unixjava.ppt

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1Angel: Interactive Computer Graphics 4E Addison-Wesley 2005

UNIT-1


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A graphics system

A computer graphics system is a

computer system.

There are five major elements

- Input devices

- Processor

- Memory

- Frame buffer- Output devices


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Basic Graphics System-

High level view

Input devices

Output device

Image formed in FB


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Pixels and frame buffer

Image produced as an array (the raster)

of picture elements (pixels) in the frame

buffer


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Frame buffer The pixels are stored in a part of memory called the frame

buffer.

core elements of a graphics system.

Its resolution- the no of pixels in the frame buffer. The

max no of points that can be displayed without overlap on aCRT.

Depth or precision of the frame buffer

The number of bits that are used for each pixeldetermines properties such as how many colorscan be

represented on a given system. For example- 1 bit deep frame buffer allows only two colors.

- 8 bit deep frame buffer allows 256 colors.


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Full color system or true color or RGB color system

There are 24 bits per pixel .such systems can displaysufficient colors to represent most images realistically.

High dynamic range applications require more than 24-bitfixed point.

The frame bufferis implemented with special types ofmemory chips.

In simple systems- the frame buffer holds only the coloredpixels

In most systems-the frame buffer holds far moreinformation.


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Color buffer

Frame buffer comprises multiple buffers, one or moreof which are color buffers that hold the colored pixelsthat are displayed.

In simple systemonly one processor- CPU must donormal processing and the graphical processing.

The main graphical function of the processor is takespecificationof graphical primitives generated byapplication programs and to assign values to the thepixels in the frame buffer.


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For example a triangle is specified by its three

vertices,but to display its outline by the three line

segments connecting the vertices, the graphics

system must generate set of pixels that appear as line

segments to the viewer. The conversion of geometric entities to pixel colors

and locations in the frame buffer is known as

rasterization or scan conversion.

In early systemframe buffer was part of the stdmemory

Today-Special purpose graphics processing

units(GPUs)-


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Output devices-CRT

Can be used either as a line-drawing device

(calligraphic) or to display contents of frame buffer

(raster mode)


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Refreshing

CRT emit light for only a short time-few milliseconds afterthe phospher is excited by the electron beam.

The same path must be retraced or refreshed by the beamat a sufficiently high rate ,the refresh rate.

Older system- 60 Hz in US and 50Hz in rest of the world

Modern -85hz.

Non interlaced or progressive display

Pixels are displayed row by row or scan line by scan line at

the refresh rate.Interlaced

Odd rows and even rows are refreshed alternately

Used in tv -60hz


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Shadow Mask CRT


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Flat panel monitors

LED,LCD PLASMA PANELS

The two outside plates contain parallel grids of

wires that are perpendicular to each other.

By sending electrical signals to the grid-tocontrol the corresponding element in the middle.

The middle plate in an LED panel contains light

emitting diods that can be turned on and off by

electrical signals sent to the grid.

LCDLiquid crystals

Plasma panel -gas


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Most projection systems are also raster devices.-DLP

Hard copy devices such as printers and plotters- also

raster based- cannot be refreshed.

Aspect ratio

Most displays had a 4:3 width to height ratio.

width to height ratio- horizontal to vertical point ratio.

VGA resolution- 640x480

Computer resolution-XGA-1024X768- SXGA-1280X1024

HDTV-16:9



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Input devices

Common input devices are-

keyboard,mouse,the joystick and the data

tablets.

Data gloves-include many sensors andcomputer vision systems.


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Images: physical and

synthetic

Traditional approach- how to construct rasterimages of simple two-dimensional geometricentities.

Two and three dimensional mathematical

objects in the computer .Modern systems- to create realistic image

- It involves many aspects of image formation, such aslighting,shading and properties of materials.

Computer generated images are synthetic orartificial, in the sense that objects beingimaged may not exist physically.


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Objects and viewers

Two basic entities must be part of any image formationprocess.

- Object

- Viewer

The object exists in space independent of any viewer andany image formation method.

In CG, where we deal with synthetic objects, we formobjects by specifying the positions in space of variousgeometric primitives such as points, lines and polygons.

Vertices is sufficient to define or approximate most objects. To form an image we need a person or a camera or adigitizer.it is the viewer that forms the image of our objects.


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In the human visual system, the image is formedon the back of the eye.

In a camera the image is formed in the filmplane.

The image is what is seen by observer A.

All three images contain same building, butimage of the building is different in all three.

The specification of object is combined with thespecification of the viewer to produce a twodimensional image is the essence of imageformation.


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Light and images

A camera system with an object and a light source


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Light and images

Light from light source strikes various surfaces

of the objects and a portion of the reflected light

enters the camera through the lens.

The details of the interaction between light andthe surfaces of the objects determine how much

light enters the camera.

Light is a form of electromagnetic radiation.

The electromagnetic energy travels as wavesthat can be characterized by either wavelengths

or their frequencies.


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The electromagnetic spectrum includes radiowaves, infrared and a portion that causes aresponse in our visual systems.

The visible spectrum which has wavelengths in

the range of 350 to 780 nanometers(nm) iscalled light.

A given light source has a color determined bythe energy that it emits at various wavelengths.

520nm green

350 blue

780 red


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Point source emits energy from a single

location at one or more frequencies

equally in all directions.

A light bulb can be characterized asemitting light over an area and by

emitting more light in one direction than

another.


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Image formation

model

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 havemultiple interactions with objects

before being absorbed or going to infinity.


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Image formation model

A ray is a semi-infinite line that emanates from a point andtravels to infinity in a particular direction.

Portion of these infinite rays contributes to the image on thefilm plane of our camera

For example if the source is visible from camera, some ofthe rays go directly from the source through the lens of thecamera and strike the film plane.

Most rays go off to infinity neither entering the cameradirectly nor striking any of the objects. These rayscontribute nothing to the image.

Reflection and scattering

Transparent surface light pass through it and may interactwith other objects.


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Ray tracing and photon mapping are image formationtechniques that are based on these ideas.

Tracing rays can provide a close approximation to thephysical word. But not suited for real time computation.

Radiositythis method works best for surfaces that scatterthe incoming light equally in all directions.

More computations.

Given time constraints in image formation, we are satisfiedwith images that look reasonable rather than that arephysically correct.

Real time needhours of computer time for each frame.

With the increased speed of present hardware, we can getcloser to physically correct images in real time systems.


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Imaging systems- 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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Pinhole camera provides an example of image formation.A pinhole camera is a box with a small hole in the center of

one side of the box.

The film is placed inside the box on the side opposite thepinhole.

Initially the pinhole is covered. It is uncovered for a shorttime to expose the film.

Suppose that we orient our camera along the Z-axis withthe pinhole at the origin of our coordinate system.

Assume hole is small that only a single ray oflight,emanating from apoint, can enter it.

The film plane is located a distance d from the pinhole.


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Side view allows us to calculate where the

image of the point(x,y,x) is on the film plane z=-d

Two triangles are similar.

We find that the y coordinate of the image is atypwhere

A similar calculation ,using a top view

A point (xp,yp, -d) is called the projection of the

point (x,y,z)

xp= - x/z/d

yp= -y/z/d


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All points along the line between(x,y,z) and

(xp,yp, -d) project to (xp,yp, -d) .

The color on the film plane at this point will be

the color of the point (x,y,z) The field or angle of view of our camera is the

angle made by the largest object that our

camera can image on its film plane.

If the h is the height of the camera, then theangle of view is

= 2tan-1h/2d


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Disadvantage Because the pinhole is so small.it admits only a single ray

from a point source-almost no light enters the camera.

The camera cant be adjusted to have a different angle ofview.

By replacing the pinhole with lens we solve the twoproblems of the pinhole camera.

1. the lens gathers more light than can pass through thepinhole. the larger aperture of the lens, the more lens cancollect.

2.By picking a lens with the proper focal length. A selectionequivalent to choosing d for the pinhole color. We canachieve any desired angle of view(180).


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Human visual system

Human visual system has two types of

sensors

- Rods: monochromatic, night vision

- Cones Color sensitive

Three types of cones

Only three values (the tristimulus

values) are sent to the brain

Need only match these three values

- Need only threeprimarycolors


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Synthetic Camera Model

center of projection

image plane

projector

p

projection of p


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Creating a computer generated image as being similar to formingan image using an optical system. This paradigm has becomeknown as the synthetic camera model.

Viewer is bellows camera. the image is formed on the film planeat the back of the camera.

To create artificial image, we need to identify a few basic

principles.1.The specification of the objects is independent of thespecification of the viewer. There will be separate functions forspecifying the object and viewer.

2.We can compute the image using simple geometric calculations.

In real camera the image of the object is flipped relative to the

object. In synthetic camera we draw another plane in front of the lens

and work in 3D

Projector

we find the image of a point on the object on the virtual imageplane by drawing a line called a projector, from the point to the

center of the lens or the center of projection(COP).


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Projection plane the virtual image plane that we have moved in front of the lens is

called the projection plane.

Clipping rectangle (or) clipping window

We must also consider the limited size of the image. Not allobjects can be imaged onto the pinhole cameras film plane.

The angle of view expresses this limitation.

In the synthetic camera, we can move this limitation to the frontby placing a clipping rectangle (or) clipping window in theprojection plane.

Rectangle acts as a window ,through which a viewer located atthe center of projection sees the world.

Given the location of the center of projection, the locationand orientation of the projection plane and size of theclipping rectangle, we can determine which objects willappear in the image.


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API(application programmers

interface)

- Programmer sees the graphics system through a

software interface: (API)

- The interface between an application program and

a graphics system.- It can be specified through a set of functions that

resides in a graphics library.

- The application programmer sees only the API .

- It is shielded from the details of both the hardwareor the software implementation of the graphics

library.


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Application programmers

model of graphics system

The software drivers are responsible forinterpreting the output of the API and

converting these data to a form that is

understood by the particular hardware.


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Pen-plotter model

A pen plotter produces imagesby moving a pen held by agantry,a structure that canmove the pen in two orthogonaldirections across the paper.

The plotter can raise and lowerthe pen as required to create thedesired image.

Still in use.

They are well suited for drawinglarge diagrams such asblueprints.


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Drawing function- moveto(x,y)

- lineto(x,y)

Execution of the moveto function moves the pen to the location(x,y)on the paper without leaving a mark.

Lineto function moves the pen to (x,y) and draws a line from the old

to the new location of the pen.Simple program

moveto(0,0);

lineto(1,0);

lineto(1,1);

lineto(0,1);lineto(0,0);

O/p of pen plotter program:square


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If we added the code, we will get cubemoveto(0,1);

lineto(0.5,1.866);

lineto(1.5,1.866);lineto(1.5,0.866);

lineto(1,0);

moveto(1,1);

lineto(1.5,1.866);


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For certain applications, such as page layout in the printingindustry, systems built on this model work well.

Alternative

2D model relies on writing pixels directly into a frame buffer.

write_pixel(x,y,color)

Where x,y is the location of the pixel in the frame bufferand color gives the color to be written there.

Suited to writing the algorithms for rasterization andprocessing of digital images.

Disadvantage

The pen- plotter model does not extend well to three

dimensional graphics systems. 2D points are the projections of points in three dimensionalspace. the mathematical process of determining projections isan application of trigonometry.

We prefer to use an API that carry out the projection processautomatically without any trigonometric calculation.


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Three dimensional APIs

The synthetic camera model is the basis for all the popularAPIs,including OpenGL,Direct3D, and Open Scene Graph.

We need functions in the API to specify the following:- Objects

- Viewer

- Light Source(s)

- Material properties .

Objects are defined by sets of vertices.

Most APIs support a limited set of primitives including- Points

- Line segments

- Polygons

- Some curves and surfaces

Quadrics Parametric polynomials

Complex object- Circle can be defined by three points on its circumference, or by its center and

one point on the circumference.


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Example

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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The function glBegin specifies the type ofprimitive that the vertices define.

Each subsequent execution of glVertex3f

specifies the x,y,z coordinates of a location. The function glEnd ends the list of vertices.

Example type

GL_LINE_STRIP

GL_POINTS


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Camera Specification

We can identify four typesOf necessary specification

1.Position-COP

2.Orientation

-Coordinate system3.Focal length

Determine size of the

image on the film plane.

4.Film plane

Back of the camerahas a height and a width


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Camera Specification

TWO WAYS

1.To develop the specifications for the camera

location and orientation uses a series of

coordinate system transformation.

2. OpenGL API

gluLookAt(cop_x,cop_y,cop_z,at_x,at_y,at_z,up_x,up_y,

up_z);

gluPerspective(field _of_view,aspect_ratio,near,far);


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The first function call points the camerafrom a center of projection toward a

desired point(the at point),with a

specified up directionfor the camera. The second selects a lens for a

perspective view(the field of view) and

how much of the world that the camerashould image.


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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

Th M d li R d i


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The Modeling-Rendering

paradigm

Modeling-rendering pipeline

Modeler Renderer

Interface file


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In CAD and movie ,we can separate the modeling

of the scene from theproduction of the image or

the rendering of the scene.

For example, consider the production of a singleframe in an animation.

- Design and position our objects . we do not need to work

with detailed images of the objects.

- Render it ,adding light sources, material properties, othereffects to form a production quality image. It requires more

computation time.


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Interface file- produced by modeler . Describes the objects and that contains additional

information important only to the renderer, such as lightsources,viewer location and material properties.

Advantage-same file for different renderer.

This paradigm has become popular as a method forgenerating computer games and images over the internet.

Models, including the geometric objects, lights, cameras,and material properties are placed in a data structurecalled a scene graphthat is passed to a renderer or agame engine.


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A SEQUENCE OF IMAGES

Color plate 1 shows an image of an artistscreation of a sun-like object.

Color plate 2 shows the object rendered usingonly line segments. The rendered object shows

only the outlines of the parts.This type ofimage is known as a wireframe image .

Color plate 3 the same object has beenrendered with flat polygons. Certain surfaces

are not visible becausethere is a solidsurface between them an the viewer.


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Color plate 4 illustrates smooth shadingof the polygonsthat approximate the object.

Color plate 5 shows a more sophisticated wireframemodel .

Color plate 6 and 7 we add surface textureto our object.

Color plate 6 use a technique called bump mappingthatgives the appearance of a rough surface.

Color plate 7 uses an environment mapwhich gives thesurface of a mirror.

Color plate 8 shows a small area of the rendering of theobject.


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THE GRAPHICS

ARCHITECTURES

Early graphics system

Display processor architecturePipeline architecture


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THE GRAPHICS ARCHITECTURES

One side of API is the application program.

On the other side is some combination of

hardware and software that implements the

functionality of the API. Fig:Early graphics system

HostDigital to

analog Monitor


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THE GRAPHICS ARCHITECTURES

The display in early graphics system wasbased on a calligraphic CRT display.

the job of the host computer was to run

the application program and to computethe endpointsof the line segments inthe image.

This information had to be sent to the

display at a rate highenough to avoidflickeron the display.

Refreshing is very slow.


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Pipeline Architectures

Pipelining is similar to an assembly line ina car plant.

As the chassis passes down the line ,

series of operations is performed on it,each using specialized tools andworkers, until at the end, the assemblyprocess is complete.

Throughput-the number of carsproduced in a given time.

Display processor


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Display processor

architecture

HostDisplay

processor

Display

list

Monitor


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These Display processors had conventional

architectures but included instructions to display

primitives on the CRT.

Advantage- The instructions to generate the image could be

assembled once in the host and sent to the display

processor, where they were stored in the display

processors own memory as a display list or display file

The display processor would then execute repetitivelytheprogram in the display list, at a rate sufficient to avoid

flicker.


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Pipeline Architectures

Fig: Arithmetic pipelineb a

c

In the pipeline ,there is an adder and a multipler.

we have to carry out the same computation with manyvalues of a,b, and c.

The multiplier can pass on the results of its calculationto the adder and can start its next multiplication while theadder carries out the second step of the calculation on thefirst set of data.

* +


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The graphics pipeline

Each object comprises a set of graphical primitives.

Each primitive comprises a set of vertices.

Process all these vertices in a similar manner to form an imagein the frame buffer.

All steps can be implemented in hardware on the graphicscard.


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The graphics pipeline

Four major steps in the imaging process:- Vertex processing

- Clipping and primitive assembly

- Rasterization- Fragment processing


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Vertex processing

Each vertex is processed independently. Major function

Coordinate transformations

Compute a color for each vertex

We can represent each change of coordinate systems

by a matrix. We can represent successive changes in coordinate

systems by multiplying, or concatenating,theindividual matrices into a single matrix.

After multiple stages of transformation, the

geometry is transformed by a projectiontransformation.

The assignment of vertex colors can besimple.

Clipping and primitive


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Clipping and primitive

assembly

Must do clipping because of the limitation that noimaging system can see the whole world at once.

The human retina-90 degree field of view.

Cameras have film of limited size.

In the synthetic camera ,clipping volume is

considered. The projections of objects in this volume appear in

the image.

Those that are outside do not and are said to beclipped out.

Clipping must be done on a primitive by primitive basisrather than on vertex by vertex basis.

The output of this stage is a set of primitives whoseprojections can appear in the image.


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Rasterization

If an object is not clipped out, the appropriate pixels in theframe 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 therasterizer.

For example,- if three vertices specify a triangle filled with a solid color,the rasterizer must

determine which pixels in the frame buffer are inside the polygon.

The output of the rasterizer is a set of fragments for each primitive. Fragment-pixel location,color,depth information.


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Fragment Processing

The final block takes in the fragments generatedby rasterizer updates the pixels in the framebuffer.

The color of the fragment may be altered bytexture mapping or bump mapping.

The color of the pixel that corresponds to afragment can also be read from the frame bufferand blended with the fragments color to createtranslucenteffect.

Fragments may be blocked by other fragmentscloser to the camera

- Hidden-surface removal


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PROGRAMMABLE PIPELINES

The commodity graphics market is dominated by graphicscardsthat have pipelines built into the graphics processingunit.

Pipeline architectures had a fixed functionality. Basic operationsavailable with in the pipeline were fixed.

For example, there was only one lighting model for how to

compute a shade using the specified light sources andmaterials.

Recently, both the vertex processor and the fragmentprocessorare now programmable by the application program.

Vertex programs can alter the location or color of each vertex asit flows through the pipeline.

Thus we can implement a variety of light material models orcreate new kinds of projections.

Fragment programs allows us to use textures.

PERFORMANCE


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PERFORMANCE

CHARACTERISTICS There are two fundamentally different types of processing in our pipeline

architecture.- Front end processing

- Back end processing

Front end processing

Geometric processing, based on processing vertices through the varioustransformations, vertex shading, clipping, and primitive assembly.

it involves floating point calculation.

VLSI implementation for many of these operations in a specialpurposechip .

Today Graphics workstations and commodity graphics cards use graphicsprocessing units(GPUs).

GPUs perform most of the graphics operations at the chip level.

Back end processing

different from front end, it moves block of bits quickly using architectures.

The overall performance of a system is characterized by how fast we canmovegeometric entities through the pipeline and by how many pixels persecond we can alterin the frame buffer.

Graphics cards use GPUs that contain the entire pipeline within a single chip.

GRAPHICS PROGRAMMING-


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SIERPINSKI GASKET

The sierpinski gasket is an object that can bedefined recursively or randomly.

Suppose start with three points in space. As

long as three the points are not collinear, they

are vertices of a unique triangleand also

define a unique plane. Assume that this plane

is the plane z=0.

Three points are (x1,y1,0),(x2,y2,0),(x3,y3,0)


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Algorithm1.pick an initial point(x,y,z) at random inside the triangle.

2.select one of the three vertices at random.

3.Find the location halfway between the initial point and therandomly selected vertex.

4.Display this new point by putting some sort ofmarker,such as a small circle,at the correspondinglocation on the display.

5.Replace the point at(x,y,z) with this new point.

6.Return to step 2.

Each time that we generate a new point,we display it on theoutput devices.

Where po is the initial location,and p1and p2are the first twolocations generated by our algorithm.


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Main(){

initialize_the_system();

for(some_number_of_points)

{

pt=generate_a_point();

display_the_point(pt);

}

cleanup();

}

PROGRAMMING TWO-


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PROGRAMMING TWO-

DIMENSIONAL APPLIACATION

Represent a point in the plane z=0 as p=(x,y,0) in the 3Dworld,or as p=(x,y) in two dimensional plane.

P=(x,y,z) or column matrix.

A vertex is a position in space. We use 2 ,3,4D spaces in CG.

Simplest geometric primitive is a point in space. which isspecified by a single vertex.

General formglVertex*();

where * can be interpreted as either nt or ntv.

n--- number of dimensions (2,3,4)

t --- datatypeinteger,float,double

VIf present ,indicates that the variable are specified through apointer to an array.


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#define GLfloat float

glVertex2i(GLint xi,GLint yi);

glVertex3f(GLfloat x,GLfloat y,GLfloat z);

array

GLfloat vertex[3];

we can use

glVertex3fv(vertex);

Gasket program- random


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Gasket program random

method

In this application,because we are working inthe plane z=0 we could use either the form

glVertex3f(x,y,0)

Or the form glVertex2f(x,y)

We could also define a new data type

typedef GLfloat point[2];

and use some thing like

point p; glVertex2fv(p);


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Void display()

{GLfloat vertices[3][3] = {{0.0,0.0,0.0}, {25.0,50.0,0.0} , {50.0,0.0,0.0}};

GLfloat p[3]={7.5,5.0,0.0};

Int j,k;

glBegin(GL_POINTS);

for(k=0;k


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Coordinate systems

The users coordinate system became known as the worldcoordinate systemor the application ,model, or objectcoordinate system.

The space in which the objects are described is calledworldcoordinate system- Cartesian x, y coordinates.

Units on the display were first called physical-device coordinatesor device coordinates.

For raster devices, such as most CRT displays,we use the termwindow coordinates or screen coordinates.

The values in the world coordinates must be mappedto screen

coordinates. Mapping is performed automatically as part of the rendering

process.

Mapping from worlddi t t


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coordinates to screen

coordinates

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