1 91.427 Computer Graphics I, Fall 2008

Image Formation

2 91.427 Computer Graphics I, Fall 2008

Objectives

•Fundamental imaging notions•Physical basis for image formation

Light Color Perception

•Synthetic camera model•Other models

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Image Formation

•In computer graphics form images generally two dimensional process analogous to how images formed by physical imaging systems

Cameras Microscopes Telescopes Human visual system

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Elements of Image Formation

•Objects•Viewer•Light source(s)

•Attributes govern how light interacts with materials in scene

•Note independence of objects, viewer, and light source(s)

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Light

•Light is the part of the electromagnetic spectrum that causes a reaction in our visual systems

•Generally these are wavelengths in the range of about 350-750 nm (nanometers)

•Long wavelengths appear as reds and short wavelengths as blues

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Ray Tracing and Geometric Optics

One way to form image:•follow rays of light

from point source

•find which rays enter lens of camera•But, each ray ==> •multiple interactions with objects•before being absorbed or going to infinity

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Luminance and Color Images

•Luminance Image Monochromatic Values are gray levels Analogous to working with black and white film or television

•Color Image Perceptional attributes: hue, saturation, and lightness

Have to match every frequency in visible spectrum? No!

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Three-Color Theory

•Human visual system: two types of sensors Rods: monochromatic, night vision Cones

• Color sensitive• Three types of cones• Only three values (tristimulus values) sent to brain

•Need only match these three values ==> Need only three primary colors

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Shadow Mask CRT

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Additive and Subtractive Color

•Additive color Form color by adding amounts of three primaries

• CRTs, projection systems, positive film

Primaries = Red (R), Green (G), Blue (B)

•Subtractive color Form color by filtering white light with Cyan (C), Magenta (M), and Yellow (Y) filters

• Light-material interactions• Printing• Negative film

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Pinhole Camera

xp= -x/z/d yp= -y/z/d

Use trigonometry to find projection of point at (x,y,z)

Equations of simple perspective

zp= d

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Synthetic Camera Model

center of projection

image plane

projector

p

projection of p

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Advantages

•Separation of objects, viewer, light sources

•2-D graphics is special case of 3-D graphics

•Leads to simple software API Specify objects, lights, camera, attributes Let implementation determine image

•Leads to fast hardware implementation

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Global vs Local Lighting

•Cannot compute color or shade of each object independently Some objects blocked from light Light can reflect from object to object Some objects might be translucent

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Why not ray tracing?

•Ray tracing more physically based ==>•Why not use to design graphics system?•Possible & simple for simple objects •E.g., polygons & quadrics with simple point sources

•In principle, can produce global lighting effects

•E.g., shadows & multiple reflections •But ray tracing slow & not well-suited for interactive applications