MEG 361 CAD Chapter 3

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MEG 361 CADChapter 3

• Basic Concepts of Graphics
• Programming

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Interactive shape manipulation plays a major role
in CAD/CAM/CAE

• Programming for graphics display on monitor is
  essential part of CAD/CAM/CAE software

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Graphics Libraries

• Graphics Software may be divided into two
  groups
• device driver and
• graphics libraries.

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Graphics Libraries

More
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Graphics Libraries

• Device Driver is a set of machine-dependant
  codes that directly controls the display
  processing unit of a graphics device so that the
  electron beam is cast at the desired location.
  Each device driver (machine-dependant) has only a
  primitive capability, and so a graphics program
  written with such commands would be very long if
  any meaningful task to be performed. A program
  with poor readability will result.

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Graphics Libraries

More
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Graphics Libraries

• Graphics Libraries
• Similar to the math libraries in conventional
  programming, the graphics library is a set of
  subroutines , each of which has a specific
  purpose. For example, a subroutine might draw a
  line or subroutine might draw a circle. The
  graphics library is built on top of the device
  driver as shown below. Each subroutine is
  created by using a supporting set of device
  driver commands, for example a subroutine for
  drawing a drawing a short straight-line segment,
  circle might be composed of series of device
  driver commands

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Graphics Libraries

• Examples of Existing Graphics Libraries
• Core graphics provided in 1977, SIGGraph of the
  Association of Computing Machinery (ACM)
• It does not provide enough commands to use all
  raster graphics systems.
• GKS was developed by (ISO) in about 1977.

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Graphics Libraries

• Examples of Existing Graphics Libraries.
• Both CORE and GKS have some deficiencies with
  regard to supporting dynamic display and
  versatile user interaction.
• OpenGL , set of libraries developed by Silicon
  Graphics Company, runs under MSwindows NT. It
  becomes the standard graphics library.

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Coordinate Systems

• Basic tasks to display an image of an object
  on a graphics device
• Specifying the location of all points on the
  object in space.
• Determining which locations on the display
  monitor to be occupied by those points .

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Coordinate Systems

• Thus a coordinate system is necessary to
  provide a reference for specifying the location
  of a point both in space and on the monitor

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Coordinate Systems

• 1 Device coordinate system
• Is used as the reference in defining a location
  on the display monitor.
• The origin of u-v-z axes is chosen arbitrary.

screen
u
v
v
v
u
u
z
More
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Coordinate Systems

• 2 Virtual device coordinate system
• Has the same origin.
• The origin of u-v-z axes is in the lower left
  part. Graphics programmer specifies a shape
  consistently regardless of the DCS.

screen
v
u
More
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Coordinate Systems

• Device coordinate system and
• Virtual device coordinate system are
  2-Dimesional

More
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World Coordinate System (WCS)

• Is used to describe how the world of interest
  looks like.
• It is 3-Dimesional.

It describes the locations and orientations of
desks, chairs, and the blackboard if our world is
a classroom.
More
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Model Coordinate System (MCS)

• Is used to describe the shape of a part w.r.t.
  MCS attached to it.
• It is 3-Dimesional.

In MCS the coordinates of points on the part do
not change their values even when the part is
translated and/or rotated .
More
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Model Coordinate System (MCS)..

• The location and orientation of each part are
  then specified by WCS.
• The coordinates of all points of the parts are
  obtained in WCS using transformation matrices.

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Two Types of Projection

Perspective Projection
Parallel Projection
More
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Transformation bet. Coordinate Systems
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Window and Viewport

• Window defines the region in space that will be
  projected onto the display monitor so that any
  object outside the space of the monitor will not
  be displayed

Viewport(s) is the area(s) on the display monitor
where we want the projected image to appear.
Parallel Projection
More
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Window and ViewportOpenGL Sample
Parallel Projection
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Output primitives

• are the graphics elements that can be displayed
  by a graphics libraray.
• Line, Polygon, marker and Text are examples of
  output primitives.

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Graphics Input

• Graphics program accepts points, lines, or
  polygons as input in addition to numbers and
  text strings (e.,g select a polygon among all
  the graphics elements on the display
• Locator and Button are two types of physical
  device.

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Graphics InputLocator and Button

• Locator transfers location of cursor to the
  graphics program (mouse ball)
• Button transfers action of user on or off at
  current cursor location (mouse button)

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Representation of Cureves

• In solid modeling and computer aided drafting
  systems, curve equation itself or its attributes
  (center point, radius,..) have to be stored .

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Representation of Cureves

• Curve equations
• nonparametric
• Parametric,(x, y, z are related with a
  parameter ( ? )

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Representation of CurevesCircle

• Nonparametric
• Parametric

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Conic Sections
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See meg361_.....doc
parametric representation of curve Line -------
--------- example fro the position vector
P112 P243plot the line t0.011 x13
t y2t plot(x,y)grid xlabel('x'),ylabel('y')
axis(0 5 0 5)
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meg361 parametric representation of curve
parabola ---------------- example Generate
the parabolic segment in the first quadrant for
1ltxlt4 for the parabola given by x1th2,
y2th, ie a1 plot the segment of the
parabola a1 xmin1xmax4 thminsqrt(xmin/a) t
hmaxsqrt(xmax/a) ththmin(thmax-thmin)/101thma
x xath.2 y2ath plot(x,y)grid xlabel('x'
),ylabel('y') axis(0 5 0 5)
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Linear TransformationTranslation
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Linear TransformationRotation
Rotation _at_ x-axis
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Linear TransformationRotation
Rotation _at_ x-axis
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Linear TransformationRotation
Rotation _at_ x-axis...
Rot_x
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Linear TransformationRotation
Similarly Rotation _at_ y-axis...
Rot_y
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Linear TransformationRotation
Similarly Rotation _at_ z-axis...
Rot_z
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Linear TransformationGraphics Library
Example how transformation matrix is called