Computer Graphics
Introduction
Introduction
Computer graphics is concerned with all aspects of producing pictures or
images using a computer. The field began humbly almost 50 years ago, with
the display of a few lines on a cathode-ray tube (CRT); now, we can create
images by computer that are indistinguishable from photographs of real
objects.
APPLICATIONS OF COMPUTER GRAPHICS
The development of computer graphics has been driven both by the needs of
the user community and by advances in hardware and software. The
applications of computer graphics are many and varied; we can, however,
divide them into four major areas:
1. Display of information
2. Design
3. Simulation and animation
4. User interfaces
Although many applications span two or more of these areas, the
development of the field was based on separate work in each.
A GRAPHICS SYSTEM
A computer graphics system is a computer system; as such, it must have all
the components of a general-purpose computer system. Let us start with the
high-level view of a graphics system, as shown in the block diagram in Figure
There are five major elements in our system:
1. Input devices
2. Processor
3. Memory
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Compu
uter Graphics
Introducti
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ion
4. Fra
ame bufffer
5. Ou
utput devvices
Pixels
s and th
he Frame Buffe
er
Prese
ently, almost all graphiccs system
ms are raster b
based. A picture is
produce
ed as an
n array —the
—
rastter— of picture
p
elements, or pixels
s, within the
t
graphiccs system
m. Each pixel corrresponds to a lo
ocation, o
or small area, intthe
image. Collectivvely, the pixels
p
are
e stored in a part of
o memorry called the frame
e
buffe
er. The frrame bufffer can b
be viewed as the core ele
ement of a graph
hics
system. Its reso
olution—tthe numb
ber of pix
xels in th
he frame
e buffer—
—determin
nes
the dettail that you
y can see
s in the
e image. The depth, or pre
ecision, of
o the fram
me
buffer, defined as
a the nu
umber off bits thatt are use
ed for eacch pixel, determin
nes
propertties such as how many co
olors can
n be reprresented on a giv
ven syste
em.
For exa
ample, a 1-bit-dee
ep frame
e buffer allows
a
only two co
olors, whe
ereas an 8bit-deep frame buffer
b
allo
ows 28 (2
256) colo
ors. In full-color syystems, th
here are 24
(or morre) bits pe
er pixel. Such
S
sysstems can
n display sufficien
nt colors to
t represe
ent
most im
mages rea
alistically
y.
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Computer Graphics
Introduction
Objects and Viewers
Two basic entities must be part of any image-formation process, be it
mathematical or physical: object and viewer. The object exists in space
independent of any image-formation process and of any viewer. In computer
graphics, where we deal with synthetic objects, we form objects by specifying
the positions in space of various geometric primitives, such as points, lines,
and polygons. In most graphics systems, a set of locations in space, or of
vertices, is sufficient to define, or approximate, most objects. For example, a
line can be specified by two vertices; a polygon can be specified by an ordered
list of vertices;
Every imaging system must provide a means of forming images from
objects. To form an image, we must have someone or something that is
viewing our objects, be it a person, a camera, or a digitizer. It is the viewer that
forms the image of our objects.
Light and Images
The preceding description of image formation is far from complete. For
example, 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.
Light travels in straight lines, from the sources to those objects with which it
interacts. An ideal point source emits energy from a single location at one or
more frequencies equally in all directions. More complex sources, such as a
light bulb, can be characterized as emitting light over an area and by emitting
more light in one direction than another.
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Compu
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Introducti
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Image
e Forma
ation Mo
odels
There
e are mu
ultiple ap
pproachess to how
w we can
n form im
mages fro
om a sett of
objectss, the light-reflectin
ng properrties of th
hese obje
ects, and the prope
erties of the
t
light so
ources in the scene. We ca
an start building an
n imaging
g model by
b following
light fro
om a sourrce.
Ray trracing and
a pho
oton ma
apping
Theyy are imag
ge-forma
ation tech niques th
hat are ba
ased on tthese ide
eas and th
hat
can forrm the ba
asis for prroducing compute
er-genera
ated imag
ges. We can
c use the
t
ray-traccing idea to simula
ate physiical effec
cts as com
mplex as we wish, as long as
we are willing to
o carry ou
ut the req uisite com
mputing.
THE P
PROGRA
AMMER
R'S INTE
ERFACE
E
The iinterface between
n an appliication prrogram and a grap
phics sys
stem can be
specifie
ed throug
gh a set of functi ons that resides in a gra phics library. The
ese
nterface (API). The
specificcations are
a called
d the ap
pplication
n program
mmer's in
T
applica
ation prog
grammer's model of the sy
ystem is shown in
n Figure below. The
T
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Compu
uter Graphics
Introducti
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applica
ation prog
grammer sees onlyy the API and is thus shiellded from
m the deta
ails
of both the hard
dware an
nd the so
oftware im
mplementtation of the graphics libra
ary.
The software drrivers are
e responssible for interpretin
i
ng the ou
utput of the
t API and
a
convertting these
.
e data to a form th
hat is und
derstood by the pa
articular hardware
h
GRAP
PHICS ARCHITE
A
ECTURES
On o
one side of
o the AP
PI is the a
application program
m. On the
e other side
s
is som
me
combin
nation of hardware
e and sofftware that implem
ments the
e function
nality of the
t
API.
1. D
Display Process
sors
The earliest attempts
s to bu ild special-purpose graph
hics sys
stems we
ere
concern
ned primarily with
h relieving
g the gen
neral-purp
pose com
mputer fro
om the ta
ask
of refrreshing the
t
display conttinuously. These display
y processors had
h
conven
ntional arrchitecturres (Figu re 1.29) but included insstructions
s to display
primitivves on the
e CRT
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Compu
uter Graphics
Introducti
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2. P
Pipeline Archite
ectures
The concept of pipelin
ning is illu
ustrated in Figure
e 1.30 forr a simple
e arithme
etic
calculation. In our
o pipeliine, there
e is an adder
a
and
d a multiiplier. If we
w use this
t
configu
uration to
o compu
ute a + (b * c),
c then the calcculation takes one
o
multiplication an
nd one addition th
he same amount of work rrequired if we use
ea
single p
processo
or to carry
y out both
h operations. How
wever, su
uppose th
hat we ha
ave
to carryy out the same co
omputatio
on with many
m
values of a,, b, and c.
c Now, the
t
multiplier can pa
ass on the results of its
calcu
ulation to the adde
er and ca
an start its
i next multiplica
m
tion while
e the add
der
carries out the second step
s
of th
he calculation on the first set of da
ata. Hence,
wherea
as it takess the sam
me amou
unt of time to calc
culate the
e results for any one
o
set of d
data, whe
en we are
e working
g on two sets
s
of da
ata at one
e time, ou
ur total tim
me
for calcculation iss shortened marke
edly. Here the rate
e at which
h data flo
ows throu
ugh
the sysstem, the throughp
put of the
e system
m, has been doubl ed. Note
e that as we
add mo
ore boxess to a pipeline, the
e latency of the sy
ystem inccreases and we must
balance
e latencyy against increase d through
hput in evaluating
g the perfformance
e of
a pipeline.
We ccan consttruct pipe
elines for more com
mplex arithmetic ccalculations that will
w
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Compu
uter Graphics
Introducti
I
ion
afford
d even greater
g
increases in throug
ghput. Off course, there is no pointt in
building
g a pipeline unles
ss we wi ll do the same operation on many
y data se
ets.
where la
But tha
at is just what
w
we do in co
omputer graphics,
g
arge sets
s of verticces
must be
e processsed in the
e same m
manner.
The G
Graphics
s Pipelin
ne
We sstart with
h a set of
o objectss. Each object comprisess a set of
o graphical
primitivves. Each
h primitiv
ve comprrises a set
s of ve
ertices. W
We can think
t
of the
t
collectio
on of prrimitive ty
ypes and
d vertice
es as defining th e geome
etry of the
t
scene. In a co
omplex scene,
s
th
here ma
ay be thousands—
—even millions—
—of
verticess that deffine the objects.
o
W
We must process all these
e vertices in a similar
manner to form an image
e in the fframe bufffer. If we
e think in terms off processing
the geo
ometry of
o our ob
bjects to obtain an
a image
e, we can
n employ
y the blo
ock
diagram
m in Figu
ure 1.31, which shows the four major stteps in the imaging
processs:
1. Ve
ertex proccessing
2. Clipping an
nd primitiv
ve assem
mbly
3. Ra
asterizatio
on
4. Fra
agment processin
p
ng
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Computer Graphics
Introduction
Exercise:
An interesting exercise that should help you understand how rapidly
graphics performance has improved is to go to the Web sites of some of the
GPU manufacturers, such as NVIDIA and ATI, and look at the specifications for
their products. Often the specs for older cards and GPUs are still there. How
rapidly has geometric performance improved? What about pixel processing?
How has the cost per rendered triangle decreased?
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