introduction - texas a&m university-corpus christi › ~sking › courses › cosc5327 › notes...
TRANSCRIPT
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INTRODUCTION Slides modified from Angel book 6e
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Objectives
• Historical introduction to computer graphics
• Fundamental imaging notions
• Physical basis for image formation
• Light, Color, Perception
• Synthetic camera model
• Other models
• Basic design of a graphics system
• Introduce a graphics pipeline architecture
• Examine software components for interactive graphics
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Computer Graphics
• Computer graphics deals with all aspects of creating
images with a computer
• Hardware
• Software
• Applications
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Example
• Where did this image come from?
• What hardware/software did we need to produce it?
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Preliminary Answer
• Application: The object is an artist’s rendition of the sun
for an animation to be shown in a domed environment
(planetarium)
• Software: Maya for modeling and rendering but Maya is
built on top of OpenGL
• Hardware: PC with graphics card for modeling and
rendering
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Basic Graphics System
Input devices Output device
Image formed in frame buffer
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CRT
Can be used either as a line-drawing device
(calligraphic) or to display contents of frame buffer
(raster mode)
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Computer Graphics: 1950-1960
• Computer graphics goes back to the earliest days of
computing
• Strip charts
• Pen plotters
• Simple displays using A/D converters to go from computer to
calligraphic CRT
• Cost of refresh for CRT too high
• Computers slow, expensive, unreliable
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Computer Graphics: 1960-1970
• Wireframe graphics
• Draw only lines
• Sketchpad
• Display Processors
• Storage tube
wireframe representation
of sun object
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Sketchpad
• Ivan Sutherland’s PhD thesis at MIT
• Recognized the potential of man-machine interaction
• Loop
• Display something
• User moves light pen
• Computer generates new display
• Sutherland also created many of the now common algorithms for
computer graphics
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Display Processor
• Rather than have the host computer try to refresh
display use a special purpose computer called a
display processor (DPU)
• Graphics stored in display list (display file) on
display processor
• Host compiles display list and sends to DPU
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Direct View Storage Tube
• Created by Tektronix
• Did not require constant refresh
• Standard interface to computers
• Allowed for standard software
• Plot3D in Fortran
• Relatively inexpensive
• Opened door to use of computer graphics for CAD community
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Computer Graphics: 1970-1980
• Raster Graphics
• Beginning of graphics standards
• IFIPS
• GKS: European effort
• Becomes ISO 2D standard
• Core: North American effort
• 3D but fails to become ISO standard
• Workstations and PCs
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Raster Graphics
• Image produced as an array (the raster) of picture
elements (pixels) in the frame buffer
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Raster Graphics
• Allows us to go from lines and wire frame images to filled
polygons
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Vector vs. Raster
• Which draws better pictures?
• Which draws better shaded pictures?
• Which draws better color pictures?
• Which draws better lines?
• Which requires more memory?
• Which has a constant display time?
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PCs and Workstations
• Although we no longer make the distinction between
workstations and PCs, historically they evolved from
different roots
• Early workstations characterized by
• Networked connection: client-server model
• High-level of interactivity
• Early PCs included frame buffer as part of user memory
• Easy to change contents and create images
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Computer Graphics 1970-1980
• Pong 1972
• The Black Hole 1979
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Computer Graphics: 1980-1990
Tron 1982
Wrath of Kahn 1982
Last Starfighter 1984
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Computer Graphics: 1980-1990
Realism comes to computer graphics
smooth shading environment
mapping
bump mapping
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Computer Graphics: 1980-1990
• Special purpose hardware
• Silicon Graphics geometry engine
• VLSI implementation of graphics pipeline
• Industry-based standards
• PHIGS
• RenderMan
• Networked graphics: X Window System
• Human-Computer Interface (HCI)
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Computer Graphics: 1990-2000
• Babylon 5 93-98 TV
• Jurrasic Park 1993
• Titanic 1997 (linux rendering)
• The Matrix 1999
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Computer Graphics: 1990-2000
• OpenGL API
• Completely computer-generated feature-length movies
(Toy Story) are successful
• New hardware capabilities
• Texture mapping
• Blending
• Accumulation, stencil buffers
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Computer Graphics: 2000-
• Photorealism
• Graphics cards for PCs dominate market
• Nvidia, ATI
• Game boxes and game players determine direction of
market
• Computer graphics routine in movie industry: Maya,
Lightwave
• Programmable pipelines
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Image Formation
• In computer graphics, we form images which are generally
two dimensional using a process analogous to how images
are 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 that govern how light interacts with the materials
in the scene
• Note the independence of the objects, the viewer, and the
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 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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Luminance and Color Images
• Luminance Image
• Monochromatic
• Values are gray levels
• Analogous to working with black and white film or television
• Color Image
• Has perceptional attributes of hue, saturation, and lightness
• Do we have to match every frequency in visible spectrum? No!
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Three-Color Theory
• Human visual system has two types of sensors
• Rods: monochromatic, night vision
• Cones
• Color sensitive
• Three types of cones: S, M, L
• Name after wavelength they respond to
• Only three values (the tristimulus
values) are sent to the brain
• Need only match these three values
• Can get by with three primary colors
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What is This?
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Color Perception
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Light Source
• What does the power spectrum from varying sources look
like?
• Fluorescent – solid
• Tungsten - dash
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Color Perception
• How is a red Cortland apple perceived under fluorescent
lighting?
• This is called the dominant wavelength
• It is the perceived hue
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Color Terms
• Hue • Dominant wavelength (spectral color)
• Brightness/Luminance • Brightness increases until colors “wash out”
• Luminance is total power of the light.
• Purity/Saturation • How close to a spectral “pure” color
• How many wavelengths make up a color
• Percent of luminance in dominant wavelength
• Chromaticity • Combination of hue and purity
• Metamer • Two colors (objects) that appear the same under one set of conditions and
different under another.
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Optical Illusions
• http://www.youramazingbrain.org.uk/supersenses/astounding.htm
• http://www.echalk.co.uk/amusements/OpticalIllusions/colourPerception/colourPerception.html
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Additive and Subtractive Color
• Additive color • Form a color by adding amounts of three primaries
• CRTs, projection systems, positive film
• Primaries are Red (R), Green (G), Blue (B)
• Subtractive color • Form a color by filtering white light with cyan (C), Magenta (M), and
Yellow (Y) filters
• Light-material interactions
• Printing
• Negative film
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CIE
• In 1931, Commission
Internationale de Eclariage
created three primaries
• They are imaginary primaries
• Adding different amounts of them will
produce all visible colors
• These three colors form a 3-
space
• All perceived hues in diagram
• Plus others
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CIE
• Colors on a line emanating from the
origin have the same hue and
saturation, but different luminance
• Boundary colors correspond to
maximum saturation of a spectral
color
• Except on purple line
• Triangle is a gamut for RGB
• All colors that can be matched by RGB
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CIE Color Regions
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Other Gamuts matched to CIE
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Color Terms
• Spectral Color • Color of the rainbow
• Non-spectral color • A perceivable color that doesn’t have a wavelength (not part of rainbow)
• Colors on the Purple line.
• What about white?
• Dominant wavelength • Line from W through the color C will intersect the dominant wavelength
• White Spot (W) • Equal parts of all primaries
• Complimentary color • Line from color through W will intersect this
• Gamut • Colors that can be represented by a group of primaries
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CIE – Some Relationships
• W – White spot • Achromatic point
• B • Complimentary to A
• Equal quantities of A & B give white
• Non-spectral
• No dominant wavelength
• C • Dominant wavelength for A
• Mix with White to get A
• Complimentary wavelength for B
• D • Complimentary color to A
• Non-spectral (on purple line)
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W
A
B
C
D
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CIE – 3D not 2D
• Sometimes a color appears in the gamut but is not
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Shadow Mask CRT
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Liquid Crystal Display
• Liquid Crystal
• Not solid, not liquid
• Can transmit and change polarized light
• Change alignment with a current
• Place liquid crystals between two polarized panels
• Applying charge to crystals will untwist them allowing light to pass
through (or stop)
• Can be backlit or reflective
• http://static.howstuffworks.com/flash/lcd-twisted.swf
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LCD
• Arrange pixels in a grid with row and column oriented
conductive substrates
• A charge on the correct row and column will light up pixel
• Amount of charge controls how much light passes through
• Generally 256 levels
• Alternating Red, Green, and Blue columns allows for color
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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)
These are 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
• Two-dimensional graphics is a special case of three-
dimensional 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 are 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 seems more physically based so why don’t we use it to design a graphics system?
• Possible and is actually simple for simple objects such as polygons and quadrics with simple point sources
• 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
• Ray tracing with GPUs is close to real time
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Image Formation Revisited
• Can we mimic the synthetic camera model to design
graphics hardware software?
• Application Programmer Interface (API)
• Need only specify
• Objects
• Materials
• Viewer
• Lights
• But how is the API implemented?
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Physical Approaches • Ray tracing: follow rays of light from center of projection until they either are absorbed by objects or go off to infinity • Can handle global effects
• Multiple reflections
• Translucent objects
• Slow
• Must have whole data base
available at all times
• Radiosity: Energy based approach • Very slow
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Practical Approach
• Process objects one at a time in the order they are generated by the application • Can consider only local lighting
• Pipeline architecture
• All steps can be implemented in hardware on the graphics card
application
program display
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Vertex Processing
• Much of the work in the pipeline is in converting
object representations from one coordinate system
to another
• Object coordinates
• Camera (eye) coordinates
• Screen coordinates
• Every change of coordinates is equivalent to a
matrix transformation
• Vertex processor also computes vertex colors
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Projection
• Projection is the process that combines the 3D viewer with
the 3D objects to produce the 2D image
• Perspective projections: all projectors meet at the center of
projection
• Parallel projection: projectors are parallel, center of projection is
replaced by a direction of projection
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Primitive Assembly
Vertices must be collected into geometric objects before
clipping and rasterization can take place
• Line segments
• Polygons
• Curves and surfaces
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Clipping
Just as a real camera cannot “see” the whole world, the
virtual camera can only see part of the world or object
space
• Objects that are not within this volume are said to be clipped out of
the scene
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Rasterization
• If an object is not clipped out, the appropriate
pixels in the frame buffer must be assigned colors
• Rasterizer produces a set of fragments for each
object
• Fragments are “potential pixels”
• Have a location in frame bufffer
• Color and depth attributes
• Vertex attributes are interpolated over objects by
the rasterizer
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Fragment Processing
• Fragments are processed to determine the color of the
corresponding pixel in the frame buffer
• Colors can be determined by texture mapping or
interpolation of vertex colors
• Fragments may be blocked by other fragments closer to
the camera
• Hidden-surface removal
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The Programmer’s Interface
• Programmer sees the graphics system through a software
interface: the Application Programmer Interface (API)
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API Contents
• Functions that specify what we need to form an image
• Objects
• Viewer
• Light Source(s)
• Materials
• Other information
• Input from devices such as mouse and keyboard
• Capabilities of system
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Object Specification
• Most APIs support a limited set of primitives including • Points (0D object)
• Line segments (1D objects)
• Polygons (2D objects)
• Some curves and surfaces
• Quadrics
• Parametric polynomials
• All are defined through locations in space or vertices
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Example (GL 2.X)
glBegin(GL_POLYGON)
glVertex3f(0.0, 0.0, 0.0);
glVertex3f(0.0, 1.0, 0.0);
glVertex3f(0.0, 0.0, 1.0);
glEnd( );
type of object
location of vertex
end of object definition
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Example (GL > 3.X - GPU based)
vec3 points[3];
points[0] = vec3(0.0, 0.0, 0.0);
points[1] = vec3(0.0, 1.0, 0.0);
points[2] = vec3(0.0, 0.0, 1.0);
• Put geometric data in an array
• Send array to GPU
• Tell GPU to render as triangle
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Camera Specification
• Six degrees of freedom
• Position of center of lens
• Orientation
• Lens
• Film size
• Orientation of film plane
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Lights and Materials
• Types of lights • Point sources vs distributed sources
• Spot lights
• Near and far sources
• Color properties
• Material properties • Absorption: color properties
• Scattering
• Diffuse
• Specular
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