CAD - Unit-1 (Fundamentals of Computer Graphics)

Presented by
C. P. Goldin Priscilla,
AP/MECH,
Kamaraj College of Engineering and Technology,
Virudhunagar.
COMPUTER AIDED DESIGN (CAD)
UNIT – I Fundamentals of Computer Graphics

CAD
• CAD also known as Computer Aided
Drafting/Design.
• There are 3 different types of CAD (2D, 2.5D
and 3D).
• The software is used to create and design
models of these types and test them.

Uses of CAD
CAD is used to design a variety of different products
for a variety of fields such as
• Architecture
• Electronics
• Automotive engineering
• Industrial Design
• Machinery
• Visual Art
• Medical Design

The Product Cycle and CAD/CAM
• The product begin with a need which is
identified based on customers and market
demands.
• In order to establish the scope and definition
of CAD/CAM in an engineering environment
and identify existing and future related tools,
a study of a typical product cycle is necessary.

5
The Manufacturing Process
The Design Process
Synthesis
Analysis The CAD Process
The CAM Process
Design
needs
Design
definitions,
specifications,
and requirements
Collecting
relevant design
information and
feasibility study
Design
conceptualization
Design
modeling and
simulation
Design
analysis
Design
optimization
Design
evaluation
Design
documentation and
communication
Process
planning
Order
materials
Design and
procurement
of new tools
Production
planning
NC, CNC, DNC
programming
Production
Quality
control
Packaging
Marketing
Shipping
Typical Product Life Cycle

• Inspection to finished product
– Two main process
– Design Process
» Synthesis
(Sketches, Layout drawings- CAD/CAM system)
» Analysis
(Design Modeling & Simulation)
– Manufacturing Process
(Process Planning & Production)
(Outcome Production Plan, tools procurement,
material order, CNC Programming)

Concurrent Engineering
• It is a strategy where all the tasks involved in
product development are done in parallel.
Collaboration between all individuals, groups and
departments within a company.
– Customer research
– Designers
– Marketing
– Accounting
– Engineering

Concurrent Engineering
Form Design
Functional
Design
Production
Design
Revising and testing
prototypes
Manufacturing
Specifications
Design
Specifications
Feasibility
Study
Idea
Generation
Suppliers R&D Customers
MarketingCompetitors
Product or Service concept
Performance Specifications
Pilot run and final
tests
Final Design
and process
plans
Product Launch
Preliminary
Design
Commercial
Design Process
Linear Process

Sequential Vs Concurrent Engineering
Traditional Process = Linear
Vs
Concurrent Engineering = Team collaboration

Benefits of Concurrent Engineering
• Reduces time from design concept to market
launch by 25%.
• Reduces Capital investment by 20%.
• Supports total quality from the start of
production with earlier opportunities for
continuous improvement.
• Simplifies after-sales service.
• Increases product life-cycle profitability
throughout the supply system.

The Design Process : Then and Now
Before CAD After CAD

CAD/CAM Systems
1. Hardware
2. Software
GUI
Client/Standalone
Database
Works on all OS [ Unix,Linux,Windows,Macintosh]

CAD/CAM Applications
• Geometric Module  Modelling/editing, documentation
• Application Module  Utilize model for Design Analysis.
• Programming Module  Customization by programming
• Communication Module  IGES, STEP file
• Collaborative Module  collaborative design via internet

Two Dimensional Viewing
• The Viewing Transformation/ Pipeline
• Line Drawing
– Several Algorithms
• Clipping
– Point clipping
– Line clipping
– Area (Polygon) clipping
– Curve clipping
– Text clipping

Windowing
When we display a scene only those objects
within a particular window are displayed
wymax
wymin
wxmin wxmax
Window
World Coordinates

Windowing
Because drawing things to a display takes time
we clip everything outside the window
wymax
wymin
wxmin wxmax
World Coordinates
Window

Clipping
• Remove objects that are outside the world window.
• For the image below consider which lines and points
should be kept and which ones should be clipped
wymax
wymin
wxmin wxmax
Window
P1
P2
P3
P6
P5P7
P10
P9
P4
P8

Point Clipping
Easy - a point (x,y) is not clipped if:
wxmin ≤ x ≤ wxmax AND wymin ≤ y ≤ wymax
otherwise it is clipped
wymax
wymin
wxmin wxmax
Window
P1
P2
P5
P7
P10
P9
P4
P8
Clipped
Points Within the Window
are Not Clipped
Clipped
Clipped
Clipped

Line Clipping
Harder - examine the end-points of each line to
see if they are in the window or not
Situation Solution Example
Both end-points inside
the window
Don’t clip
One end-point inside the
window, one outside
Must clip
Both end-points outside
the window
Don’t know!

Cohen-Sutherland Clipping Algorithm
Salient Features
•An efficient line clipping algorithm
•The key advantage of the algorithm is
that it vastly reduces the number of line
intersections that must be calculated
Dr. Ivan E. Sutherland co-developed
the Cohen-Sutherland clipping
algorithm. Sutherland is a graphics
giant and includes amongst his
achievements the invention of the
head mounted display.

Cohen-Sutherland: World Division
World space is divided into regions based on the
window boundaries
– Each region has a unique four bit region code
– Region codes indicate the position of the regions with
respect to the window
1001 1000 1010
0001
0000
Window
0010
0101 0100 0110
above below right left
4 3 2 1
Region Code Legend

Cohen-Sutherland: Labelling
Every end-point is labelled with the appropriate region
code
wymax
wymin
wxmin wxmax
Window
P3 [0001]
P6 [0000]
P5 [0000]
P7 [0001]
P10 [0100]
P9 [0000]
P4 [1000]
P8 [0010]
P12 [0010]
P11 [1010]
P13 [0101] P14 [0110]

Cohen-Sutherland: Lines In The Window
Lines completely contained within the window
boundaries have region code [0000] for both end-
points so are not clipped.
wymax
wymin
wxmin wxmax
Window
P3 [0001]
P6 [0000]
P5 [0000]
P7 [0001]
P10 [0100]
P9 [0000]
P4 [1000]
P8 [0010]
P12 [0010]
P11 [1010]
P13 [0101] P14 [0110]

Cohen-Sutherland: Lines Outside The Window
Any lines with a common set bit in the region codes of
both end-points can be clipped
– The AND operation can efficiently check this
wymax
wymin
wxmin wxmax
Window
P3 [0001]
P6 [0000]
P5 [0000]
P7 [0001]
P10 [0100]
P9 [0000]
P4 [1000]
P8 [0010]
P12 [0010]
P11 [1010]
P13 [0101] P14 [0110]

Cohen-Sutherland: Other Lines
Lines that cannot be identified as completely inside or outside
the window may or may not cross the window interior
These lines are processed as follows:
– Compare an end-point outside the window to a boundary
(choose any order in which to consider boundaries e.g.
left, right, bottom, top) and determine how much can be
discarded
– If the remainder of the line is entirely inside or outside the
window, retain it or clip it respectively
– Otherwise, compare the remainder of the line against the
other window boundaries
– Continue until the line is either discarded or a segment
inside the window is found

Cohen-Sutherland: Other Lines (cont…)
We can use the region codes to determine which window
boundaries should be considered for intersection
– To check if a line crosses a particular boundary we
compare the appropriate bits in the region codes of its
end-points
– If one of these is a 1 and the other is a 0 then the line
crosses the boundary

Cohen-Sutherland:- Examples
Consider the line P9 to P10 below
– Start at P10
– From the region codes
of the two end-points we
know the line doesn’t
cross the left or right
boundary
– Calculate the
intersection of the line with the bottom
boundary to generate point P10’
– The line P9 to P10’ is completely inside the window so is
retained
wymax
wymin
wxmin wxmax
Window
P10 [0100]
P9 [0000]
P10’ [0000]
P9 [0000]

Cohen-Sutherland Examples (cont…)
– Start at P4
– From the region codes
of the two end-points
we know the line
crosses the left
boundary so calculate
the intersection point to
generate P4’
– The line P3 to P4’ is completely
outside the window so is clipped
wymax
wymin
wxmin wxmax
Window
P4’ [1001]
P3 [0001]
P4 [1000]
P3 [0001]

– Start at P7
– From the two region
codes of the two
end-points we know
the line crosses the
left boundary so
calculate the
intersection point to
generate P7’
wymax
wymin
wxmin wxmax
Window
P7’ [0000]
P7 [0001] P8 [0010]
P8’ [0000]

Consider the line P7’ to P8
– Start at P8
– Calculate the
intersection with the
right boundary to
generate P8’
– P7’ to P8’ is inside
the window so is
retained
wymax
wymin
wxmin wxmax
Window
P7’ [0000]
P7 [0001] P8 [0010]
P8’ [0000]

Area Clipping
• Similarly to lines, areas
must be clipped to a
window boundary
• Consideration must be
taken as to which portions
of the area must be clipped

Sutherland-Hodgman: Area Clipping Algorithm
• A technique for clipping areas developed by
Sutherland & Hodgman.
• Put simply the polygon is clipped by
comparing it against each boundary in turn.
Original Area Clip Left Clip Right Clip Top Clip Bottom

Sutherland-Hodgman:
Area Clipping Algorithm (cont…)
To clip an area against an individual boundary:
– Consider each vertex in turn against the boundary
– Vertices inside the boundary are saved for clipping against the
next boundary
– Vertices outside the boundary are clipped
– If we proceed from a point inside the boundary to one
outside, the intersection of the line with the boundary is saved
– If we cross from the outside to the inside intersection point
and the vertex are saved

Sutherland-Hodgman Example
Each example shows the
point being processed (P)
and the previous point (S)
Saved points define area
clipped to the boundary in
question
S
P
Save Point P
S
P
Save Point I
I
P
S
No Points Saved
S
P
Save Points I & P
I

41
Other Clipping
Curve clipping
• Use bounding rectangle to test for overlap with
a rectangular clip window.
Text clipping
• All-or-none string-clipping
• All-or-none character-clipping
• Clip the components of individual characters

CAD - Unit-1 (Fundamentals of Computer Graphics)

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