· Web viewThe image size is limited i.e., not all objects can be imaged onto film plane. A clipping rectangle or clipping window, in the projection plane, placed to the front indicates

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UNIT-1Graphics Systems and Models

Computer graphics is concerned with all aspects of producing pictures or images using a computer.

The field began with the display of a few lines on a cathode-ray tube (CRT) and now, the field generates photograph-equivalent images.

The development of computer graphics has been driven both by the needs of the user community and by advances in hardware and software.

The combination of computers, networks, and the complex human visual system, through computer graphics, has led to new ways of displaying information, seeing virtual worlds, and communicating with both other people and machines.

Applications of Computer Graphics: Four major areas of application are:

1. Display of information: Computer-based drafting systems produce the information

needed for architects, mechanical designers, and drafts people. Computer plotting packages provide a variety of plotting

techniques and color tools, and that can handle multiple large data sets.

Medical imaging systems such as Computed Tomography (CT), Magnetic Resonance Imaging (MRI), ultrasound, and positron-emission tomography (PET) – can generate three-dimensional data that must be subjected to algorithmic manipulation to provide useful information.

Graphical tools provided by the field of scientific visualization help the researchers to interpret the vast quantity of data that the supercomputers generate while solving previously intractable problems.

Data can be converted to geometric entities and then the images can be produced. This capability has yielded new insights into complex processes in fields such as fluid flow, molecular biology, and mathematics.

Internet can develop & manipulate maps relevant to geographic information systems in real time.

2. Design: Professions such as engineering and architecture are concerned

with design. Design starts with a set of specifications and ends with a cost-

effective and esthetic solution that satisfies the specifications. But to end-up with such a solution, design process takes several

iterations. Thus, the designer generates a possible design, tests it, and then uses the results as the basis for exploring other solutions.

Normally the solutions are not unique. Moreover design problems are either over-determined, such

that they possess no optimal solution, or under-determined, such that they have multiple solutions.

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Computer graphics aids iterative process and also obtain optimal solution.

Computer-aided design (CAD) uses interactive graphical tools. CAD is used in architecture and the design of mechanical parts and of very-large-scale integrated (VLSI) circuits. In many such applications, the graphics is used in a number of distinct ways. For example, in a VLSI design, the graphics provides an interactive interface between the user and the design package, usually via tools such as menus and icons. In addition, after the user evolves a possible design, other tools analyze the design and display the analysis graphically.

3. Simulation: Graphics systems are capable of generating sophisticated

images in real time and the engineers and researchers use them as simulators. Uses of simulation include:

Graphical flight simulators: Are proved cost-effective and safety in training the pilots.

Arcade games: are as sophisticated as flight simulators. Games and educational software: for home computers are

almost as impressive. Robot designing and planning its path, and behavior in complex

environments The television, motion-picture, and advertising industries use

computer graphics to generate the photorealistic images. Entire animated movies can be made by computer at a cost

comparable to that of movies made with traditional hand-animation techniques.

In the field of virtual reality (VR), a human viewer is equipped with a display headset with which separate images can be seen with the right and left eyes. This effect on the viewer is called the effect of stereoscopic vision. In addition, the body location and position, possibly including head and finger positions, of the viewer are tracked by the computer. The viewer can with other interactive devices available, including force-sensing gloves and sound, can then act as part of a computer-generated scene, limited only by the image-generation ability of the computer.

For example, a surgical intern might be trained to do an operation in this way, or an astronaut might be trained to work in a weightless environment.

4. User interfaces: Visual paradigm includes windows, icons, menus, and a pointing

device, such as a mouse. This paradigm has increased human-computer interaction through windowing systems such as the X Window system, Microsoft Windows, and the Macintosh operating systems.

Graphical network browsers, such as Netscape and Internet Explorer have created millions of users on the Internet. There are graphical interfaces other than Graphical User Interfaces.

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A Graphics System A computer graphics system is a computer system having a special

component called frame buffer in addition to the common components of a general-purpose computer system.

Block Diagram & 5 Major components of a Graphics system 1. Processor2. Memory3. Frame buffer4. Output devices5. Input devices

Raster-Graphics Display: It is a point-plotting output device based on the cathode ray tube.

Raster: A matrix (arranged in rows in columns) of discrete cells (pixels) which can be illuminated in a Raster-Graphics display is called a raster.

Pixel: Each discrete cell in a raster is called a pixel (Pi(x)cture element).

Frame Buffer: A part of memory used by the graphics system to store pixels is called frame

buffer. The frame buffer can be viewed as the core element of a graphics system. In simpler systems, the frame buffer is part of standard memory. In high-end systems, the frame buffer is implemented with special types of

memory chips-video random-access memory (VRAM) or dynamic random-access memory (DRAM)-that enable fast redisplay of the contents of the frame buffer.

Depth of the frame buffer: The number of bits that are used for each pixel which determines properties

of each pixel (say, color of a pixel) is called the depth of the frame buffer. E.g.

1-bit-deep frame buffer allows the frame buffer only two colors. 8-bit-deep frame buffer allows 28 (=256) colors. Full / true / RGB-color systems: Display systems with a frame buffer

depth of 24-bits per pixel, in which, individual three groups of 8-bits are assigned to each of the three primary colors - red, green, and blue used in most displays. Such systems can display sufficient colors to represent most images realistically.

Resolution of frame buffer: The number of pixels in the frame buffer which determines the detail of an

image is called the resolution.

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Processor: In a simple system, there may be only one processor, which must do both

the normal processing and the graphical processing. Sophisticated graphics systems are characterized by various special-purpose processors, each custom-tailored to specific graphics functions.

Graphical Processing / Functions? – Rasterization / Scan conversion: The graphical process of conversion of geometric entities (such as lines,

circles, and polygons) generated by application programs to pixel assignments in the frame buffer for the best representation of those entities.

Output Devices – say Cathode-Ray Tube (CRT):An electron gun produces a beam of electrons.

The output of the computer is converted, by digital-to-analog converters, to voltages across the x and y deflection plates which control the direction of the electron beam.When electrons strike the phosphor coating on the tube, light is emitted. Light appears on the surface of the

CRT when a sufficiently intense beam of electrons is directed at the phosphor.TYPES OF CRTs

Random-scan or calligraphic CRT:The CRTs used in early graphics systems in which an electron beam can be moved directly from any position to any other position. If the voltages steering the beam change at a constant rate, the beam will trace a straight line, visible to a viewer. If intensity of the beam is turned off, the beam can be moved to a new position without changing any visible display.A typical CRT will emit light for only a short time-usually, a few milli-seconds after the phosphor is excited by the electron beam. For a human to see a steady image on most CRT displays, the same path must be retraced, or refreshed, by the beam at least 50 times per second.

Raster system: The CRTs in present graphics system, in which the pixels are taken from the frame buffer and displayed as points on the surface of the display.

Refresh Rate: The high rate at which the entire contents of the frame buffer are displayed on the CRT to avoid flicker.

Two types of Raster Systems: These types are according to the two ways of displaying pixels.

Interlaced display: Odd rows and even rows are refreshed alternately. Interlaced displays are used in commercial television. In an interlaced display operating at 60 Hz, the screen is redrawn

in its entirety only 30 times per second, although the visual system is tricked into thinking the refresh rate is 60 Hz rather than 30 Hz.

Non-interlaced system:

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The pixels are displayed row by row, or scan line by scan line, at the refresh rate, which is usually 50 to 85 times per second, or 50 to 85 hertz (Hz).

Non-interlaced displays are becoming more widespread, even though these displays process pixels at twice the rate of the interlaced display.

Viewers located near the screen, however, can tell the difference between the interlaced and non-interlaced displays.

Color CRTs: They have three different colored phosphors ,(red, green, and blue), arranged in small groups, One common style arranges the phosphors in triangular groups called triads, each triad consisting of three phosphors, one of each primary. Most color CRTs have three electron beams, corresponding to the three types of phosphors.

Shadow-mask CRT: A metal screen with small holes-the shadow mask-ensures that an electron beam excites only phosphors of the proper color.

The other output devices such as the liquid-crystal displays (LCDs) , must be refreshed, whereas hard-copy devices, such as printers, do not need to be refreshed, albeit both are raster-based.

Input Devices (Chapter 3 covers these devices) Positional Input Devices: Provide positional information to the system and

usually is equipped with one or more buttons to provide signals to the processor. E.g. Mouse, Joystick, and Data tablet.

Pointing devices: Allow a user to indicate a particular location on the display. E.g. Light pen.

Other: say Keyboard

Computer Generated Images: S y n t h e t i c ( A r t i f i c i a l ) I m a g e s Computer-generated images are synthetic or artificial, in the sense that the

objects being imaged do not exist physically. They can be formed similar to the traditional imaging methods say optical

systems, such as cameras and the human visual system. Hence, to understand and develop computer generated imaging systems,

following sequence of study is needed: 1. Traditional imaging systems.2. A model (paradigm) of the image formation process needs to

be constructed. This model is based on the traditional imaging methods.

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3. Computer architecture for implementing that model (paradigm). (Covered in subsequent chapters with relevant equations).

Basic entities of image formation: Objects and ViewersIn computer graphics, graphic objects (various geometric primitives, such as points, lines, and polygons) are synthetic and are specified/defined/approximated with their positions (locations) and sometimes relationships among them, in space. They exist in space independent of viewer or any image formation process.E.g.

A line can be defined by two vertices. Polygon can be defined by an ordered list of vertices. Sphere can be specified by two vertices that specify its center and any point on

its circumference.Viewer: It forms the image of objects. Viewer may be a human, a camera, or a digitizer. It is easy to confuse images and objects. Usually an object is seen from an individual’s single perspective by forgetting that other viewers, located in other places, will see the same object differently. In a camera system viewing a building, both the object (i.e. building) and the viewer exist in a three-dimensional world. However, the image that they define and find on the film plane is two-dimensional.Thus the process by which the specification of the object is combined with the specification of the viewer to produce a two-dimensional image is the essence of image formation.Other entities of image formation:

Light source: It makes the objects visible in the image without which the objects would be dark and there would be nothing visible in the image.

Color: The way the color enters the picture Different kinds of surfaces on objects affecting an image

A simple physical imaging system – A camera system with a light source: taking a more physical approach:

It consists of physical object and a viewer (the camera) and a light source in the scene. Light from the source strikes various surfaces of the object, and a portion of the reflected light enters the camera through the lens. The details of the interaction between light and the surfaces of the object determine how much light enters the camera.Light sources emit at a fixed rate of light energy or intensity. Light travels in straight lines, from the sources to those objects with which it interacts. A particular light source is characterized by the intensity of light that it emits at each frequency, and by that light's directionality.

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. More complex sources often can be modeled by a number of carefully placed point sources (Chapter 6).

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Note: Here only the monochromatic (means a source of single frequency), point sources are considered for simplicity. This is analogous to discussing black-and-white television before examining color television.

Imaging systems/ Models / Paradigms of Graphics SystemsRay TracingBuilding an imaging model: by following light from a source.

Consider the scene in the figure.It is illuminated by a single point source. Viewer is included because the light that reaches the eye of viewer is of interest. The viewer can also be camera as shown below:

Ray: It is a semi-infinite line that emanates from a point and travels to infinity in a particular direction because light travels in straight lines. A portion of these infinite rays contributes to the image on the film plane of the camera. E.g. If the source is visible from the camera, some of the rays go directly from the source through the lens of the camera, and strike the film plane. Most rays go off to infinity, neither entering the camera directly, nor striking any of the objects.These rays contribute nothing to the image, although they may be seen by some other viewer. The remaining rays strike and illuminate objects. These rays can interact with the objects surfaces in a variety of ways. E.g.

Mirror Surface: If the surface is a mirror, a reflected ray might-depending on the orientation of the surface enter the lens of the camera and contribute to the image.

Diffuse surfaces: They scatter light in all directions. Transparent surfaces: Allow the light ray from the source to pass through it;

perhaps being bent or refracted, and may interact with other objects, enter the camera, or travel to infinity without striking another surface.

Ray tracing is an image-formation technique that is based on the ideas (aforesaid) of tracing rays of light to form an image. This paradigm is useful in understanding the interaction between light and materials that is essential to physical image formation. Only a small fraction of all the rays leaving a source enter the imaging system and the time spent tracing most rays is wasted. Ray tracing is an alternative way to develop a

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computer graphics system. It can be used to simulate even complex physical effects with expense of requisite computing. It is a close approximation to the physical world. But it is not well suited for fast computation. However by further simplification, it is possible to reduce the computational burden. E.g.

By assuming that all objects are uniformly bright- say from the perspective of the viewer, a red triangle appears to have the same shade of red at every point, and is indistinguishable from a uniformly red emitter of light. Given this assumption, sources can be neglected, and simple trigonometric methods can be used to calculate the image.

From a physical perspective, if an object emits light, one cannot tell whether the looking object is reflecting light, or whether the object is emitting light from internal energy sources. It also reduces computations.

The Synthetic-Camera ModelIt is a modern model of three-dimensional computer graphics in which creating a computer-generated image is similar to forming an image using an optical system.

Consider the imaging system shown in figure containing objects and a viewer. The viewer is a bellows camera. In a bellows camera, the lens is located at the front plane and the film plane is located at the back of the camera. These two are connected by flexible sides. Thus, the back of the camera can be moved independently of the front of the camera, introducing additional flexibility in the image-

formation process.The image is formed on the film plane at the back of the camera so that this process emulates the creation of artificial images.Basic Principles:

1. The specification of the objects is independent of the specification of the viewer. => Within a graphics library, there will be separate functions for specifying the objects and the viewer.

2. Image can be computed using simple trigonometric calculations in a straightforward manner. Consider the side view of the camera and a simple object in figure below:

View in (b) is obtained by noting the similarity of the two triangles in (a).

Image/Film plane is moved in front of the lens. In three dimensions, it is possible to work with the arrangement of figure.

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The image of a point on the object is obtained by drawing a line, called a projector, from the point to the center of the lens, or the center of projection. All projectors are rays emanating from the center of projection. The film plane that is moved in front of the lens is called the projection plane. The image of the point is located where the projector passes through the projection plane. (Chapter 5 discusses more and derives relevant mathematical formulas).

3. The image size is limited i.e., not all objects can be imaged onto film plane. A clipping rectangle or clipping window, in the projection plane, placed to the front indicates this limitation. This rectangle acts as a window through which a viewer, located at the center of projection, sees the world. Given the location of the center of projection, the location and orientation of the projection plane, and the size of the clipping rectangle, it is possible to determine which objects will appear in the image.

4. Synthetic-camera model leads to the notion of a pipeline architecture in which each of the various stages in the pipeline performs distinct operations on geometric entities, then passes on the transformed objects to the next stage.

The Modeling-Rendering ParadigmIt assumes the image formation, a two-step process:

1. Modeling of the scene: This process designs and positions the objects of the scene. This step is highly interactive. The details of images of the objects need not be specified at this stage. Hence this step is carried out on a graphical workstation.

2. Rendering/ Production of the scene: It renders the designed scene by adding light sources, material properties, and a variety of other detailed effects, to form a production-quality image. This step requires a tremendous amount of computation, and hence requires a number-cruncher machine.

These two steps not only differ in the required optimal hardware but also the in their required software.The interface between the modeler and renderer can be as simple as a file produced by the modeler that describes the objects, and that contains additional information important to only the renderer, such as light sources, viewer location, and material properties. Pixar's Renderman Interface follows this approach and uses a file format that allows modelers to pass models to the render in text format.

Modeling-Rendering Pipeline

It suggests that the modeler and the renderer can be implemented with different software and hardware.

Advantages: It allows developing modelers that, although they use the same renderer, are

custom-tailored to particular applications.

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Likewise, different renderers can take as input the same interface file. It is even possible, at least in principle, to dispense with the modeler completely,

and to use a standard text editor to generate an interface file.Disadvantages: For complex scenes it is difficult for the users to edit lists of information for a renderer. Hence an interactive modeler is used. Such modelers are based upon the simple synthetic-camera model. Applications: In CAD applications and in development of realistic images, such as for movies.The Programmer’s InterfaceInterface provides ways that a user can interact with a graphics system. With completely self-contained packages, using mouse and keyboard, the menus and icons representing possible actions can be selected and the user can guide the software and produce images without having to write programs.To write own graphics application interfaces: Application programmer's interface (API) : A set of functions that resides in a graphics library with which the interface between an application program and a graphics system is specified is called API. Application programmer's model of the system is below:

The application programmer sees only the API, and is thus shielded from the details of both the hardware and the software implementation of the graphics library. From the perspective of the writer of an application program, the functions available through

the API should match the conceptual model that the user wishes to employ to specify images. The synthetic-camera model is the basis for a number of popular APIs, including OpenGL. There are functions to specify:

Objects:Objects are usually defined by sets of vertices. For simple geometric objects such as line segments, rectangles, and polygons-there is a simple relationship between a list of vertices and the object. For more complex objects, there may be multiple ways of defining the object from a set of vertices. A circle, for example, can be defined by three points on its circumference, or by its center and one point on the circumference.Most APIs provide similar sets of primitive objects for the user. These primitives are usually those that can be displayed rapidly on the hardware. The usual sets include points, line segments, polygons, and, sometimes, text. OpenGL defines primitives through lists of vertices.E.g. To define a triangular polygon in OpenGL through five function calls:

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( ):

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Note: By adding additional vertices, an arbitrary polygon can be defined. Same vertices can be used to define a different geometric primitive simply by

changing the type parameter, GL_POLYGON. GL_LINE_SCRIPT uses the vertices to define two connected line segments, whereas the type GL_POINTS uses the same vertices to define three points.

Some APIs let the user work directly in the frame buffer by providing functions that read and write pixels.

Some APIs provide curves and surfaces as primitives; often, however, these types are approximated by a series of simpler primitives within the application program. OpenGL provides access to the frame buffer, curves, and surfaces.

Viewer Viewer or camera can be defined in a variety of ways. Available APIs differ in both how much flexibility they provide in camera selection, and in how many different methods they allow.

There are four types of necessary specifications:1. Position: The camera location usually is given by the position of the center of the lens (the center of projection).2. Orientation: Once the camera is positioned, a camera coordinate system can be placed with its origin at the center of projection. Then the camera can be rotated independently around the three axes of this system.3. Focal length: The focal length of the lens determines the size of the image on the film plane or, equivalently, the portion of the world the camera sees.4. Film plane: The back of the camera has a height and a width. On the bellows camera, and in some APIs, the orientation of the back of the camera can be adjusted

independently of the orientation of the lens.These specifications can be satisfied in various ways:

1. Developing the specifications for the camera location and orientation uses a series of coordinate system transformations. These transformations convert object positions represented in the coordinate system that specifies object vertices to object positions in a coordinate system centered at the center of projection. This approach is useful, both for doing implementation and for getting the full set of views that a flexible camera can provide. (Chapter 5)

2. The synthetic-camera model emphasizes object is independent of the view. But, the classical viewing techniques, stress the relationship between the object and the viewer. Thus, the classical two-point perspective of a cube in below is a two-point perspective because of a particular relationship between the viewer and the planes of the cube.

3. In OpenGL API, all transformations can be set with complete freedom. In addition to that, OpenGL also provides helpful extra functions. E.g. Function call

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gluLookAt(cop_x, cop_y, cop_z, at_x, at_y, at_z, ...);points the camera from a center of projection toward a desired point.

Function callglPerspective(field_of_view, ...);selects a lens for a perspective view.

4. However, none of the APIs built on the synthetic-camera model - OpenGL provides functions for specifying desired relationships between the camera and an object.

Light sourcesLight sources can be defined by their location, strength, color, and directionality. APIs provide a set of functions to specify these parameters for each source.

Material PropertiesMaterial properties are characteristics, or attributes, of the objects, and such properties are usually specified through a series of function calls at the time that each object is defined. Both light sources and material properties depend on the models of light-material interactions supported by the API. (Chapter 6)Graphics Architectures

A model of early graphics systems:

They used general-purpose computers with the standard von Neumann architecture. Such computers are characterized by a single processing unit that processes a single instruction at a time. The display in these systems was based

on a CRT display that included the necessary circuitry to generate a line segment connecting two points. The job of the host computer was to run the application program, and to compute the endpoints of the line segments in the image (in units of the display). This information had to be sent to the display at a rate high enough to avoid flicker on the display. Computers were so slow that refreshing even simple images, containing a few hundred line segments, would burden an expensive computer.

Display Processor Architecture: It relieves the general-purpose computer from the task of refreshing the display continuously by incorporating a special display processor. These display processors had conventional architectures as that of

general-purpose, but included instructions to display primitives on the CRT.

The main advantage of the display processor was that 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 processor's own memory as a display list or display file. The display processor would then execute repetitively the program in the display list, at a rate sufficient to avoid flicker, independently of the host, thus freeing the host for other tasks. It is similar to client-server architecture.

Pipeline ArchitecturesThe major advances in graphics architectures parallel closely the advances in workstations. In both cases, the ability to create special-purpose VLSI circuits was the

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key enabling technology development. In addition, the availability of cheap solid-state memory led to the universality of raster displays.For computer graphics applications, the most important use of custom VLSI circuits has been in creating pipeline architectures. The concept of pipelining is illustrated in the

figure below for a simple arithmetic calculation.In this pipeline, there is an adder and a multiplier. If this configuration is used to compute a + (b * c), the calculation takes one multiplication and one addition, - the same amount of work required if a single processor is used to carry out both operations. However, suppose

that the same computation is to be performed with many values of a, b, and c. The multiplier can pass on the results of its calculation to the adder, and can start its next multiplication while the adder carries out the second step of the calculation on the first set of data. Here, the rate at which data flows through the system, the throughput of the system, has been doubled.Pipelines can be constructed for more complex arithmetic calculations that will afford even greater increases in throughput. There is no point in building a pipeline unless the same operation is to be performed on many data sets.Pipeline architecture suits Computer Graphics because in computer graphics, large sets of vertices need be processed in the same manner.Geometric Pipeline and four major steps in Image Procesing: Suppose a set of geometric primitives are defined by a set of vertices. Set of primitive types and vertices can be referred to as the geometry of the data. In a complex scene, there may be thousands-even millions of vertices that define the objects. These entire vertices muse be processed in a similar manner to form an image in the frame buffer. Processing the geometry of objects to obtain an image can be pipelined as follows:

This pipelining shows four major steps in the imaging process:1. Vertex Processing:

Each vertex is processed independently. This block includes two major functions: Transformation:

Representation of same object in different coordinate system requires transformation. E.g.

In the synthetic camera model:

While putting an image onto CRT or output display:

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Representation in terms of the coordinate system of the camera.

Representation of objects from the coordinate system

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Each change of coordinate systems can be represented by a matrix. Successive changes in coordinate systems can be represented by multiplying, or concatenating, the individual matrices into a single matrix. (Chapter 4)Because multiplying one matrix by another matrix yields a third matrix, a sequence of transformations is an obvious candidate for pipeline architecture. In addition, because the matrices that used in computer graphics will always be small (4 x 4), there is opportunity to use parallelism within the transformation blocks in the pipeline.Eventually, after multiple stages of transformation, the geometry is transformed by a projection transformation. This step can be implemented using 4 x 4 matrices (Chapter 5) and thus projection fits in the pipeline. In general, 3-D information needs to be kept as long as possible, as objects pass through the pipeline. Further, there is a variety of projections that can be implemented (Chapter 5).

Assignment of Vertex Color:The assignment of vertex colors can be as simple as the program specifying a color or as complex as the computation of a color from a physically realistic lighting model that incorporates the surface properties of the object and the characteristic light sources in the scene. (Chapter 6).

2. Primitive assembly and Clipping:Clipping is done because of the limitation that no imaging system can see the whole world at once. E.g. Cameras have film of limited size and their fields of view can be adjusted by selecting different lenses. Equivalent property can be obtained in the synthetic camera model, by considering a clipping volume, such as the pyramid in front of the lens. The projections of objects in this volume appear in the image. Those that are outside do not and are said to be clipped out. Objects that straddle the edges of the clipping volume are partly visible in the image.

Clipping must be done on a primitive by primitive basis rather than on a vertex by vertex basis. Thus, sets of vertices must be assembled in to primitives, such as line segments and polygons before clipping can take place within this stage of the pipeline. Consequently the output of this stage is a set of primitives whose projections can appear in the image. (Chapter 7 – covers efficient clipping algorithms).Clipping can occur at various stages in the imaging process. For simple geometric objects, whether or not an object is clipped out can be determined from their vertices. Because clippers work with vertices, clippers can be inserted with transformers into the pipeline. Clipping can even be subdivided further into a sequence of pipelined clippers.

3. Rasterization or Scan ConversionThe primitives that emerge from the clipper are still represented in terms of their vertices and must be further processed to generate pixels in the frame buffer. E.g. 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. (Chapter 8 discusses

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Representation in terms of the coordinate system of the camera.

Representation of objects from the coordinate system

Representation in Display Coordinate System

The internal representation of objects - whether in the camera coordinate system or perhaps in a system used by the graphics software.

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rasterization for line segments and polygons). The output of the rasterizer is a set of fragments for each primitive. A fragment can be thought of as a potential pixel that carries with it information, including its color and location, that is used to update the corresponding pixel in the frame buffer. Fragments can also carry along depth information that allows later stages to determine if a particular fragment lies behind other previously rasterized fragments for a given pixel.

4. Fragment ProcessingIt updates the pixels in the frame buffer for the fragments generated by the rasterizer. If the application generated 3-D data, some fragments may not be visible because the surfaces that they define are behind other surfaces. The color of a fragment may be altered by texture mapping or bump mapping. The color of the pixel that corresponds to a fragment can also be read from the frame buffer and blended with the fragment's color to create translucent effects. (These effects will be covered in Chapters8 and 9.)

Performance Characteristics: Two fundamentally different types of processing in Graphics architecture:

Front end geometric processing, based on processing vertices through the various clippers and transformers. This processing is ideally-suited tor pipelining, and usually involves floating-point calculations.

The geometry engine developed by Silicon Graphics was a VLSI implementation for many of these operations in a special-purpose chip that became the basis for a series of fast graphics workstations.

Later, floating-point accelerator chips, such as the Intel i860, put 4 x 4 matrix-transformation units on the chip, reducing a matrix multiplication to a single instruction.

Graphics workstations and add-on graphics boards use Application Specific Integrated Circuits (ASICS) that perform many of the graphics operations at the chip level.

Pipeline architectures are the dominant type of high-performance system. As more boxes are added to the pipeline, however, it takes more time tor a single datum to pass through the system. This time is called the latency of the system; Latency must be balanced against increased throughput in evaluating the performance of a pipeline.

Back end direct manipulation of bits in the frame buffer: Beginning with rasterization including many other features, process directly the bits in the frame buffer. It is fundamentally different from front-end processing, and can be implemented most effectively using architectures that have the ability to move blocks of bits quickly.

The overall performance of a system is characterized by how fast the geometric entities are moved through the pipeline, and by how many pixels per second can be altered in the frame buffer.

Consequently, the fastest graphics workstations are characterized by pipelines at the front ends and parallel bit processors at the back ends.

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Pipeline architectures dominate the graphics field, especially where real-time performance is of importance. Note: Pipelining architecture can be implemented not only in hardware but also in a software system implementation of an API. The power of the synthetic-camera paradigm is that the pipelining works well in both cases.

1. What is Computer Graphics? Explain its applications.2. Describe the components of a graphics system.3. Explain different types of CRTs.4. Comment on Synthetic Camera Model of Imaging System.5. What do you mean by API? Discuss the nature of functions supported by API to

specify objects and camera. 6. Discuss different Graphics Architectures. 7. What do you mean by geometric pipeline? What are the major steps involved in

it? Explain.8. Write a note on performance characteristics of graphics system.

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· Web viewThe image size is limited i.e., not all objects can be imaged onto film plane. A clipping rectangle or clipping window, in the projection plane, placed to the front indicates