Digital Media

Dr. Jim Rowan

ITEC 2110

Vector Graphics II

Vector Graphics3-D... 3 dimensions

• x, y and z• x & y form the ground plane• z is the height

3D

• 3D shapes (objects) are defined by their surfaces

• Made even more complicated by the fact that a 3D object inside the computer must be translated into 2D to be rendered…

• This results in the need to – specify the viewpoint, a camera– specify the lighting

3D

• But… having to specify the camera and the lighting buys you one huge advantage that you do not get when doing 2D

• rendering automatically generates all of shadows

• BUT… rendering is extremely computationally expensive (demanding, time consuming)

3-D: even more complexity

• 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

http://wiki.ggc.usg.edu/mediawiki/index.php/Content_group_2#Lecture_8_Content_group_2_Vector_Graphics_3D

issues of focus/atmosphereexamples from Sintel

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: Extrusion

• Meat balls

• 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 2 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 playdoh factory• Draw a 2 dimensional shape through

space along a line• The line can be straight or curved

Meat balls

• Reacts like soft objects rather than hard, solid ones

• Think of the objects as having surface tension• Kinda like an uncooked meatball, press two

of them together and they “squish”

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– http://en.wikipedia.org/wiki/Fractal

• Particle systems... many particles, few controls• 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

3-D rendering

• Shading... how light reacts with surface– Based roughly on physics but modified by

heuristics (rules derived from experience)– Examples ===>

• Texture mapping– An image is mathematically wrapped around the

object

3-D rendering• Light reflecting off objects of one color affect the color

and lighting of surrounding objects– Ray tracing Complex... must be repeated for pixel

in the image... photo-realistic results – Wayne Wooton Pixar– Radiosity

Questions?