Topic 2 Graphics Systems and Models.ppt

Topic 2 Graphics Systems and
Models
Dr. Collins Oduor
Kenyatta University
Chapter 1 -- Graphics Systems and Models 1

2
Chapter 1 -- Graphics Systems and
Models
2. A Graphics System
 There are 5 major elements in our system
 Processor, Memory, Frame Buffer, Input Devices, Output
Devices

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Models
 2.1 Pixels and the Frame Buffer
 At present, almost all graphics systems are raster-
based
 A picture is produced from an array (the raster) of
picture elements (pixels)
 Def: depth of the frame buffer

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Models
 Definitions:
 Depth of the frame buffer
 1-bit
 8-bit deep
 full-color is 24-bit or more
 Resolution
 the number of pixels in the frame buffer
 Rasterization or scan conversion
 The converting of geometric primitives into pixel
assignments in the frame buffer

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Models
 2.2 Output Devices
 The dominate type of display is the Cathode-ray
tube (CRT)
 Def: refresh rate

 Graphics System
 Output Device
 The general display is cathode-ray tube (CRT)
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Chapter 1 -- Graphics Systems and Models

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Models
 Color CRT’s
 have three different colored phosphors arranged in
small groups (typically triads).

 Graphics System
 Input Devices
 The system is usually equipped with pointing devices (location
+ buttons).
 The common devices are mouse, joystick, and data tablet.
 Data gloves or computer vision system can provide higher-
dimensional data.
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Introduction
 Graphics System
 Input Devices
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 Graphics System
 Input Devices
10

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Models
 2.3 Input Devices
 Most graphics systems provide a keyboard and at
least one other input device
 mouse
 joystick
 data tablet
 We will study these devices in Chapter 3

 Graphics System
 Processors
 In a simple system, there are only one processor for normal
processing and graphical processing.
 Today, all graphics system are characterized by special-
purpose graphical processing unit (GPU)
 It takes charge of a conversion of geometric entities to pixel
in the frame buffer (Rasterization).
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 Graphics System
 Pixels
 An image is produced as an array (raster) of picture elements
(pixel).
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row
column

 Graphics System
 Frame Buffer
 It is a part of memory to store the set of pixels.
 It is usually implemented with special type of memory chips
to enable redisplaying.
 Resolution: the number of pixels in frame buffer
 Depth or Precision: the number of bits used for each pixel.
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Image Formation
 Description
 Image formation is a process analogous to how images
are formed by physical systems
 Human Visual System
 Camera
 The process combining the specification of objects with
viewers to produce 2D image.
 Object is an entity existing in space independent of any
image-formation process and any viewer.
 Viewers is a way to from images from objects.
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Image Formation
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Images seen by three different views
Camera View
Object

Image Formation
 Light Description
 Light is a form of electromagnetic radiation
 Wavelengths
 Frequencies
 The wavelengths in the range of 350 (nm) to 780 nm is
called visible spectrum
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Image Formation
 Pinhole Camera Model
 A pinhole camera is a box with a small hole in the center of
one side.
 The film is placed inside the box on the side opposite the
pinhole
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Image Formation
 Point Projection
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( , , )
p p
x y d

/
p
x
x
z d

/
p
y
y
z d


Image Formation
 Field or View of Angle
 The angle made by the largest object that our image can
image on its film plane.
20
/ 2
tan
2
h
d


1 / 2
tan
2
h
d
 
 
1
2tan
2
h
d
 
 


Image Formation
 Depth of Field
 The longest distance that the objects can appear on its
film plane.
 The ideal pinhole camera model has an infinite depth of
field.
 Limitations
 The pinhole is so small that it admits only a single ray from
a point source
 The camera cannot be adjusted to have different angle of
view.
21

Image Formation
22
視網膜上兩種視覺接受器：
• 錐細胞錐細胞 (cone): 6~7 million
• 桿細胞桿細胞 (rod):75~150
million

Image Formation
23
人類的視覺神經路徑人類的視覺神經路徑
網膜→視交叉→側膝核→視皮質網膜→ 視交叉→
側膝核→視皮質

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Models
3. Images:
Physical and Synthetic
 The Usual pedagogical approach:
 construct raster images
 simple 2D entities (points, lines, polygons)
 Define objects based upon 2D
 Because such functionality is supported by most present
computer graphics systems, we are going to learn to
create images here, rather than expand a limited model.

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Models
 3.1 Objects and Viewers
 Two basic entities must be part of any image-
formation process:
 the object
 the viewer
 The object exists in space independent of any image-
formation process, and of any viewer.

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Models
 3.2 Light and Images
 Much information was missing from the preceding
picture:
 We have yet to mention light!
 If there were no light sources the objects would be dark,
and there would be nothing visible in our image.
 We have not mentioned how color enters the picture
 Or, what are the effects of different kinds of surfaces
on the objects.

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Models
 Taking a more physical approach, we can start with
the following arrangement:
 The details of the interaction between light and the
surfaces of the object determine how much light
enters the camera.

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Models
 Light is a form of electromagnetic radiation:

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Models
 3.3 Ray Tracing
 We can start building an imaging model by following
light from a source.

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Models
 Ray tracing is an image formation technique that is
based on these ideas and that can form the basis for
producing computer generated images.

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Models
4. The Human Visual System
 Our extremely complex visual system has all the components
of a physical imaging system, such as a camera or a
microscope.

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Models
5. The Pinhole Camera
 What is one?
 A Pinhole camera is a box
 with a small hole in the center on one side,
 and the film on the opposite side

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Models
 Pinhole camera near Cliff House in San Francisco

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Models
6. The Synthetic Camera Model
 Consider the imaging system shown here:

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 A few Basic principles:
 First, the specification of the objects is independent of
the specification of the viewer.
 In a graphics library we would expect separate functions
for specifying objects and the viewer.
 Second, we can compute the image using simple
trigonometric calculations

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Models
 Because of this, we can move the image plane in front
of the lens (call this the projection plane) and end up
with:
 In Chapter 5, we discuss this process in detail and
derive the relevant mathematical formulas

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Models
 We must also consider the limited size of the image.
 Therefore, we must discuss clipping:

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7. The Programmer’s Interface
 There are numerous ways that a user can interact with a
graphics system.
 In a typical paint program it would look like:

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Models
 7.1 Application Programmer’s Interfaces
 What is an API?
 Why do you want one?

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Models
 If we are to follow the synthetic camera model, we
need functions in the API to specify:
 Objects
 Viewer
 Light Sources
 Material Properties

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Models
 We can define a viewer or camera in a variety of
ways
 We can identify four types of necessary specifications:
 Position
 Orientation
 Focal length
 Film plane

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Models
 Light sources can be defined by their location,
strength, color, and directionality.
 API’s provide a set of functions to specify these
parameters for each source.
 Material properties are characteristics, or attributes,
of the objects
 such properties are usually defined through a series of
function calls at the time that each object is defined.

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Models
 7.2 A Sequence of Images
 In Chapter 2 , we begin our detailed discussion of
the OpenGL API
 Color Plates 1 through 8 show what is possible with
available hardware and a good API, but also they are
not difficult to generate

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Models
8. Graphics Architectures
 On one side of the API is the application program. On the
other is some combination of hardware and software that
implements the functionality of the API
 Researchers have taken various approaches to developing
architectures to support graphics APIs

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 Early systems used general purpose computers
 single processing unit
 In the early days of computer graphics, computers
were so slow that refreshing even simple images,
containing a few hundred line segments, would
burden an expensive computer

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Models
 8.1 Display Processors
 The earliest attempts to build a special purpose
graphics system were concerned primarily with
relieving the task of refreshing the display

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 8.2 Pipeline Architectures
 The major advances in graphics architectures parallel
closely the advances in workstations.
 For computer graphics applications, the most
important use of custom VLSI circuits has been in
creating pipeline architectures
 A simple arithmetic pipeline is shown here:

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Models
 If we think in terms of processing the geometry of our
objects to obtain an image, we can use the following
block diagram:
 We will discuss the details of these steps in
subsequent chapters.

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Models
 8.3 Transformations
 Many of the steps in the imaging process can be
viewed as transformations between representations
of objects in different coordinate systems
 for example: from the system in which the object was
defined to the system of the camera
 We can represent each change of coordinate
systems by a matrix
 We can represent successive changes by multiplying
(or concatenating) the individual matrices into a single
matrix.

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Models
 8.4 Clipping
 Why do we Clip?
 Efficient clipping algorithms are developed in
Chapter 7

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Models
 8.5. Projection
 In general three-dimensional objects are kept in
three dimensions as long as possible, as they pass
through the pipeline.
 Eventually though, they must be projected into two-
dimensional objects.
 There are various projections that we can
implement.
 We shall see in Chapter 5 that we can implement
this step using 4 x 4 matrices, and, thus, also fit it
into the pipeline.

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Models
 8.6 Rasterization
 Finally, our projected objects must be represented
as pixels in the frame buffer.
 We discuss this scan-conversion or rasterization
process in Chapter 7

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Models
 8.7 Performance Characteristics
 There are two fundamentally different types of
processing
 Front end -- geometric processing, based on the
processing of vertices
 ideally suited for pipelining, and usually involves floating-
point calculations.
 The geometry engine developed by SGI was a VLSI
implementation for many of these operations in a special
purpose chip. These became the basis for a series of
graphics workstations.
 Back end -- involves direct manipulation of bits in the
frame buffer.
 Ideally suited for parallel bit processors.

Programmer Interface
 Description
 There are numerous ways that a user can interact with a
graphics system.
 A user develops images through interactions with display
using input devices.
 Someone develops the code for the application
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 API Functions
 It is to provide interfaces between an application program
and a graphics system.
 API in OpenGL
 It is to specify how to form an image: (1) Objects, (2) Viewer,
(3) Light Source(s), (4) Materials
 The APIs provided by OpenGL is based on the synthetic-
camera model.
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 Objects
 Objects are usually defined by sets of vertices
 Most APIs provide sets of primitive objects (primitives) for
the user.
 Points (0D object)
 Line segments (1D objects)
 Polygons (2D objects)
 Some curves and surfaces
 OpenGL programs define primitives through lists of
vertices.
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 Example
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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

 Example
 glBegin() and glEnd()brackets the calls to define the
object to be drawn.
 OpenGL Commands
 Prefix: gl
 Initial capital letters for each word
 Optional Character: number of arguments or type
 OpenGL Constants
 Prefix: GL
 All characters are capitilized
 Underscores among Words.
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glVertex3f(0.0, 0.0, 0.0);
glBegin(GL_POLYGON)

 Viewer
 Available APIs differ in how much flexibility in camera
selection.
 Camera Specifications
 Position
 Orientation
 Focal Length
 Film Plane.
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 Light
 Point sources v.s distributed sources
 Spot lights
 Near and far sources
 Color properties
 Material Properties
 Absorption: color properties
 Scattering
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Graphics Architecture
 General Description
 Two Sides of API
 Application Program
 Combination of hardware and software that implements the
functionality of the API.
 Various approaches are to develop graphics architectures
for supporting graphics APIs.
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 Early Graphics System
 It used general-purposed computers characterized by a
CPU.
 CPU Task
 Run the application program
 Compute the endpoints of line segment.
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 Display Processors
 The display processors include instructions to display
primitives.
 The display primitives are stored in its own memory
denoted as display list.
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 Pipeline Architecture
 The creation of special-purpose VLSI chips enables the
pipeline technology.
 The pipeline is workable in graphics because it processes
data in the same manner.
 For two stage pipeline, the throughput of the system has
been doubled.
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a+(b*c)

 Scene Geometry
 Scene geometry is the collection of primitives and vertices.
 A scene consists of a set of objects.
 Each object comprises a set of primitives.
 A primitive comprises a set of vertices.
 In a complex scene, there are may be thousands of
vertices.
 We must process all these vertices in a similar manner to
form an image in the frame buffer.
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 Graphics Pipeline
 The imaging processing includes four steps.
 Vertex Processing
 Clipping and Primitive Assembly
 Rasterization
 Fragment Processing
 These four steps can be performed in a pipeline manner.
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 Vertex Processing
 Coordinate Transformation: imaging process is a
transformation between different coordinate system.
 Vertex Coloring
 Program Specification
 Computation from a realistic shading model.
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 Determine the area in the world that can be seen by the
camera.
 Clipping Volume
 The projection in this volume appear in the image.
 Those that are outside are to be clipped out.
 The clipping is done in primitive level rather than vertex
level.
 The output of this stage is a set of primitives whose
projections can appear in the image.
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3D Clipping Volume
2D Clipping Volume

 Rasterization (Drawing)
 It converts the primitives in terms of vertices to pixels in
the frame buffer.
 The output is a set of fragments for each primitive, that
is, pixel with color or location
 Fragment Processing
 It takes the generated fragments to update the pixels in
the frame buffer.
 The pixels in frame buffer can be read to blend with the
fragment.
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Topic 2 Graphics Systems and Models.ppt

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