1 91.427 computer graphics i, fall 2008 image formation
TRANSCRIPT
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
3 91.427 Computer Graphics I, Fall 2008
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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91.427 Computer Graphics I, Fall 2008
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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91.427 Computer Graphics I, Fall 2008
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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91.427 Computer Graphics I, Fall 2008
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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91.427 Computer Graphics I, Fall 2008
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