Graphics Systems and Models Chapter 1. CS 480/680 2Chapter 1 -- Graphics Systems and Models...

Graphics Systems and Graphics Systems and ModelsModels

Chapter 1Chapter 1

CS 480/680 2Chapter 1 -- Graphics Systems and Models

Introduction:Introduction: Computer GraphicsComputer Graphics

What is it?What is it?

Overview of what we will cover:Overview of what we will cover: A graphics overviewA graphics overview Graphics TheoryGraphics Theory A graphics Software System: OpenGLA graphics Software System: OpenGL

Our approach will be top-down.Our approach will be top-down. We want you to start writing application programs We want you to start writing application programs

that generate graphical output as quickly as possiblethat generate graphical output as quickly as possible


1. Applications of Computer 1. Applications of Computer GraphicsGraphics

The development of Computer Graphics has been The development of Computer Graphics has been driven by the needs of the user community and by driven by the needs of the user community and by the advances in hardware and software.the advances in hardware and software.

Applications can be split into four major areas:Applications can be split into four major areas: Display of informationDisplay of information DesignDesign SimulationSimulation User InterfacesUser Interfaces


1.1 Display of Information1.1 Display of Information Classical graphics techniques Classical graphics techniques

arose as a medium to convey arose as a medium to convey information among people:information among people:

4,000 years ago -- Babylonians: 4,000 years ago -- Babylonians: floor plans of buildings on stonesfloor plans of buildings on stones

2,000 years ago -- Greeks: 2,000 years ago -- Greeks: Architectural ideas Architectural ideas

Now we have Computer-based Now we have Computer-based drafting programsdrafting programs


For centuries -- Cartographers have For centuries -- Cartographers have developed maps to display celestial developed maps to display celestial and geographical information.and geographical information.

Now maps can be developed and Now maps can be developed and manipulated in real-time over the manipulated in real-time over the internet.internet.


Over the past 100 years -- workers in statistics have Over the past 100 years -- workers in statistics have explored techniques for generating plots to convey explored techniques for generating plots to convey informationinformation

Now we have computer plotting packagesNow we have computer plotting packages


Medicine poses interesting and important data-Medicine poses interesting and important data-analysis problemsanalysis problems

CAT Scans, MRI’s ultrasound, and other 3D data CAT Scans, MRI’s ultrasound, and other 3D data producing technologiesproducing technologies


The field of Scientific Visualization provides The field of Scientific Visualization provides graphical tools that help these researchers and others graphical tools that help these researchers and others interpret the vast quantities of data generatedinterpret the vast quantities of data generated


1.2 Design1.2 Design Engineering and Architecture are concerned Engineering and Architecture are concerned

with designwith design starting with a set of specificationstarting with a set of specification seek a cost-effective (and esthetic) solutionseek a cost-effective (and esthetic) solution This is an iterative processThis is an iterative process

The power of interacting with images on the The power of interacting with images on the screen screen

has been known for at least 40 years.has been known for at least 40 years. and today the use of interactive tools pervades the and today the use of interactive tools pervades the

CAD field in areas such as architecture and VLSI CAD field in areas such as architecture and VLSI designdesign


1.3 Simulation1.3 Simulation When did graphics begin to be used?When did graphics begin to be used? Why?Why? The field of VR has opened up many new The field of VR has opened up many new

horizons.horizons. Same (or different) image in each eyeSame (or different) image in each eye position trackingposition tracking interactive devicesinteractive devices This has led to training capabilitiesThis has led to training capabilities

NASA research grantNASA research grant


1.4 User Interfaces1.4 User Interfaces Our interaction with computers has become Our interaction with computers has become

dominated by a visual paradigm that includes:dominated by a visual paradigm that includes: windows,windows, icons,icons, menus, and menus, and a pointing devicea pointing device

We have become so accustomed to this style of We have become so accustomed to this style of interface that we often forget that what we are doing interface that we often forget that what we are doing is working with computer graphics.is working with computer graphics.


2. A Graphics System2. A Graphics System

There are 5 major elements in our systemThere are 5 major elements in our system

Processor, Memory, Frame Buffer, Input Devices, Processor, Memory, Frame Buffer, Input Devices, Output DevicesOutput Devices


2.1 Pixels and the Frame Buffer2.1 Pixels and the Frame Buffer At present, almost all graphics systems are At present, almost all graphics systems are

raster-basedraster-based A picture is produced from an array (the raster) of A picture is produced from an array (the raster) of

picture elements (pixels)picture elements (pixels)

Def: depth of the frame bufferDef: depth of the frame buffer


Definitions:Definitions: Depth of the frame bufferDepth of the frame buffer

1-bit 1-bit 8-bit deep8-bit deep full-color is 24-bit or morefull-color is 24-bit or more

ResolutionResolution the number of pixels in the frame bufferthe number of pixels in the frame buffer

Rasterization or scan conversionRasterization or scan conversion The converting of geometric primitives into pixel The converting of geometric primitives into pixel

assignments in the frame bufferassignments in the frame buffer


2.2 Output Devices2.2 Output Devices The dominate type of display is the Cathode-The dominate type of display is the Cathode-

ray tube (CRT)ray tube (CRT)

Def: refresh rateDef: refresh rate


Color CRT’s Color CRT’s have three different colored phosphors arranged in have three different colored phosphors arranged in

small groups (typically triads).small groups (typically triads).


2.3 Input Devices2.3 Input Devices Most graphics systems provide a keyboard and Most graphics systems provide a keyboard and

at least one other input deviceat least one other input device mousemouse joystickjoystick data tabletdata tablet

We will study these devices in Chapter 3We will study these devices in Chapter 3


3. Images: 3. Images: Physical and SyntheticPhysical and Synthetic

The Usual pedagogical approach:The Usual pedagogical approach: construct raster imagesconstruct raster images

simple 2D entities (points, lines, polygons)simple 2D entities (points, lines, polygons) Define objects based upon 2DDefine objects based upon 2D

Because such functionality is supported by most Because such functionality is supported by most present computer graphics systems, we are going present computer graphics systems, we are going to learn to create images here, rather than expand a to learn to create images here, rather than expand a limited model.limited model.


3.1 Objects and Viewers3.1 Objects and Viewers Two basic entities must be part of any image-Two basic entities must be part of any image-

formation process:formation process: the objectthe object the viewerthe viewer

The object exists in space independent of any The object exists in space independent of any image-formation process, and of any viewer.image-formation process, and of any viewer.


What we will study NowWhat we will study Now Both the object and the viewer exist in a Both the object and the viewer exist in a

3D world. However, the image they 3D world. However, the image they define is 2Ddefine is 2D

The process by which the specification The process by which the specification of the object is combined with the of the object is combined with the specification of the viewer to produce an specification of the viewer to produce an image is the essence of image formation.image is the essence of image formation.

Future:Future: In Chapter 2, we show how OpenGL In Chapter 2, we show how OpenGL

allows us to build simple objectsallows us to build simple objects In Chapter 9 we learn how to define In Chapter 9 we learn how to define

objects in a manner that incorporates objects in a manner that incorporates relationships among objects.relationships among objects.


3.2 Light and Images3.2 Light and Images Much information was missing from the Much information was missing from the

preceding picture:preceding picture: We have yet to mention light!We have yet to mention light!

If there were no light sources the objects would be dark, If there were no light sources the objects would be dark, and there would be nothing visible in our image.and there would be nothing visible in our image.

We have not mentioned how color enters the pictureWe have not mentioned how color enters the picture Or, what are the effects of different kinds of Or, what are the effects of different kinds of

surfaces on the objects.surfaces on the objects.


Taking a more physical approach, we can start Taking a more physical approach, we can start with the following arrangement:with the following arrangement:

The details of the interaction between light and the The details of the interaction between light and the surfaces of the object determine how much light surfaces of the object determine how much light enters the camera.enters the camera.


Light is a form of electromagnetic radiation:Light is a form of electromagnetic radiation:


3.3 Ray Tracing3.3 Ray Tracing We can start building an imaging model by We can start building an imaging model by

following light from a source.following light from a source.


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


4. The Human Visual System4. The Human Visual System

Our extremely complex visual system has all the Our extremely complex visual system has all the components of a physical imaging system, such as a components of a physical imaging system, such as a camera or a microscope.camera or a microscope.


5. The Pinhole Camera5. The Pinhole Camera

What is one?What is one?

A Pinhole camera is a box A Pinhole camera is a box with a small hole in the center on one side, with a small hole in the center on one side, and the film on the opposite sideand the film on the opposite side


Pinhole camera near Cliff House in San FranciscoPinhole camera near Cliff House in San Francisco


6. The Synthetic Camera Model6. The Synthetic Camera Model

Consider the imaging system shown here:Consider the imaging system shown here:


A few Basic principles:A few Basic principles: First, the specification of the objects is independent First, the specification of the objects is independent

of the specification of the viewer.of the specification of the viewer. In a graphics library we would expect separate functions In a graphics library we would expect separate functions

for specifying objects and the viewer.for specifying objects and the viewer.

Second, we can compute the image using simple Second, we can compute the image using simple trigonometric calculationstrigonometric calculations


Because of this, we can move the image plane in Because of this, we can move the image plane in front of the lens (call this the projection plane) and front of the lens (call this the projection plane) and end up with:end up with:

In Chapter 5, we discuss this process in detail and In Chapter 5, we discuss this process in detail and derive the relevant mathematical formulasderive the relevant mathematical formulas


We must also consider the limited size of the image. We must also consider the limited size of the image.

Therefore, we must discuss clipping:Therefore, we must discuss clipping:


7. The Programmer’s Interface7. The Programmer’s Interface

There are numerous ways that a user can interact There are numerous ways that a user can interact with a graphics system.with a graphics system.

In a typical paint program it would look like:In a typical paint program it would look like:


7.1 Application Programmer’s Interfaces7.1 Application Programmer’s Interfaces What is an API?What is an API?

Why do you want one?Why do you want one?


If we are to follow the synthetic camera model, If we are to follow the synthetic camera model, we need functions in the API to specify:we need functions in the API to specify:

ObjectsObjects ViewerViewer Light SourcesLight Sources Material PropertiesMaterial Properties


Objects are usually defined by a set of verticesObjects are usually defined by a set of vertices

The following code fragment defines a triangular The following code fragment defines a triangular polygon in OpenGLpolygon in OpenGL

glBegin(GL_POLYGON);glBegin(GL_POLYGON); glVertex3f(0.0,0.0,0.0);glVertex3f(0.0,0.0,0.0); glVertex3f(0.0,1.0,0.0);glVertex3f(0.0,1.0,0.0); glVertex3f(0.0,0.0,1.0);glVertex3f(0.0,0.0,1.0); glEND();glEND();


We can define a viewer or camera in a variety We can define a viewer or camera in a variety of waysof ways

We can identify four types of necessary We can identify four types of necessary specifications:specifications:

PositionPosition OrientationOrientation Focal lengthFocal length Film planeFilm plane


Light sources can be defined by their location, Light sources can be defined by their location, strength, color, and directionality.strength, color, and directionality.

API’s provide a set of functions to specify these API’s provide a set of functions to specify these parameters for each source.parameters for each source.

Material properties are characteristics, or Material properties are characteristics, or attributes, of the objectsattributes, of the objects

such properties are usually defined through a series such properties are usually defined through a series of function calls at the time that each object is of function calls at the time that each object is defined.defined.


7.2 A Sequence of Images7.2 A Sequence of Images In Chapter 2 , we begin our detailed discussion In Chapter 2 , we begin our detailed discussion

of the OpenGL APIof the OpenGL API Color Plates 1 through 8 show what is possible Color Plates 1 through 8 show what is possible

with available hardware and a good API, but with available hardware and a good API, but also they are not difficult to generatealso they are not difficult to generate


8. Graphics Architectures8. Graphics Architectures

On one side of the API is the application program. On one side of the API is the application program. On the other is some combination of hardware and On the other is some combination of hardware and software that implements the functionality of the software that implements the functionality of the APIAPI

Researchers have taken various approaches to Researchers have taken various approaches to developing architectures to support graphics APIsdeveloping architectures to support graphics APIs


Early systems used general purpose computersEarly systems used general purpose computers single processing unitsingle processing unit

In the early days of computer graphics, computers In the early days of computer graphics, computers were so slow that refreshing even simple images, were so slow that refreshing even simple images, containing a few hundred line segments, would containing a few hundred line segments, would burden an expensive computerburden an expensive computer


8.1 Display Processors8.1 Display Processors The earliest attempts to build a special purpose The earliest attempts to build a special purpose

graphics system were concerned primarily with graphics system were concerned primarily with relieving the task of refreshing the displayrelieving the task of refreshing the display


8.2 Pipeline Architectures8.2 Pipeline Architectures The major advances in graphics architectures The major advances in graphics architectures

parallel closely the advances in workstations.parallel closely the advances in workstations. For computer graphics applications, the most For computer graphics applications, the most

important use of custom VLSI circuits has been in important use of custom VLSI circuits has been in creating pipeline architecturescreating pipeline architectures

A simple arithmetic pipeline is shown here:A simple arithmetic pipeline is shown here:


If we think in terms of processing the geometry of If we think in terms of processing the geometry of our objects to obtain an image, we can use the our objects to obtain an image, we can use the following block diagram:following block diagram:

We will discuss the details of these steps in We will discuss the details of these steps in subsequent chapters.subsequent chapters.


8.3 Transformations8.3 Transformations Many of the steps in the imaging process can be Many of the steps in the imaging process can be

viewed as transformations between viewed as transformations between representations of objects in different representations of objects in different coordinate systemscoordinate systems

for example: from the system in which the object for example: from the system in which the object was defined to the system of the camera was defined to the system of the camera

We can represent each change of coordinate We can represent each change of coordinate systems by a matrixsystems by a matrix

We can represent successive changes by multiplying We can represent successive changes by multiplying (or concatenating) the individual matrices into a (or concatenating) the individual matrices into a single matrix.single matrix.


8.4 Clipping8.4 Clipping Why do we Clip?Why do we Clip?

Efficient clipping algorithms are developed in Efficient clipping algorithms are developed in Chapter 7Chapter 7


8.5. Projection8.5. Projection In general three-dimensional objects are kept in In general three-dimensional objects are kept in

three dimensions as long as possible, as they three dimensions as long as possible, as they pass through the pipeline. pass through the pipeline.

Eventually though, they must be projected into Eventually though, they must be projected into two-dimensional objects.two-dimensional objects.

There are various projections that we can There are various projections that we can implement.implement.

We shall see in Chapter 5 that we can We shall see in Chapter 5 that we can implement this step using 4 x 4 matrices, and, implement this step using 4 x 4 matrices, and, thus, also fit it into the pipeline.thus, also fit it into the pipeline.


8.6 Rasterization8.6 Rasterization Finally, our projected objects must be Finally, our projected objects must be

represented as pixels in the frame buffer.represented as pixels in the frame buffer. We discuss this scan-conversion or rasterization We discuss this scan-conversion or rasterization

process in Chapter 7process in Chapter 7


8.7 Performance Characteristics8.7 Performance Characteristics There are two fundamentally different types of There are two fundamentally different types of

processing processing Front end -- geometric processing, based on the Front end -- geometric processing, based on the

processing of verticesprocessing of vertices ideally suited for pipelining, and usually involves ideally suited for pipelining, and usually involves

floating-point calculations.floating-point calculations. The geometry engine developed by SGI was a VLSI The geometry engine developed by SGI was a VLSI

implementation for many of these operations in a special implementation for many of these operations in a special purpose chip. These became the basis for a series of purpose chip. These became the basis for a series of graphics workstations.graphics workstations.

Back end -- involves direct manipulation of bits in Back end -- involves direct manipulation of bits in the frame buffer.the frame buffer.

Ideally suited for parallel bit processors.Ideally suited for parallel bit processors.


9. Summary9. Summary

In this chapter we have set the stage for our top-In this chapter we have set the stage for our top-down development of computer graphics.down development of computer graphics.

Computer graphics is a method of image formation that Computer graphics is a method of image formation that should be related to classical methods -- in particular to should be related to classical methods -- in particular to camerascameras

Our next step is to explore the application side of Our next step is to explore the application side of Computer Graphics programmingComputer Graphics programming

We will use the OpenGL APIWe will use the OpenGL API


10. Suggested Readings10. Suggested Readings

Journals:Journals: Computer GraphicsComputer Graphics -- ACM -- ACM IEEE Compute Graphics and ApplicationsIEEE Compute Graphics and Applications

Textbooks:Textbooks: Foley et.al.Foley et.al. Hearn and BakerHearn and Baker HillHill

Graphics Systems and Models Chapter 1. CS 480/680 2Chapter 1 -- Graphics Systems and Models...

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