Digital Media

Lecture 7: Vector Graphics 3D

Georgia Gwinnett CollegeSchool of Science and Technology

Dr. Jim Rowan

3D

X & Y like 2D graphics Z is Height (or depth)

3D

3D shapes (objects) are defined by their surfaces

Complicated because a 3D object (inside the computer) must be translated into 2D to be viewed…

And you need to: – specify the viewpoint, a camera– specify the lighting

3D

Specifying the camera and the lighting has one huge advantage over 2D– Automatically generates all of shadows

BUT… rendering (converting 3D to 2D) is extremely computationally expensive (demanding, time consuming)– It can be slow!

3D

Lighting has different characteristics and must be specified– natural or artificial– spot or flood– color– multiple sources– reflections off other objects in the scene

Atmosphere must be intentionally included

Surface texture must be specified

Issues of focus/atmosphereExamples from Sintel

Sintel Example

Managing Complexity

Structural hierarchy Things in the real world are compositions of

smaller things

Things in the 3-D graphics world are also compositions of smaller things

Hierarchical structure is an excellent way of coping with complexity

Also seen in object-oriented programming like Java and Squeak!

3D Models

So…

How do you build a model inside a computer when you can’t touch it?

Constructive Solid Geometry Free Form: Bezier Surfaces Free Form: Extrusion Procedural modeling

3D models

Constructive solid geometry– building things from known shapes– uses geometric solids: cube, cylinder,

sphere and pyramid– objects build by squishing and

stretching those objects– objects joined using union,

intersection and difference

Intersection

Difference

Union or just two objects?

Free Form

Building things one side at a time Uses an object’s surface (it’s boundary with the

world) to define it Build surfaces from flat polygons or curved patches

– flat polygons are easier to render and therefore frequently used in games where computational power is limited

Results in an object drawn as a “mesh” Can be done using Bezier surface patch but they

have 16 control points! More tractable (do-able) patch uses a surface called

a non-rational B-spline

Free Form: Extrusion

Building things using a play-doh factory

Move a 2 dimensional shape through space along a line

The line can be straight or curved

Procedural modeling

Best known is based on Fractals– Fractals– exhibit the same structure at all levels of

detail aka “self similar”– used to model natural objects

Particle systems... many particles, few controls– Fur, hair, grass…

Physics... distribution of mass, elasticity, optical properties, laws of motion

3D Rendering

Rendering engine handles the complexity

Wire frames are used to preview objects and their position– can’t tell which surface is closer to us and

which surface is hidden To save computation time, hidden

surfaces are removed before rendering– Why render what can’t be seen?

3D rendering

Lighting– Added to scene much like an object– spot light, point source, floodlight...– position and intensity

Direct relationship between rendering quality and computational burden(render time)