygons is that, due to their straight and hard edges, they have a very segmented appearance which George Avgerakis describes as looking “boxy and artificial” (Avgerakis,G. 2004, pg233). To overcome this, one can further subdivide these polygons “until the edges [sic] smooth out and disappear” (Avgerakis,G. 2004, pg233). This, however, presents another disadvantage associated with polygons and that is that in creating a more complex mesh to smoothen out one’s polygon shape, the polygon becomes a much more render intensive object which will inevitably take a substantially longer amount of time to fully render.
In order to overcome excessive polygon meshes, one can also make use of splines – and more specifically for this essay – NURBS. A spline, firstly, is defined as a “flexible line segment defined by edit points called vertices” (Ratner, P. 2004, pg 4). NURBS, an abbreviation for Non-uniform rational B-spline, are a type of spline which is “characterised by a set of control vertices (CV’s)” and which makes the creation of smooth surfaces and curves possible (Ratner, P. 2004, pg 4). NURBS are therefore mostly used in the modelling of organic objects (e.g. flowers).
The first advantage of using NURBS is that users can avoid the complex polygon arrangements used to achieve smooth edges and surfaces which allows for a smoother looking object with a lower render time. Tom Meade notes that NURBS use fewer control points which “translates into smaller file sizes” and, due to this, is a more efficient use of one’s computer’s RAM. (Meade, T. 2007, pg134). He also notes that it is easier to map textures onto these surfaces due to the “corresponding coordinate systems” between UV directions on a NURBS and XY coordinates on, say, a Photoshopped bitmap image. (Meade, T. 2007, pg135). Lastly, he notes that NURBS have a very accurate “conversion ability” from a NURBS surface to a polygon surface which also produces a very neat polygon geometry. (Meade, T. 2007, pg136).
The first disadvantage of using NURBS is the occurrence of seams. These occur, as Meade explains, due to the fact that NURBS surfaces are generally made from “multiple individual surfaces” (Meade, T. 2007, pg136). This results in one being able to see the spaces between different NURBS surfaces. These seams may be ‘patched’ together in a variety of ways but, once again, this increases the render time of the object. Another disadvantage is known as branching which refers to “parts of models that extend, or branch off, from the main surface or a group of surfaces” (Meade, T. 2007, pg137). In this scenario, “five-point junctions” become evident which then need to be modified because NURBS surfaces “can only contain a quad layout” (Meade, T. 2007, pg137). Lastly, while Meade also listed it as an advantage, he also notes that texture mapping can be a disadvantage of NURBS too, especially across multiple surfaces due to textures being applied to a variety of uneven surfaces, further complicated by the fact that UVs “cannot be edited separately from the mesh” (Meade, T. 2007, pg137).
Subdivision surface modelling was developed almost as a meeting point for polygons and NURBS. As Peter Ratner defines it, “this method uses a low polygon control mesh that applies a smoothing algorithm to bend the edges of polygons, giving them a curved appearance.” (Ratner, P. 2004, pg 5). Subdivision surfaces are useful for creating organic shapes from single surfaces and polygons.
The advantage, then, of a subdivision surface is that the user can achieve the smoothness of NURBS with the versatility of polygons and still avoid an excessive polygon count. The disadvantages of subdivision surfaces include, according to Dariush Derakhshani, requiring “even more computation than NURBS, and keeping models in subdivisions will cost you a lot of memory.” (Derakhshani, D. 2010, pg110). Due to them existing between polygon surfaces and NURBS surfaces, they are also difficult to apply UV texturing upon.
In conclusion, this essay has defined and discussed three of the modelling techniques dominantly used in 3D software today – polygons, NURBS and subdivision surfaces. It has explored the properties of each of these, their specific applications in modelling as well as looking at the advantages and disadvantages of all three. It has shown that, due to the wide variety of methods available to use when approaching modelling, it is useful to plan and map one’s ideas with regards to the implementation of these techniques in order to achieve the best result with the least amount of strain on the computer and, indeed, with the most efficient working strategy.
(1009 words)
References:
1. Avgerakis, George. Digital Animation Bible: Creating Professional Animation with 3Ds Max, Lightwave, and Maya. 2004. The McGraw-Hill Companies, Inc. USA.
2. Derakhshani, Dariush. Introducing Maya 2011. 2010. Wiley Publishing. Indianapolis, Indiana.
3. Meade, Tom. Maya 8: The Complete Reference. 2007. The McGraw-Hill Companies, Inc. USA.
4. Ratner, Peter. Mastering 3D Animation. 2004. 2nd Ed. Allworth Press, New York.
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