SKM 4200Computer Animation

Chapter 4: Animation(3D Computer Animation – Part 1)

Understanding Polygons• Polygons are is a closed 2D shape formed by straight lines (edges)• The building blocks of a polygon… For example, a cube (adapted from [1]):

• The number of vertices (plural of vertex) is always equals to the number of edges

• Question: What is the minimum number of edges to make a polygon? ____ (please fill in the blank… )

3D Computer Animation

The breakdown… • Vertex: Has no Length, Width or Height. In short, it has ZERO dimensions;

• Edge: Has ONE dimension… Length;

• Face (i.e. the polygon): TWO dimensions… Width and Length;

vertex

edge

face/polygon

3D Computer Animation

http://www.mathsteacher.com.au/year7/ch09_polygons/05_polygon/Image11206.gif

Note:

Four-side polygons are referred to as quadrangles, whereaspolygons with more than 4-sidesare n-gons [1]. The ‘n’ representsthe number of sides…

Quadrangle – 4 sides

Understanding Polygons (continued)• Planar polygons

– When all the vertices lie on the same plane (a plane is like the floor we stand on. It has no curves and infinitely (straightly) extends in each direction )

– A triangle is always planar • Non-planar polygons

– When the vertices lie on different planes– Polygons other than triangles can be non-planar

3D Computer Animation

Planar Non-planarhttp://softimage.wiki.softimage.com/xsidocs/poly_basic_PolygonMeshes.htm

3D Computer Animation

3D Primitives• Instant 3D objects… with polygons as building blocks• 3D Primitives: cube, cylinder, sphere, cone, polyhedron, wedge etc. • Some 3D packages offer different types of primitives

– Upgrades add on to existing primitives

For polyhedron, goto:

http://docs.autodesk.com/ACD/2011/ENU/filesAUG/WS1a9193826455f5ffa23ce210c4a30acaf-68e7.htm

http://en.wikipedia.org/wiki/Polyhedron

Polygon Meshes• A polygon mesh is a 3D object composed of one or more

polygons– A group of polygons forming a model– The 3D primitives are examples of polygon meshes

• The 3D cube primitive:

3D Computer Animation

The breakdown… • Vertex: Has no Length, Width or Height. In short, it has ZERO dimensions;

• Edge: Has ONE dimension… Length;

• Face (polygon): Has TWO dimensions… Width and Length;

• The 3D mesh/model (the CUBE): Has ALL THREE… Height, Width and Length.Reference [1]

3D Computer Animation

• Closed polygon meshes– Edges are shared by all faces– An example is the cube, sphere and (closed) cones and

cylinders, etc.– No hole to get into the mesh…

• Open polygon meshes– Not all faces share the edges– Can result in a 2D look (for a leaf?)… but it’s 3D of course– The outside edges are referred to as ‘boundary edges’ or

‘border edges’ [1]

3D Computer Animation

• An example of an open polygon mesh

Boundary/Border Edgeshttp://cdn.ronenbekerman.com/wp-content/uploads/2011/06/single-leaf-mesh.jpg

http://files.turbosquid.com/Preview/Content_2010_12_08__22_43_43/Leaf_red_mesh_2.jpgce037a0c-b7c8-480a-a5fe-fd6d447db19dLarger.jpg

3D Computer Animation

Modeling • Two modeling approaches will be discussed– BOX MODELING– EXTRUSION

3D Computer Animation

Box modeling• Modeling begins with a box (a cube primitive)• Artist then scales the object, moves

(translates) vertices and edges, and adds more faces by adding edges– This results in the 3D shape they want to create

http://upload.wikimedia.org/wikibooks/en/9/96/W3D_quadcar_Step6.jpg

http://red-zombie.com/wp-content/uploads/2008/04/lion2.jpg

3D Computer Animation

Extrusion modeling• Extrude basically means to push or thrust out– Modeling begins with an edge or a polygon, or

even a primitive– Then you extrude new polygons from what you

started with

3D Computer Animation

• A basic tutorial on how to model a car can be found here: A basic tutorial of how to box model a car can be found at http://en.wikibooks.org/wiki/Wings_3D/Tutorials/Box_modeling_a_car_with_all_Quad_topography

• Some videos– Box modeling -1– Box modeling-2– Extrude modeling-1– Extrude modeling-2

3D Computer Animation

Orthogonal Drawings

3D Computer Animation

Orthogonal DrawingsTOP

FRONT

SIDE

PERSPECTIVE

3D Computer AnimationOrthogonal Drawings- Normally used as reference for modeling- Does not have perspective- Normally set up as planes in the FRONT VIEW, SIDE VIEW and TOP

VIEW of a particular 3D modeling application

IMPORTANT NOTE!!!• Make sure that each drawing have the same aspect ratio• Make sure each are set up so the edges of each drawing match each

other where they would meet

3D Computer Animation

Mirroring- Model one side of the character/object, and then duplicate

its ‘mirror’ to obtain the other side- So you won’t have to remodel different sides of your

object/character- Many 3D applications allow mirroring to be done

automatically- So if you model one side, the other side automatically gets

modeled as well

• Video 1 Concept of mirroring• Video 2 Head modeling animation

3D Computer Animation

Editing the Mesh• Things you can basically edit– Vertices– Edges– Faces

• Can either choose to edit one at a time, or as a group

http://en.wikipedia.org/wiki/Polygon_mesh

3D Computer AnimationEditing the Mesh• Soft selection– Allows you to control how strongly the vertices and polygons surrounding

selected parts (for editing) are affected– Polygons from far away feel the effect of changing a face’s position; then the

change will create a smoother, less steep slope– The mesh behaves like soft clay, and so this is typically called soft selection– VIDEO

3D Computer Animation

• Extruding– A way of adding more polygons to a mesh– Applicable to edges and faces only– You can extrude more than once depending on

what you want to model

3D Computer Animation• Subdividing– Adding edges (or subtracting them) can be done across the

entire mesh• Adding edges however increases the number of vertices and also the

number of faces/polygons– Subdividing allows you model to become more detailed• Smoothing can then be applied…• Smoothing averages out the vertices, so that it has fewer sharp

angles

• Simplifying– The opposite of subdividing– Involves deleting of unwanted components (e.g. vertices, faces

or edges)

3D Computer Animation

3D Computer Animation

• Boolean - SUBTRACT– FIRSTLY, select the mesh you want to keep – SECOND, select then the other mesh– Then perform the subtract operation– This will cause the second mesh to be “removed”

3D Computer Animation

• Boolean - INTERSECTION– Combine both meshes that you want to have

intersection– Then perform the intersection operation

3D Computer Animation

• Boolean - UNION– As the name suggests… both meshes are united – For the example given below… the two shapes

involved are a cube and a sphere…

3D Computer Animation

• Welding– The vertices to be joined (welded) on the border

edges from two different meshes are specified– Welding results in those vertices and the edges to

combine into one

3D Computer Animation

• Bridging– A new edge will be drawn between the selected

vertices, so that new bridging polygons are created between the border edges of the two meshes

3D Computer Animation

• Polygon Count– The more polygons you have, the more detail a

mesh will be – Will take up more processing power to manipulate

the mesh• The longer it will take to render

3D Computer Animation• Normals

– Polygons have a back face and a front face– When a primitive is created, the side of the faces that you can see are the front faces.

You can also think of this as the direction that the polygon is pointed in– This is called the normal– To visualize it, imagine a line that points straight out, perpendicular to your polygon– Normals are important for:

• Lighting - how it interacts with the model• Rendering

References[1] Chopine, A., “3D Art Essentials: The Fundamentals of 3D Modeling,

Texturing and Animation”, Focal Press, 2011.