Computer graphics are pictures and films created using computers. Usually, the term refers to computer-generated image data created with help from specialized graphical hardware and software. It is a vast and recent area in computer science.
2. Objectives
โข Fundamental imaging notions
โข Physical basis for image formation
โข Light
โข Color
โข Perception
โข Synthetic camera model
โข Other models
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3. Image Formation
โข In computer graphics, we form images which are generally two
dimensional using a process analogous to how images are formed by
physical imaging systems
โข Cameras
โข Microscopes
โข Telescopes
โข Human visual system
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4. Elements of Image Formation
โข Objects
โข Viewer
โข Light source(s)
โข Attributes that govern how light interacts with the materials in the
scene
โข Note the independence of the objects, the viewer, and the light
source(s)
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5. Light
โข Light is the part of the electromagnetic spectrum that causes a
reaction in our visual systems
โข Generally these are wavelengths in the range of about 350-750 nm
(nanometers)
โข Long wavelengths appear as reds and short wavelengths as blues
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6. Ray Tracing and
Geometric Optics
One way to form an image is to
follow rays of light from a
point source finding which
rays enter the lens of the
camera. However, each
ray of light may have
multiple interactions with objects
before being absorbed or going to infinity.
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7. Luminance and Color Images
โข Luminance Image
โข Monochromatic
โข Values are gray levels
โข Analogous to working with black and white film or television
โข Color Image
โข Has perceptional attributes of hue, saturation, and lightness
โข Do we have to match every frequency in visible spectrum? No!
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8. Three-Color Theory
โข Human visual system has two types of sensors
โข Rods: monochromatic, night vision
โข Cones
โข Color sensitive
โข Three types of cones
โข Only three values (the tristimulus
values) are sent to the brain
โข Need only match these three values
โข Need only three primary colors
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10. Additive and Subtractive Color
โข Additive color
โข Form a color by adding amounts of three primaries
โข CRTs, projection systems, positive film
โข Primaries are Red (R), Green (G), Blue (B)
โข Subtractive color
โข Form a color by filtering white light with cyan (C), Magenta (M), and Yellow
(Y) filters
โข Light-material interactions
โข Printing
โข Negative film
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11. Pinhole Camera
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xp= -x/z/d yp= -y/z/d
Use trigonometry to find projection of point at (x,y,z)
These are equations of simple perspective
zp= d
13. Advantages
โข Separation of objects, viewer, light sources
โข Two-dimensional graphics is a special case of three-dimensional
graphics
โข Leads to simple software API
โข Specify objects, lights, camera, attributes
โข Let implementation determine image
โข Leads to fast hardware implementation
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14. Global vs Local Lighting
โข Cannot compute color or shade of each object independently
โข Some objects are blocked from light
โข Light can reflect from object to object
โข Some objects might be translucent
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15. Why not ray tracing?
โข Ray tracing seems more physically based so why donโt we use it to
design a graphics system?
โข Possible and is actually simple for simple objects such as polygons
and quadrics with simple point sources
โข In principle, can produce global lighting effects such as shadows and
multiple reflections but ray tracing is slow and not well-suited for
interactive applications
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