ME6501 Unit 1 introduction to cad

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ME6501-COMPUTER AIDED DESIGN Unit-I
V.R.S. College of Engineering & Technology, Arasur-607107
(Accredited by NAAC and an ISO 9001:2008 Recertified Institution)
Subject Name : COMPUTER AIDED DESIGN
Subject code : ME6501
Year : IIIrd
year
Semester : Vth
semester
AL AMEEN ENGINEERING COLLEGE
DEPARTMENT OF MECHANICAL ENGINEERING
Prepared by :
J.S.JAVITH SALEEM,
AP/MECH-AEC

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ME6501 COMPUTER AIDED DESIGN L T P C 3 0 0 3
OBJECTIVES: To provide an overview of how computers are being used in mechanical
component design
UNIT I FUNDAMENTALS OF COMPUTER GRAPHICS 9
Product cycle- Design process- sequential and concurrent engineering- Computer aided design –
CAD system architecture- Computer graphics – co-ordinate systems- 2D and 3D transformations
homogeneous coordinates - Line drawing -Clipping- viewing transformation
UNIT II GEOMETRIC MODELING 9
Representation of curves- Hermite curve- Bezier curve- B-spline curves-rational curves
Techniques for surface modeling – surface patch- Coons and bicubic patches- Bezier and B
spline surfaces. Solid modeling techniques- CSG and B-rep
UNIT III VISUAL REALISM 9
Hidden – Line-Surface-Solid removal algorithms – shading – colouring – computer animation.
UNIT IV ASSEMBLY OF PARTS 9
Assembly modelling – interferences of positions and orientation – tolerance analysis
massproperty calculations – mechanism simulation and interference checking.
UNIT V CAD STANDARDS 9
Standards for computer graphics- Graphical Kernel System (GKS) - standards for exchange
images-Open Graphics Library (OpenGL) - Data exchange standards - IGES, STEP, CALSetc.
communication standards.
TOTAL : 45 PERIODS
OUTCOMES: Upon completion of this course, the students can able to use computer and CAD
software's for modeling of mechanical components
TEXT BOOKS:
1. Ibrahim Zeid “Mastering CAD CAM” Tata McGraw-Hill Publishing Co.2007
REFERENCES:
1. Chris McMahon and Jimmie Browne “CAD/CAM Principles", "Practice and Manufacturing
management “ Second Edition, Pearson Education, 1999.

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2. William M Neumann and Robert F.Sproul “Principles of Computer Graphics”, McGraw Hill
Book Co. Singapore, 1989.
3. Donald Hearn and M. Pauline Baker “Computer Graphics”’. Prentice Hall, Inc, 1992.
4. Foley, Wan Dam, Feiner and Hughes - "Computer graphics principles & practice" Pearson
Education - 2003.
UNIT 1
FUNDAMENTALS OF COMPUTER GRAPHICS
CLASS NOTES
1. Explain Product life cycle with design process. (Or) Briefly explain the conventional
process of the product cycle in the conventional manufacturing environment. (Nov
08)
Product Life Cycle:
Figure 1.1.shows the life cycle of a typical product. The product begins with a need
which is identified based on customers’ and markets’ demands. The product goes through two
main processes from the idea conceptualization to the finished product: The design process and
the manufacturing process.
Design process: The product goes through two main processes from inception to a finished
product: the design process and the manufacturing process. Synthesis and analysis are the two
main subprocesses of the design process.
Synthesis: The philosophy, functionality, and uniqueness of the product arc all determined
during synthesis. During synthesis, a design takes the form of sketches and layout drawings
that show the relationship among the various product parts. These sketches and drawings can
be created using a CAD/CAM system or simply hand-drawn on paper. They are used during
brainstorming discussions among various design teams and for presentation purposes.
Analysis: The analysis subprocess begins with an attempt to put the conceptual design into the
context of engineering sciences to evaluate the performance of the expected product. This
requires design modeling and simulation. An important aspect of analysis is the "what if'
questions that help us to eliminate multiple design choices and find the best solution to each
design problem. The outcome of analysis is the design documentation in the form of engineering
drawings (also known as blueprints).

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Manufacturing Process: The manufacturing process begins with the process planning and ends
with the actual product. Process planning is considered the backbone or the manufacturing
process since it attempts to determine the most efficient sequence in which to produce the
product. A process planner must be aware of the various aspects of manufacturing to plan
properly. The planner typically works with the blueprints and may communicate with the design
team to clarify or request changes in the design to fit manufacturing requirements. The outcome
of the process planning is a production plan, tools procurement, material order, and machine
programming. Other special manufacturing needs such as design or jigs and fixtures or
inspection gages are planned.
Figure 1.1.Typical product life cycle
Product Cycle Model:
There are several Product cycle models in industry to be considered, one of the possible
product cycle is given below (Figure 1.2):
Step 1: Conceive-Imagine, Specify, Plan, Innovate
The first step is the definition of the product requirements based on company, market and
customer. From this requirement, the product's technical data can be defined. In parallel, the
early concept design work is performed defining the product with its main functional features.

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Various media are utilized for these processes, from paper and pencil to clay mock-up to 3D
Computer Aided Industrial Design.
Step 2: Design-Describe, Define, Develop, Test, Analyze and Validate
This is where the completed design and development of the product begins, succeeding to
prototype testing, through pilot release to final product. It can also involve redesign and ramp for
Improvement to existing products as well as planned obsolescence. The main tool used for
design and development is CAD. This can be simple 2D drawing / drafting or 3D parametric
feature based solid/surface modeling.
This step covers many engineering disciplines including: electronic, electrical, mechanical, and
civil. Besides the actual making of geometry there is the analysis of the components and
assemblies.
Optimization, Validation and Simulation activities are carried out using Computer Aided
Engineering (CAE) software. These are used to perform various tasks such as: Computational
Fluid Dynamics (CFD); Finite Element Analysis (FEA); and Mechanical Event Simulation
(MES). Computer Aided Quality (CAQ) is used for activities such as Dimensional tolerance
analysis.
Figure 1.2.Product Cycle Model
Step 3: Realize-Manufacture, Make, Build, Procure, Produce, Sell and Deliver
Once the design of the components is complete the method of manufacturing is finalized.
This includes CAD operations such as generation of CNC Machining instructions for the

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product’s component as well as tools to manufacture those components, using integrated
Computer Aided Manufacturing (CAM) software.
It includes Production Planning tools for carrying out plant and factory layout and production
simulation. Once details components are manufactured their geometrical form and dimensions
can be verified against the original data with the use of Computer Aided Inspection Equipment
(CAIE). Parallel to the engineering tasks, sales and marketing work take place. This could
consist of transferring engineering data to a web based sales configuration.
Step 4: Service-Use, Operate, Maintain, Support, Sustain, Phase-out, Retire, Recycle and
Disposal
The final step of the lifecycle includes managing of information related to service for
repair and maintenance, as well as recycling and waste management information. This involves
using tools like Maintenance, Repair and Operations Management software.
2. Briefly explain about design process.
DesignProcess
The design process includes series of steps that engineers apply in making functional
products and processes. The parts of the process often need to be repeated many times before
production of a product can start. The parts that get iterated and the number of such design cycles
in any given project can be highly changeable.
One method of the engineering design process focuses on the following common aspects:
1. Research
A considerable amount of time is used on research, or finding information. Consideration
should be given to the available applicable literature, issues and successes linked with available
solutions, and need of marketplaces.
The basis of information should be significant, including existing results. Reverse engineering
can be a successful technique if other solutions are available in the market. Added sources of
information include the trade journals, available government documents, local libraries, vendor
catalogs and personal organizations.
2. Feasibility assessment
The feasibility study is an analysis and assessment of the possible of a proposed design
which is based on detail investigation and research to maintain the process of decision creation.
The feasibility assessment helps to focus the scope of the project to spot the best situation. The
purpose of a feasibility assessment is to verify whether the project can continue into the design
phase.

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Fig.2.1. Design Process
3. Conceptualization
A Concept Study is the stage of project planning that includes developing ideas and
taking into account the all features of executing those ideas. This stage of a project is done to
reduce the likelihood of assess risks, error and evaluate the potential success of the planned
project.
4. Establishing the design requirements
Establishing design requirements is one of the most essential elements in the design
practice, and this task is usually performed at the same time as the feasibility analysis. The
design requirements control the design of the project all over the engineering design process. A
few design requirements comprise maintainability, hardware and software parameters,
availability, and testability.
5. Preliminary design
The preliminary design fills the gap between the design concept and the detailed design
phase. During this task, the system configuration is defined, and schematics, diagrams, and
layouts of the project will offer early project configuration. In detailed design and optimization,
the parameters of the part being produced will change, but the preliminary design focuses on
creating the common framework to construct the project.
6. Detailed design
The next phase of preliminary design is the Detailed Design which may include of
procurement also. This phase builds on the already developed preliminary design, aiming to
further develop each phase of the project by total description through drawings, modeling as well
as specifications.
The advancement CAD programs have made the detailed design phase more competent. This is
because a CAD program can offer optimization, where it can shrink volume without
compromising the part's quality. It can also calculate displacement and stress using the FEM to

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find stresses throughout the part. It is the responsibility of designer to find whether these stresses
and displacements are acceptable, so the part is safe.
7. Production planning and tool design
The production planning and tool design is more than planning how to mass-produce the
project and which tools should be used in the manufacturing of the component. Tasks to
complete in this stage include material selection, identification of the production processes,
finalization of the sequence of operations, and selection of jigs, fixtures, and tooling. This stage
also includes testing a working prototype to confirm the created part meets qualification
standards.
With the finishing of qualification testing and prototype testing, the design process is completed.
(Or)
Designprocess---Shingley Model
It involves six steps
1. Recognition of need
 Realization of problem exists in the design or in the product
 Identification of some defect in a current machine design
 New product opportunity
2. Definition of problem
 Specification of the item to be designed
 Functional characteristics, cost, quality, performance, etc.
Recognitionof need
Definitionof problem
Synthesis
Analysisand
Optimization
Evaluation
Presentation
Change the design
Can the designbe improved
Designimpossibleforthe
givenspecifications
Yes
NoNo

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3. Synthesis
Preliminary ideas are developed through research of similar product or designs in use.
4. Analysis and Optimization
 Suitability for the specified design constraints
 If not suitable or design fails to satisfy the constraints
 Then redesign or modified iteration continues until the proposed design meet the
specifications (or) until feasibility is achieved.
 Then components, sub-assemblies or sub-systems are then synthesized into the
final overall system in a similar iterative manner.
5. Evaluation
 Prototyping
 Testing
 Quality
 Reliability testing
6. Presentation
Documentation of the design through drawings, material specifications, assembly lists,
etc.
Pahl and Beitz Model:
1. Clarification of the task: Collection of information, constraints on the design.
2. Conceptual design: establishment of the functions to be included in the design,
3. Embodiment design: problems are resolved and weak aspects are eliminated
4. Detail design: The dimensions, tolerance, materials and form of individual
components of the design are specified in design for subsequent manufacturing.
3. Write briefly on Sequential and Concurrent Engineering.
Sequential Engineering:
 Sequential engineering is the term used to explain the method of production in a linear
system. The various steps are done one after another, with all attention and resources
focused on that single task.
 Sequential engineering is a system by which a group within an organization works
sequentially to create new products and services.
 The sequential engineering is a linear product design process during which all stages of
manufacturing operate in serial. Both process and product design run in serial and take
place in the different time.
 Process and Product are not matched to attain optimal matching.
 Decision making done by only group of experts.

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Sequential Engineering
Concurrent Engineering:
 In concurrent engineering, various tasks are handled at the same time, and not essentially
in the standard order. This means that info found out later in the course can be added to
earlier parts, improving them, and also saving time.
Concurrent engineering
 Concurrent engineering is a method by which several groups within an organization work
simultaneously to create new products and services.
 The concurrent engineering is a non-linear product design process during which all stages
of manufacturing operate at the same time. Both product and process design run in
parallel and take place in the same time.
 Process and Product are coordinated to attain optimal matching of requirements for
effective quality and delivery.
 Decision making involves full team involvement.

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4. Write briefly on Computer Aided Design.
CAD is the intersection of Computer Graphics, Geometric modeling and Design tools (Fig.4.1.).
The concepts of computer graphics and geometric modeling and must be used innovatively to serve the
design process. CAD is the function of computer systems to support in the creation, modification,
analysis, or optimization of a design.
Fig.4.1.CAD
 CAD software for design uses either vector-based graphics to explain the objects of traditional
drafting, or may also develop raster graphics showing the overall look of designed objects.
During the manual drafting of engineering drawings, the output of CAD must convey
information, like dimensions, materials, processes,and tolerances.
 CAD is a significant industrial art used in many purposes, including industrial and architectural
design, shipbuilding, automotive, and aerospace industries, and many more. CAD is also
extensively used to create computer animation for special effects in movies, and technical
manuals, frequently called as Digital Content Creation.
 CAD software packages provide the designer with a multi window environment with animation
which is regularly used in Digital Content Creation. The animations using wire frame modeling
helps the designer to see into the interior of object and to observe the behaviors of the inner
components of the assembly during the motion.
CAD Tools
The CAD tools are mainly using for graphics applications and modeling. Aids such a color, grids,
geometric modifiers and group facilitate structural geometric models. Visualization is achieved
through shaded components and animation which focus design conceptualization, communication and
interference detection. FEM packages provide optimization in shape and structure. Adding tolerances,
tolerance analysis and investigating the effect of manufacturing on the design can perform by
utilizing CAD tools.
CAD Tools Design Process
Geometric modeling, Graphics aids, visualization and
manipulation
Conceptualization

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Geometric modeling, Graphics aids, visualization and
manipulation, animation, assemblies
Modeling and Simulation
Analysis packages, customized programs Design Analysis
Structural optimization Design Optimization
Dimensioning, tolerance, bill of materials Design evaluation
Drafting and detailing , Shaded images
Communication and Documentation
Uses ofCAD
CAD is one of the tools used by designers and engineers and is used in different ways depending
on the profession of the customer and the type of software.
CAD is one of the Digital Product Development activities within the Product Lifecycle
Management practices with other tools, which are either integrated modules or individual, such as:
 Computer Aided engineering (CAE) and Finite Element Analysis (FEA)
 Computer Aided Manufacturing (CAM)
 Realistic Rendering and Simulation.
 Product Data Management (PDM).
CAD is also used for the development of photo simulations that are frequently necessary in the
preparation of Environmental Impact Reports, in which proposed CAD buildings are superimposed into
photographs of existing situation to represent what that conditions will be like, where the proposed
services are allowed to be built.
Parameters and constraints can be used to get the size, shape, and other properties of the modeling
elements. The features of the CAD system can be used for the several tools for measurement; such as
yield strength, tensile strength and electrical or electro-magnetic properties.
CAD System Architecture
Computer architecture is a pattern describing how a group of software and hardware technology
standards relate to form a computer system. In general, computer architecture refers to how a computer is
designed and what technologies it is compatible with. Computer architecture is likened to the art of
shaping the needs of the technology, and developing a logical design and standards based on needs.
5. Write short notes on Scope and applications of CAD/CAM
Scope of CAD/CAM
At the core of the CAD and CAM processes is a geometric model of the product under
design. Activities of the CAD process include mass properties, finite element analysis,
dimensioning. tolerancing, assembly modeling, generating shaded images, and documentation
and drafting. Activities of the CAM process include CAPP (Computer Aided Process Planning),
NC (numerical control) programming, design of injection mold, CMM (coordinate measuring
machine) verification, inspection, assembly via robots, and packaging.

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The CAD process and its tools utilize three disciplines: geometric modeling, computer
graphics, and design. The CAM process utilizes the disciplines of CAD itself, manufacturing,
and automation.
CAD/CAM Applications
There are considerable numbers of applications for the various existing CAD/CAM
systems. For example, there are mechanical, electrical, and architectural CAD and CAM
products. An inspection of these various systems reveals that they have a generic structure and
common modules. Awareness of such structure and modules enables users to better
understand system functions for both evaluation and training purposes.
The major available modules are
The geometric engine (module) is the heart of a CAD/CAM system. It provides users
with functions to perform geometric modeling and construction. Editing and manipulation of
existing geometry, drafting and documentation. The typical modeling operations that users can
engage in arc model creation, cleanup, documentation, and printing/plotting. Shaded images can
be generated as part of model documentation.
Fig.5.1 CAD and CAM disciplines
The applications module varies from one software system to another. However, there
are common applications shared by most CAD/CAM systems. Mechanical applications include
mass property calculations, assembly analysis, tolerance analysis and synthesis, sheet metal
design, finite element modeling and analysis, mechanisms analysis, animation techniques,
and simulation and analysis of plastic injection molding. Manufacturing applications include
CAPP, NC, CIM, robot simulation, and group technology.

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The programming module allows users to customize systems by programming them to
fit certain design and manufacturing tasks. A CAD/CAM system requires advanced knowledge
of the system architecture, its database format, and a high-level programming language such as
C, C++, Java, Scheme, or others.
The communications module is crucial if integration is to be achieved between
the CAD/CAM system, other computer systems, and manufacturing facilities. It is common to
network the system to transfer the CAD database of a model for analysis purposes or to transfer
its CAM database to the shop floor for production. This module also serves the purpose of
translating databases between CAD/CAM systems using graphics standards such as IGES and
STEP.
The collaborative module is emerging as an outcome of the widespread of the World
Wide Web and the Internet. This module supports collaborative design. Various design teams in
different geographical locations can work concurrently on the same part, assembly, or drawing
file in real time over the Web. One team can make changes that other teams can view and accept
or reject.
The various CAD/CAM systems support engineering design, analysis, and manufacturing
applications. They all have a flexible pricing structure that allows customers to add on
applications as needed.
6. Write short notes on: CAD, CAD process, components of CAD system, and
advantages of CAD systems.
Computer-Aided Design (CAD)
 CAD can be defined as the use of computer systems to perform certain functions
in the design process. Use of computer systems to assist in the creation,
modification, analysis, and optimization of a design
In general, a Computer Aided Design (CAD) package has three components: a) Design, b)
Analysis, and c) Visualization. A brief description of these components follows.
a) Design: Design refers to geometric modeling, i.e., 2-D and 3-D modeling, including, drafting,
part creation, creation of drawings with various views of the part, assemblies of the parts, etc.
b) Analysis: Analysis refers to finite element analysis, optimization, and other number crunching
engineering analyses. In general, a geometric model is first created and then the model is
analyzed for loads, stresses, moment of inertia, and volume, etc.
c) Visualization: Visualization refers to computer graphics, which includes: rendering a model,
creation of pie charts, contour plots, shading a model, sizing, animation, etc.
Each of these three areas has been extensively developed in the last 30 years Several books are

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written on each of these subjects and courses are available through the academic institutions and
the industry.
 Typical tools:
– Tolerance analysis
– Mass property calculations
– Finite-element modeling and visualization
 Defines the geometry of the design
Computer-Aided Design Process
Two types of activities: synthesis and analysis
 Synthesis is largely qualitative and hard to capture on computer.
 Analysis can be greatly enhanced with computers.
 Once analysis is complete, design evaluation- rapid prototyping are done.
Fig 6.1.Components of CAD/CAM/CAE Systems
CAD Hardware
There are basically two types of devices that constitute CAD hardware: a) Input
devices, and b) Output devices.
Input Devices
These are the devices that we use for communicating with computer, and providing our
input in the form of text and graphics. The text input is mainly provided through keyboard. For
graphic input, there are several devices available and used according to the work environment. A
brief description of these devices is given here.

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Mouse: This is a potentiometric device, which contains several variable resistors that send
signals to the computer. The functions of a mouse include locating a point on the screen,
sketching, dragging an object, entering values, accepting a software command, etc. Joystick and
trackballs are analogous to a mouse device, and operate on the same principle.
Digitizers: Digitizers are used to trace a sketch or other 2-D entities by moving a cursor over a
flat surface (which contains the sketch). The position of the cursor provides a feedback to the
computer connected with the device. There are electrical wires embedded in orthogonal
directions that receive and pass signals between the device and the computer. The device is
basically a free moving pen shaped stylus, connected to a tablet.
Light Pens: Lockheed’s CADAM software utilized this device to carry out the graphic input. A
light pen looks like a pen and contains a photocell, which emits an electronic signal. When the
pen is pointed at the monitor screen, it senses light, which is converted to a signal. The signal is
sent to the computer, for determination of the exact location of the pen on the monitor screen.
Touch Sensitive Screens: This device is embedded in the monitor screens, usually, in the form
of an overlay. The screen senses the physical contact of the user. The new generation of the
Laptop computers is a good example of this device.
Other Graphic Input Devices: In addition to the devices described above, some CAD software
will accept input via Image Scanners, which can copy a drawing or schematic with a camera and
light beam assembly and convert it into a pictorial database.
The devices just described are, in general, independent of the CAD package being used.
All commercial CAD software packages contain the device drivers for the most commonly used
input devices. The device drivers facilitate a smooth interaction between our input, the software,
and the computer. An input device is evaluated on the basis of the following factors:
• Resolution
• Accuracy
• Repeatability
• Linearity
Output Devices
After creating a CAD model, we often need a hard copy, using an output device. Plotters and
printers are used for this purpose.
A plotter is often used to produce large size drawings and assemblies, whereas, a laser jet printer
is adequate to provide a 3-D view of a model. Most CAD software require a plotter for producing
a shaded or a rendered view.
CAD Software
 CAD software is written in FORTRAN and C languages. FORTRAN provides the
number crunching, whereas, C language provides the visual images.
 The modern CAD software utilizes the open architecture system, i.e., software vendors
do not design and manufacture their own hardware. Third party software can be used to
augment the basic CAD package.

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 Most popular CAD package will facilitate integration of the Finite Element Analysis and
other CAD software from more than one vendor. For example, IDEAS preprocessor can
work with almost all the FEA packages for pre and post analyses.
 Networking is an important consideration in applications of CAD software. A model
created by one engineer must be readily accessible to others in an organization, which is
linked by a LAN or other means. The designer, analyst, management, marketing, vendor,
and others generally share a model. This is the concurrent engineering in action,
mentioned earlier.
CAD Platform
 In general, we can run CAD software on three different CAD platforms: Mainframe,
Workstation, and PC.
 When the CAD programs first became available, they could only be run on a mainframe
computer. However, as the PCs have become faster and cheaper, almost all the CAD
vendors have introduced a version of their CAD software that will effectively run on a
Pentium or higher computer. Currently, the most popular platforms are PCs and
Workstations.
 Popularity of Workstations stems from their ability to network easily with other
computers, and also, due to their large memory storage capability. However, PC platform
is still the most preferred medium for most engineers.
 Increasing popularity of the PC platform can be attributed to several factors, including,
total user control, the speed, capability of storing large memory, ease of hardware
upgrading and maintenance, and the overall reasonable cost.
CAD PLATFORMS
MAINFRAME WORKSTATIONS PCs
MAINFRAME WORKSTATIONS PCs
Large Data storage Medium size data storage Limited data storage
Networked Networked Can be networked
Expensive Relatively inexpensive Inexpensive
Need interface language Runs on Unix Run on MS-Windows
No user control No user control User controlled
Good for Large corporations
Good for small companies Good for all users

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Advantages of CAD/CAM systems
 Greater flexibility, Reduced lead times, Reduced inventories, Increased Productivity,
Improved customer service, Improved quality, Improved communications with suppliers,
Better product design, Greater manufacturing control, Supported integration, Reduced
costs, Increased utilization, Reduction of machine tools, Less floor space
CAD/CAM Applications – Ref Q.No:5
7. Write short notes on: Concurrent Engineering, Characteristics of Concurrent
Engineering with example.
Characteristics of Concurrent Engineering
In concurrent engineering functional divisions like design, manufacturing and quality are
integrated in a compatible environment. The integration comes in the form of instant delivery
of information about business processes across the enterprise. Cross functional units co-operate
concurrently rather than sequentially. The real-time sharing of information enables teams to
make design modifications early in a product development cycle, thus reducing unwanted
rework and engineering changes that increase cost of operations, reduce product quality and
delay the time-to-market.
The concurrent engineering approach can be characterized by the following factors:
• Integration of product and process development and logistics support
• Closer attention to the needs of customers
• Adoption of new technologies
• Continuous review of design and development process
• Rapid and automated information exchange
• Cross functional teams
• Rapid prototyping
Example of Concurrent Engineering
The story of the development of Neon Car in USA is a typical example of success of
concurrent engineering. The planning of the car started in August 1990.
For each major item product teams were made. Supporting teams were organized for such
activities like dimension control, materials etc. The composition of a typical team included
representatives from engineering, stamping, manufacturing processes, assembly, design,
purchase, finance, product planning, materials handling and vendor development. Even
suppliers were part of the product development team. Each team took approximately one year
to complete a task. Subsequently process teams were organized to manufacture the product.
Four months before the launch the process teams were converted into launch teams to
successfully introduce the product in the market.

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Another example for successful implementation of concurrent engineering is the
development of Scooty moped and other products by TVS Motors Ltd. in India. Before taking
up the design cross functional teams were formed to design and engineer the product. This
reduced not only the product development time but also helped the manufacturer to introduce
the quality product in the market.
8. Briefly explain the Coordinate Systems and its types
Coordinate Systems
Three types of coordinate systems are needed in order to input, store, and display model
geometry and graphics. These are Model Coordinate System (MCS), Working Coordinate
System (WCS) and Screen Coordinate System (SCS), respectively. Other names for MCS are
database, master, or world coordinate system. Another name for SCS is device coordinate
system.
Model Coordinate System
 The model coordinate system is defined as the reference space of the model with respect
to which all the model geometrical data is stored.
 It is a Cartesian system which forms the default coordinate system used by a particular
software program. The X, Y, and Z axes of the MCS can be displayed on the computer
screen. The origin of the MCS can be arbitrarily chosen by the user while its orientation
is established by the software.
 The three default sketch planes of a CAD/CAM system define the three planes of the
MCS, and their intersection point is the MCS origin. When a CAD designer begins
sketching, the origin becomes a corner point of the profile being sketched. The sketch
plane defines the orientation of the profile in the model 3D space.
 In order for the user to communicate properly and effectively with a model database, the
relationship between the MCS orthogonal (sketch) planes and the model views must be
understood by the user. Typically, the software chooses one of two possible orientations
of the MCS in space.
Fig 8.1.Possible orientation

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 As shown in Figure8.1a, the XY plane is the horizontal plane and defines the model top
view. The front and right side views are consequently defined by the XZ and YZ planes,
respectively. Figure 8.1b shows the other possible orientation of the MCS where the
XY plane is vertical and defines the model front view. As a result, the XZ and the
YZ planes define the top and the right side views, respectively.
 Existing CAD/CAM software uses the MCS as the default WCS. In both orientations,
the XY plane is the default construction (sketch) plane. If the user utilizes such a plane,
the first face of a model to be constructed becomes the top or front view, depending on
which MCS is used.
 The MCS is the only coordinate system that the software recognizes when storing or
retrieving graphical information in or from a model database. Many existing software
packages allow the user to input coordinate information in cartesian (x, y. z) and
cylindrical (r, θ, z) coordinates. This input information is transformed to (x, y. z)
coordinates relative to the MCS before being stored in the database.
 Obtaining views is a form of retrieving graphical information relative to the MCS. If the
MCS orientation does not match the desired orientation of the object being modeled,
users become puzzled and confused.
 Another form of retrieving information is entity verification. Coordinates of points
defining the entity are given relative to MCS by default. However, existing software
allows users to obtain the coordinates relative to another system (WCS) by using the
proper commands or modifiers
Working Coordinate System
 It is often convenient in the development of geometric models and the input of geometric
data to refer to an auxiliary coordinate system instead of the MCS. This is usually useful
when a desired plane (face) of construction is not easily defined as one of the MCS
orthogonal planes, as in the case of inclined faces of a model. The user can define a
Cartesian coordinate system whose XY plane is coincident with the desired plane of
construction. That system is the Working Coordinate System, WCS. It is a convenient
user-defined system that facilitates geometric construction.
 It can be established at any position and orientation in space that the user desires. While
the user can input data in reference to the WCS, the CAD software performs the
necessary transformations to the MCS before storing the data.
 The ability to use two separate coordinate systems within the same model database in
relation to one another gives the user great flexibility. Some commercial software refers
to the WCS as is Unigraphics offers an example. Other software refers to it as a sketch
plane (Pro/E and SolidWorks) or construction plane.
 A WCS requires three noncollinear points to define its XYplane. The first defines the
origin, the first and the second define the X axis, and the third points with the first
define the Y axis. The Z axis is determined as the cross product of the two unit vectors in

P a g e | 21
the directions defined by the lines connecting the first and the second (the X axis), and
the first and the third points (Y axis). We will use the subscript w to distinguish the
WCS axes from those of the MCS. The XwYwplane becomes the active sketch (working)
or construction plane if the user defines a WCS.
 In this case, the WCS and its corresponding Xw,Yw plane override the MCS and the
default sketch plane, respectively. As a matter of fact, the MCS with its default sketch
plane is the default WCS with its Xw,Yw plane. All CAD/CAM software packages
provide users with three standard WCSs {sketch planes) that correspond to the three
standard views: Front, Top, and Right sides. The user can define other WCSs or sketch
planes.
 There is only one active WCS (sketch plane) at any one time. If the user defines multiple
WCSs in one session during a model construction, the software recognizes only the last
one and stores it with the model database if the user stores the model. The model tree
displayed by the CAD software shows the last selected (activated) sketch plane.
Once a WCS is defined, user coordinate inputs are interpreted by the software in reference to this
system. The software calculates the corresponding homogeneous transformation matrix between
the WCS and the MCS to convert these input values into coordinates relative to the MCS before
storing them in the database. The transformation equation can be written as:
Pm = T Pw
where Pm is the position vector of a point relative to the MCS and Pw is the vector of the
point relative to the active WCS. Each vector is given by:
P = [x y z 1] T
The matrix [T] is the homogeneous transformation matrix. It is a 4 x 4 matrix and is
given by:
Where [R]w
m
is the rotation matrix that definesthe orientation of the WCS relative to the MCS
and [P]w org
m
is the position vector that describes the origin of the WCS relative to the MCS. The
columns of[R]w
m
: give the direction cosines of the unit vectors in the Xw, Yw and Zw directions
relative to the MCS as shown in Figure .These direction cosines are the components of the unit
vectors along the axes of the MCS.

P a g e | 22
If the WCS axes are along the MCS axes, then the direction cosines become 1, -1, or 0, and if Xw
and Yw are aligned along the Z and X axes of the MCS, respectively, the transformation
between the WCS and MCS is given by:
Observe that one of the matrix in eqn. is inverse of the other; their manipulation produces the
identity Matrix, I.
Screen Coordinate System
 In contrast to the MCS and WCS, the screen coordinate system (SCS) is defined as a 2D
device-dependent coordinate system whose origin is usually located at the lower left
comer of the graphics display, as shown in Figure.
 The physical dimensions of a device screen (aspect ratio) and the type of device (raster)
determine the range of the SCS. The SCS is mostly used in view-related clicks such as
definitions of view origin and window or clicking a view to select it for graphics
operations.
 A 1024 x 1024 display has an SCS with a range of (0, 0) to (1024, 1024). The center of
the screen has coordinates of (512,512). This SCS is used by the CAD/CAM software to
display relevant graphics by converting directly from MCS coordinates to SCS (physical
device) coordinates. A normalized SCS can also be utilized. The range of the SCS can be
chosen from (0, 0) to (I, 1).
 Such representation can be translated by device-dependent codes to the appropriate
physical device coordinates. The third method of defining an SCS is by using the drawing
size that the user chooses.
 If a size A drawing is chosen, the range of the SCS becomes (0,0) to (11,8.5) while
size B produces the range (0,0) to (17,11). The rationale behind this method stems from
the conventional drawing board so that the drafting paper is represented by the device
screen.

P a g e | 23
Typical SCS
A transformation operation from MCS coordinates to SCS coordinates is performed by
the software before displaying the model views and graphics. Typically, for a geometric model,
there is a data structure to store its geometric data (relative to MCS), and a display file to store its
display data (relative to SCS).
9. Explain the concepts of translation, scaling and rotators in 2-D transformation.
(JAN 2014)
Geometric transformations have numerous applications in geometric modeling, e.g.,
manipulation of size, shape, and location of an object.
In CAD, transformation is also used to generate surfaces and solids by sweeping curves
and surfaces, respectively. The term ‘sweeping’ refers to parametric transformations, which are
utilized to generate surfaces and solids. When we sweep a curve, it is transformed through
several positions along or around an axis, generating a surface.
The appearance of the generated surface depends on the number of instances of the
transformation.
There are two types of transformations:
Modeling Transformation:
This transformation alters the coordinate values of the object. Basic operations are
scaling, translation, rotation and, combination of one or more of these basic transformations.
Examples of these transformations can be easily found in any commercial CAD software. For
instance, AutoCAD uses SCALE, MOVE, and ROTATE commands for scaling, translation,
and rotation transformations, respectively.
Visual Transformation:
In this transformation there is no change in either the geometry or the coordinates of
the object. A copy of the object is placed at the desired sight, without changing the coordinate
values of the object. In AutoCAD, the ZOOM and PAN commands are good examples of
visual transformation.
Two-Dimensional (2D) Geometric Transformation:

P a g e | 24
(0,0)
∆x
∆y
𝑃′(𝑥′,𝑦′)
𝑃(𝑥 , 𝑦 )
𝑃′(𝑥′,𝑦′)
A geometric transformation is an operation that modifies its shape, size, position, orientation
etc. with respect to its current configuration operating on the vertices (position vectors).
Some of the important 2D transformations include:
1. Translation
2. Scaling
3. Rotation
4. Reflection
5. Shear
6. Twist
2D translation
Translation is nothing but moving an object across the screen from one position to another.
The translation transformation positions the object to a new location.
 Translation is the process of moving an object from one position to another.
 The translation transformation positions the object to a new location.
 The translation is accomplished by adding the coordinates of each corner point the
distance through which the object is to moved.
Translation of a point:
Consider a 2D point P, having coordinates x, y. To translate point P by a distance ∆𝑥 in
x-direction and ∆𝑦 in y-direction, a translation matrix T is added to the original matrix. Now the
point has new coordinates
The translation distance (∆𝑥,∆𝑦) is called translation vector.
Translation of a point
Let
P= Original position of the point,
P’= New position of the point, and
T= Translation matrix
tx(∆𝑥), ty(∆𝑦)are the Translation distance along the x and y axis respectively.

P a g e | 25
(0,0)
𝐵(𝑥 , 𝑦 )
A(x ,y )
C(x ,y )
∆x
∆y
𝐶′(𝑥′,𝑦′)
𝐴′(𝑥′, 𝑦′)
𝐵′(𝑥′,𝑦′)
Translation of an object:
When the object is to moved, all the points of it are to be translated. The translation of
keypoints and connections of these points by other geometric entities like lines, arcs, etc.
Object Translation
𝐴𝑥′ 𝐴𝑦′
𝐶𝑥′ 𝐵𝑦′
𝐶𝑥′ 𝐶𝑦′
=
𝐴𝑥 𝐴𝑦
𝐶𝑥 𝐵𝑦
𝐶𝑥 𝐶𝑦
+ [∆𝑥 ∆𝑦]
Example: Translate a triangle ABC with co-ordinates A (1,1), B(5,2), C(3,3), about the origin
by 3 units in the x-direction and 2 units in the y-direction.
Solution:
Given that ∆𝑥 = 3 𝑎𝑛𝑑 ∆𝑦 = 2
And
𝐴𝑥 𝐴𝑦
𝐶𝑥 𝐵𝑦
𝐶𝑥 𝐶𝑦
=
1 1
5 2
3 3
=
𝐴𝑥 𝐴𝑦
𝐶𝑥 𝐵𝑦
𝐶𝑥 𝐶𝑦
+ [∆𝑥 ∆𝑦]
,
, ,
x y
x
y
x x t y y t
tx x
ty y
    
     
            
  
P P T
P P T

P a g e | 26
=
1 1
5 2
3 3
+ [3 2] =
4 3
8 4
6 5
The new coordinates of the translated triangle are A’(4,3), B’(8,4) and C’(6,5).
2D Rotation
 Rotation refers to the movement an object in such a way that the distance between a
certain fixed point and any given point of that body remains constant.
 Rotation transformation techniques is commonly used in rendering and animation
tehniques.
 Consider the rotation of a point from initial position a P(x,y) to a new position P’(x’,y’)
by an angle θ about the origin
 Here counterclockwise rotation is consider as positive and clockwise rotation as
negative.

P a g e | 27
Mathematically, the coordinates of P’ are given by
X’ = r cos (α+θ) = r cos α cos θ – r sin α sin θ
Y’ = r sin (α+θ) = r sin α cos θ + r cos α sin θ
Substituting x = r cos α and y = r sin α into the equations gives
X’ = x cos θ – y sin θ
Y’ = x cos θ + y sin θ
2D Scaling
 Scaling is the transformation used to change, increase or decrease, the size of an object.
 Scaling can be achieved by multiplying the original coordinates of an object by the
scaling factor Sx along x-direction and Sy along y-direction.
 Scaling factor is always positive, if scaling factor is less than 1, the object is
compressed; if more than 1, the object is stretched.
 If scale factors are equal i.e., Sx=Sy=S, the object changes in size only and not in shape.
This scaling is known as uniform scaling.
 If scale factors are different i.e., Sx≠Sy, the object changes in size only and not in shape.
This scaling is known as non-uniform scaling.
 
Rotation in angle about a
pivot (rotation) point , .r rx y


P a g e | 28
Example:
If a triangle A(1,1), B(3,1), C(1,3) is scaled by a factor 2, find the new coordinates of the
triangle.
Given: A(1,1), B(3,1), C(1,3); uniform scale factor =2, the effect of scling is represented
graphically in Fig.
,
0
0
x y
x
y
x x s y y s
sx x
sy y
    
     
         
  P S P
 Scaling about a fixed point ,f fx y

P a g e | 29
2D reflection
 Reflection is a transformation in which the direction of one axis is reversed.
 Reflection transformation produces a mirror image of an object.
 The reflection transformation is useful in the construction of symmetric objects. If the
object is symmetric with respect to plane, only half of the geometry is created and then
the half model is copied by reflection to develop the full model.
P’=M.P
M=
±𝑚11 0 0
0 ±𝑚22 0
0 0 ±𝑚33
Reflection about x axis:
M=
1 0 0
0 −1 0
0 0 1
Reflection about y axis:
M=
−1 0 0
0 1 0
0 0 1
Reflection about xy plane (origin):
M=
−1 0 0
0 −1 0
0 0 1

P a g e | 30
In scaling transformation, the original coordinates of an object are multiplied by the
given scale factor. There are two types of scaling transformations: uniform and non-uniform.
In the uniform scaling, the coordinate values change uniformly along the x, y, and z
coordinates, whereas, in non-uniform scaling, the change is not necessarily the same in all the
coordinate directions.
Let us consider some typical cases
 Case 1: a=d=1 and b=c=0 – No Change (identity)
 Case 2: d=1, b=c=0 – Scaling in x coordinate
 Case 3: b=c=0 – Scaling in both x and y coordinates
 Case 4; a=d =|s|>1 – Enlargement of the original entity

P a g e | 31
 Case 5: 0<a=d=|s|<1 – Compression of the entity
 Case 6: b=c=0, a=1,d=-1 – Reflection about x- axis
 Case 7: b=c=0, a=-1,d=1 – Reflection about y- axis
 Case 8: b=c=0, a=d<0 – Reflection about the origin
Homogeneous Coordinates
Each of the above transformations with the exception of translation can be represented as a
row vector X, Y and a 2 X 2 matrix. However, all the four transformations discussed above can
be represented as a product of a 1 X 3 row vector and an appropriate 3 X 3 matrix. The
conversion of a two-dimensional co-ordinate pair (X, Y) into a 3-dimensional vector can be
achieved by representing the point as [X Y 1].

P a g e | 32
After multiplying this vector by a 3 X 3 matrix, another homogeneous row vector is
obtained [X1 Y1 1]. The first two terms in this vector are the co-ordinate pair which is the
transform of (X, Y). This three dimensional representation of a two dimensional plane is called
homogeneous coordinates and the transformation using the homogeneous co-ordinates is
called homogeneous transformation.
10. Explain the concept of 3D transformation. (JAN 2014)
A three-dimensional object has a three-dimensional geometry, and therefore,
it requires a three-dimensional coordinate transformation. A right handed coordinate
system is used to carry out a 3-D transformation.
The scaling and translation transformations are essentially the same as two-
dimensional transformations. However, the points matrix will have a non-zero 3rd
column. Additionally, the transformation matrices contain some non-zero values in
the third row and third column.
Translation
In three-dimensional homogeneous coordinate representation, a point is transformed from
position P(x,y,z) to P’= (x’,y’,z’) this can be written as:
x’ = x+ tx
y’ = y+ ty
z’ = z+ tz

P a g e | 33
P’=T.P
Rotation

P a g e | 34
Parallel to one of the Coordinate Axis
In special cases where an object is to be rotated about an axis that is parallel to one of
the coordinate axis, we can obtain the desired rotation with the following transformation
sequence.
1. Translate the object so that the rotation axis coincides with the parallel coordinate
axis (for simplicity, let us take x-axis).
2. Perform the specified rotation about that axis.
3. Translate the object so that the rotation axis is moved back to its original position.

P a g e | 35
Scaling
The matrix expression for the scaling transformation of a position P = (x, y, z)
relative to coordinate origin can be written

P a g e | 36
The matrix representation for an arbitrary fixed-point (xf, yf, zf) can be expressed as:
Reflection
 The matrix expression for the reflection transformation of a position P = (x, y, z)
relative to x-y plane can be written as:
 Transformation matrices for inverting x and y values are defined similarly, as
reflections relative to yz plane and xz plane, respectively.
Shears:
The matrix expression for the shearing transformation of a position P = (x, y, z), t
produce z-axis shear, can be written as:
• Parameters a and b can be assigned any real values. The effect of this transformation

P a g e | 37
is to alter x- and y- coordinate values by an amount that is proportional to the z value,
while leaving the z coordinate unchanged.
• Shearing transformations for the x axis and y axis are defined similarly.
11. Explain computer graphics. (Or) What is meant by Interactive Computer
Graphics? Explain its various elements. (Nov 08)
Traditionally drawings are prepared on plane drawing sheets. This has several limitations.
The sketches have to be made only in two dimensions. Though the depth can be represented
by pictorial projections like isometric and perspective projections, the projections have to be
necessarily reduced to two dimensions.
Use of computer graphics has opened up tremendous possibilities for the designer.
Some of them are listed below:
 The object is represented by its geometric model in three dimensions (X, Y and Z).
 The mathematical representation reduces creation of views like orthographic, isometric,
axonometric or perspective projections into simple viewing transformations.
 Though the size of the screen is limited, there is no need to scale the drawings.
 Drawings can be made very accurate.
 The geometric models can be represented in color and can be viewed from any angle.
 Sections can be automatically created.
 The associativity ensures that any change made in one of the related views will
automatically reflect in other views.
 Revision and revision control are easy.
 Drawings (geometric models) can be modified easily.
 More important than all, drawings can be reused conveniently.
 Storage and retrieval of drawings are easy.
Modern computer graphics displays are simple in construction. They consist of basically
three components.
i. Monitor
ii. Digital Memory or Frame Buffer iii. Display Controller
Most of the computer graphics displays use raster CRT which is a matrix of discrete cells

P a g e | 38
each of which can be made bright. A graphic entity like line or circle is represented as a
series of “points or dots” on the screen. Therefore, it is called as a point plotting device.
The video display screen is divided into very small rectangular elements called a
picture element or pixel. This happens to be the smallest addressable screen element.
Graphic images are formed by setting suitable intensity and color to the pixels which
compose the image.
Depending upon the resolution screens may have varying number of pixels. For
example, an SVGA monitor with a resolution of 1024 x 768 will have 1024 pixels in
every row (X - direction) and 768 pixels in every column (Y-direction). Monitors of
larger size will have resolution of 1024 x 1024 or more. A raster scan system displays the
image on a CRT in a certain fixed sequence.
The refresh rate is the number of complete images or frames scanned per second.
In the case of interlaced refresh cycle odd numbered raster lines are refreshed during
1/60th of a second. Even numbered raster lines are refreshed during the next 1/60th of a
second. In non-interlaced displays, all lines are refreshed in 1/60th of a second. The
quality of non- interlaced display is hence, superior. These systems, however, require
expensive frame buffer memory and display controller.
Graphic Primitives
A drawing is created by an assembly of points, lines, arcs, circles. For example,
drawing shown in Fig consists of several entities. In computer graphics also drawings are
created in a similar manner. Each of these is called an entity.
The drawing entities that a user may find in a typical CAD package include :
point line, construction line, multi-line, polyline,circle spline, arc ellipse polygon,
rectangle.
A simple drawing
Graphics System
Graphics system consists of four subsystems:
a. Geometry engine subsystem b. Scan conversion subsystem
c. Raster subsystem
d. Display subsystem
These subsystems are shown in Fig schematically.

P a g e | 39
GE OM E TRY
SCAN CONVERSIO N
RASTER
DISPLAY
Subsystems of a Graphics Engine
Geometry Engine
The geometry engine accepts 3-D world co-ordinate data and converts them into
X, Y screen co-ordinates. Depth information is manipulated using Z-buffer. Colors are
also processed. The geometry pipeline facilitates among other functions lighting,
clipping, and 3D to 2D projection, viewing transformations, rotation, scaling and
translation.
Scan Conversion
The scan conversion subsystem carries out polygon decomposition, edge slope
calculations, span slope calculations and span interpolation. The output of the scan
conversion is the pixel information to the raster subsystem.
RasterSubsystem
The raster subsystem will have usually 24 bit planes. This will provide eight bit planes
for each primary color (RGB) so that (28) shades of a single color can thus obtain.
Since the different colors are obtained by the three primary colors a total of (28)3colors
are available on the screen.
In a typical raster engine five 256K X 4D RAM provide 4 bits of Z-depth. The raster
information is stored in the frame buffer. Twenty 64 K X 4 video RAM provide 4 bits for
each pixel of 1280 X 1024 resolution. Entry level systems will have 12 bit planes and
high end systems will have 32 bit planes for the frame buffer. These provide the color
and depth for the images.
Display Subsystem
The display subsystem has multi-mode graphics processors which manage the display,
send the Red, Green, Blue color (RGB) data to the respective digital to analog converters
as well as provide a video output.
12. Write short notes on clipping, view ports, line drawing. (JAN 2014) (Or)
Explain the Cohen - Sutherland line-clipping approach with proper
sketches. (M.E. JAN 2010)

P a g e | 40
Clipping
 Clipping is the process of determining the visible portions of a drawing lying
within a window. In clipping each graphic element of the display is examined to
determine whether or not it is completely inside the window, completely outside
the window or crosses a window boundary.
 Portions outside the boundary are not drawn. If the element of a drawing crosses
the boundary the point of inter-section is determined and only portions which lie
inside are drawn. Readers are advised to refer to books on computer graphics for
typical clipping algorithms like Cohen-Sutherland clipping algorithm. Fig shows
an example of clipping.
 There is something missing between projection and viewing.
 Before projecting, we need to eliminate the portion of scene that is outside the viewing
frustum
Modify end points of lines to lie in rectangle
Method: Is end-point inside the clip region - half-plane tests
–If outside, calculate intersection between the line and the clipping rectangle and make
this the new end point
 Both endpoints inside: trivial accept
 One inside: find intersection and clip
 Both outside: either clip or reject (tricky case)

P a g e | 41
Cohen-Sutherland Algorithm
• Uses outcodes to encode the half-plane tests results
Rules:
– Trivial accept: outcode(end1) and outcode (end2) both zero
– Trivial reject: outcode(end1) & (bitwise and) outcode (end2)
Nonzero Else subdivide
If neither trivial accept nor reject:
–Pick an outside endpoint (with nonzero outcode)
–Pick an edge that is crossed (nonzero bit of outcode)
–Find line's intersection with that edge
–Replace outside endpoint with intersection point
–Repeat until trivial accept or reject
Polygon clipping

P a g e | 42
Convert a polygon into one or more polygons that form the intersection of the
original with the clip window.
Sutherland-Hodgman
Polygon Clipping Algorithm
 Subproblem:
Clip a polygon (vertex list) against a single clip plane
Output the vertex list(s) for the resulting clipped polygon(s)
• Clip against all four planes
–generalizes to 3D (6 planes)
–generalizes to any convex clip polygon/polyhedron
To clip vertex list against one half-plane:
• if first vertex is inside - output it
• loop through list testing inside/outside transition - output
depends on transition:
in-to-in: output vertex
out-to-in: output intersection and vertex
out-to-out: no output
in-to-out: output intersection
View port
Drawing of Lines
Straight line segments are used a great deal in computer generated pictures. The
following criteria have been stipulated for line drawing displays:
i. Lines should appear straight

P a g e | 43
ii. Lines should terminate accurately
iii. Lines should have constant density
iv. Line density should be independent of length and angle
v. Line should be drawn rapidly
The process of turning on the pixels for a line segment is called vector generation. If the end
points of the line segment are known, there are several schemes for selecting the pixels between
the end pixels. One method of generating a line segment is a symmetrical digital differential
analyzer (DDA).

ME6501 Unit 1 introduction to cad

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