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CAP 4703
Computer Graphic
Methods
Prof. Roy Levow
Lecture 1
Computer Graphics
 Broadly
concerned with producing
images by computer
 Many applications
– Still image generation and editing
– Animation and film, Computer games
– Design
– Scientific and medical data visualization
– Simulation and Virtual reality
– Publishing, …
Graphical System

Main Components
1.
2.
3.
4.
5.
Processor
Memory
Frame Buffer
Output Devices
Input Devices
Processor and Memory
 Depending
on the application, the
system can range from
Capable
personal computer
to a
Super-computer
Image Output
 Images
may be
– Vector graphics
 Formed
by drawing lines to create image
– Raster graphics
 Composed
of individual picture elements in
an array or raster
 Individual elements are pixels
 Stored in a frame buffer
 Raster
graphics is most common
– Vector graphics provides better
scalability
Graphic System Diragram
Frame Buffer
 Normally
– Video RAM (VRAM) – designed to
support fast transfer of large amounts
of data
– Dynamic RAM (DRAM)
 Number
of bits per pixel is the depth
of the frame buffer
– Determines number of distinct color
values
 B&W=1
 Full
color >= 24
Frame Buffer .2
 True
RGB
color systems are also known as
– Number of bits per pixel is divided by 3
– Each group represents one of the colors
 Red,
Green, Blue
 Resolution
is the number of pixels
per frame buffer
– Usually reported as width x height
Frame Buffer .3
 To
reduce number of bits per pixel, a
color buffer may be used
– Color buffer holds actual RGB values
– Frame buffer holds index into color
buffer
– Reduces size when number of colors
used is small relative to total number
possible
Rasterization Example
Grapihic Processor
 Processing
of graphic information
may be done by
– Normal CPU
– Specialized Graphic Processor
 High
performance graphic display
systems generally use separate
graphic processor
– Optimized for image processing and
display
 Specialized
throughput
operations and high data
Output Devices
 Previously,
most common output
device was the cathode-ray tube or
CRT
– Image is created by a moving electron
beam that causes a coating on the tube
face to glow
– Most commonly CRT scans a line at a
time across the screen
 But
may be random scan
CRT Operation
CRT
 The
image is not retained but must
be refreshed (recreated) periodically
– Most commonly at least 50 times per
second
 This
 Scan
is the refresh rate
lines may be produced
consecutively, non-interlaced
 Or half at a time, interlaced
Color CRT
 Each
screen pixel consists of three
closely spaced color dots, a RGB
triad
 Shadow mask helps focus electron
beam on a single dot
Color CRT Image
LCD Display
 Each
pixel is a layered solid state
device that can be turned on or off to
produce that color component
 Generally cannot support as rapid
refresh as CRT
Input Devices
 Keyboard
 Pointing
devices
– Mouse
– Joystick
– Game console
– Data tablet
Images
 Physical
image is generated by
physical object
 Computer graphic systems generate
synthetic image
– Produced by program
Objects and Viewers
 World
is populated with three
dimensional objects
 How object is seen depends on
relationship to viewer and other
attributes
 Generally a viewer can see only
some parts of an object
 Different relationships between
object and viewer can produce
different images
Views of an Object
Camera System to Define Image
Light
 Without
light all would be uniformly
black
 Light makes images visible and
changes features through
– Intensity
– Color
– Direction
 Interaction
with light is complex
Camera withLight
Light: Electromagnetic Radiation
Light
 Visible
spectrum has wavelengths
between 350 nm and 780 nm
 Color depends on wavelength
– Blue ~450 nm
– Green ~520 nm
– Red ~650 nm
Modeling Light
 Geometric
Optics
– Light assumed to come from point
source
– Fixed intensity
– Travels in straight lines
 Assume
monochromatic (single
color)
 More complex sources can be viewed
as collection of point sources
Ray Tracing
 Effect
of light can be viewed by
following ray from source to viewer
 Light travels in straight line until it
hits surface
 Surface point then acts as new point
source
 Ray behavior determined by trig laws
 Infinite possibilities but only those
that reach viewer matter
Ray Tracing
Radiosity
 When
surface scatters incoming light
in all directions, ray tracing does not
produce accurate results
– Energy conservation based calculation is
more accurate but also more
computationally intensive
Human Visual System
 Light
enters eye through lens
 Focused on retina at back of eye
 Retina contains light sensitive cells
– Rods – brightness only
– Cones – three kinds for three colors
– RGB cones allow RGB displays to
function
 Sensitivity
is not uniform
Sensitivity Curves
Pinhole Camera
A
simple model of a camera
– Assumes light enters through a pin hole
 Small
enough that only one ray enters in
any direction
Pinhole Camera Image
 Can
calculate point where ray from
point source is projected on back of
camera yp = -y/(z/d), similar for x
Pinhole Camera Image .2
 Point
at back of camera is called the
projection of the source
 Field or angle of view is angle made
by triangle from lens to image plane
 Θ=2 arctan(h/2d)
Imaging System
Synthetic Camera Model
 Standard
approach for threedimensional computer graphics
 Compute image that would be
captured by bellows camera
– Bellows allows depth of camera to be
changed as desired
 Image
is formed on projection plane,
where back of camera would be
Views of Image Formation
Synthetic Camera Image
Clipping Window or Rectangle
 Determines
the image
the edge boundaries of
Programmer’s Interface
 Visual
interface
– Allows image creation by arranging
visual components
 Programming
interface
– Function library
– Application Programmer’s Interface
(API)
 Defines
available operations
 Images build combining operations
Paint Program Interface
Pen-Plotter Model
Typical of early vector drawing systems
 Move pen from point to point drawing lines
to create image
moveto(0, 0);
lineto(1, 0);
lineto(1, 1);
lineto(0, 1);
lineto(0, 0);

Pen-Plotter Image
 Adding
additional lines could produce
the image of a cube
Pen-Plotter Model
 Limited
functionality
 Difficult to produce threedimensional images
Three-Dimensional APIs
 Synthetic
– OpenGL
– PHIGS
– Direct3D
– VRML
– JAVA-3D
Camera model is common
3-D API Components
 Objects
 Viewer
 Light
Sources
 Material Properties
Primitive Objects
 Points
 Line
segments
 Polygons
 Text
 Curves
 Surfaces
OpenGL Object Definition
 List
of components bounded by
function calls
 A triangle
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();
Camera or Viewer
 Specification
attributes
– Position
– Orientation
– Focal length
– Film plane
requires a number of
Camera Specification
Sequential Design Process
 Images
are usually generated as a
sequence of successively more
refined images
 See color plates 1-8 in text
Model-Rendering Paradigm
Graphic Architectures
 Display
Processors
 Pipeline Architectures
 Transformations
 Clipping
 Projection
 Rasterization