Introduction to Modeling

What is CG Modeling?

• Combination of Sculpting, Architecture, Drafting, and Painting.

• The core component of computer animation.

What are the challenges: – Time!

• Time to create the models.

• Time to render the models.

• Time to animate the models.

What are our options?

• To improve rendering time reduce polygon count.• To improve realism increase polygon count.• To create models faster reduce polygon count.• To reduce animation time reduce number of manipulators.• To improve animation realism increase the number of manipulators.

Low polygon model

High polygon model

Few manipulators

Many manipulators

How to create models?

• Hand code the model – no one does this anymore.

• Use a modeling tool – eg Maya.

• Scan physical models using a 3D scanner.

Generate by Hand

Pros/Cons

Very very exact.You know exactly what you get.

Very very time consuming.Not possible to build large orComplex models.

Use a Modeling Tool - Maya

Pros/Cons

Quick to build models.Can build very complex modelsLots of controls and manipulators

Easy to build overly complex modelsTools can be difficult t learn.Can be difficult to control output.Can be hard to port to other tools

Use a 3D Scanner

Pros/Cons

Very quick to build models.Can be done automatically.Can hire students to do it.

Few controls and manipulators.Requires extensive re-editing.Generates models with too many polygons.Limits animators flexibility.

What are the Parts of Modeling?

• Scene – when modeling work on a scene at a time.– Spaces – Where your models sit in space.– Objects – the primitives used to build your

models.– Structures – combine objects to make

structures.– Operators – Tools to build structures from

objects.

Structure

Object

Space

Spaces

• Coordinate Spaces• X Y Z• Cartesian

Coordinates• Rectangular

Coordinates• Global Coordinates• Local Coordinates

Projections

• How we go from 3D to 2D

• Two types: Perspective and Orthographic

Objects

How can you model objects?

• Polygonal Models.

• NURBS

• Subdivision Surfaces

• Parametric Models

• Implicit Surfaces

Polygonal ModelsPolygonal Models consist of:•Vertices•Edges•Faces

Edge

Face

Vertex

Polygon

Pros/Cons

Simple to understand.Simple to render.Simple to combine to make shapes.Hardware supported rendering

Need lots of polygons for complex shapes.Difficult to animate lots of polygons.Lots of controls needed.Advanced modeling operations are difficult.Continuity problems.

NURB Models

• NURBs are made of:– Patches– Curves– Control Points

Non-Uniform Rational B-Splines

Pros/Cons

Need fewer patches for complex objects.Easier to animate patches.Fewer controls needed.No Continuity problems.Advanced modeling operations are easier.

Hard to create corners!Difficult to understand.Difficult to render.Simple to combine to make shapes.Render by converting to polygons

Subdivision Surfaces•Provides `Localized Refinement•Allows you to mix sharp and smooth corners.•Continuity controlled locally.

•Increased rendering time•Requires clear understanding to generate good models.•Can be difficult to animate.

Parametric Models and Implicit Surfaces

Mathematical representations of complex surfaces.X^2*Y^3-Z^4- 12 X=r(t), Y=r(t)+2*r^2(t), Z=t

Not in production use at this time.Have very beneficial mathematical properties.Are extremely difficult to render, modify, and animate.

Structures

What Kinds of Modeling Operations Exist?

• Insert and Delete point, edge, face.• Add Polygon/NURB• Boolean Operations: Union, Intersect, Difference.• Advanced Operations:

– Extrude along path.– Loft surface– Revolve– Bend– Split

• Very Advanced Operations– Smooth Model

What kinds of modeling jobs are there?

• General Modelers

• Photo-realistic modelers

• High poly-count modelers

• Low poly-count modelers

• Character Animators

• Prop Modelers

What types of animation controls are available?

• Simple – directly keying specific components.

• Moderate – An I/K skeleton, Solvers.

• Complex – Programmed Mel Scripts, advanced solvers. Linked Solvers.

How do you control color and appearance.

• Assign Materials.

• Assign Shaders.

• Assign Textures.

• Advanced Shaders – eg Renderman.

What about rendering?

• Everything is translated to Triangles.

How do you Triangulate?

• Polygons

• NURBS

• Subdivision Surfaces

• Parametric Surfaces

• Implicit Surfaces

How is everything rendered.

• First everything is converted to polygons

• Second everything is converted to triangles.

• Then everything is rendered thru the graphics pipeline discussed in the prior lecture.

Good References

• 3D Computer Graphics– Alan Watt

• The Art of 3-D Computer Animation and Imaging– Isaac Victor Kerlow

• 3D Animation: From Models to Movies– Adam Watkins