1. Study Lesson 3
Orthographic Projection

2. Contents
Projection theory
Multiview drawing
Line convention

3. Projection Thoery
Contents

4. Purpose To graphically represent a 3-D object on 2-D media
(paper, screen etc.).
Object (3D) placing
on the paper (2D)
Transparent plate is placed
between object and
observer’s eyes.
A view of an object on
2D media
Inconvenient to
communicate
Object’s features are transferred
through projection.
Convenient to
communicate

5. Concept A projection theory is based on 2 variables: 1) Line of sight
2) Plane of projection (image or picture plane)
Line of sight (LOS)
is an imaginary ray of light
between an observer’s eye
and an object.
LOS
projection
line
Plane of projection
is an imaginary flat plane upon
which the image created by the
LOS is projected.

6. Line of sight Lines of sight can be parallel or converge.
Parallel projection
Converge projection
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7. Line of sight
The parallel projection lines can be normal (orthogonal) or oblique to the plane of projection.
In this course, we consider only a parallel and orthogonal projection, i.e. orthographic projection.
Orthogonal
Oblique
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8. View Image on a projection plane.
View depends on a relative orientation between an object and a plane.
Rotate
Multiview drawing
shows a 2D view of an object.
Tilt
Axonometric drawing shows a virtual 3D view of an object.
Multiview
Axonometric

9. Summary : Types of views
Projections
Parallel
Converge
Orthogonal
Oblique
Multiview
Axonometric
Multiview drawing
Pictorial drawing
Perspective
drawing
(This chapter)
(Later chapter)

10. View comparison Type Advantage Disadvantage Multiview drawing
Accurately presents object’s details, i.e.
size and shape.
Require training
to visualization.
Pictorial drawing
Easy to visualize.
Shape and angle distortion
Circular hole becomes ellipse
Right angle becomes obtuse angle.
Perspective drawing
Object looks more like what our eyes
perceive.
Difficult to create
Size and shape distortion
Distorted
width

11. Multiview drawing
Contents

12. Definition
Multiview drawing is a set of related images that are created by viewing the object from a different direction.
Depth
Height
Adjacent view(s)
is needed to fulfill the object
description.
Depth
Width
Height
Width
Depth

13. Methods 1. Revolve the object with respect to observer
2. The observer moves around the object.
Glass box concept
Top view
Top view
Front
view
Right side
view
Front view
Right side view
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14. Glass box : Revolution of the planes of projection
Rear view
Left side view
Bottom view

15. Relative orientation of views
Top view
Depth
Front view
Height
Width
Rear view
Left side view
Right side view
Bottom view

16. Summary : Problem solving steps
Given 1 2 3 4

17. Object features
Edge is a line that represent the boundary between two faces of an object.
Surface limit is a line that represents the last visible part of the curve surface.
Surface is an area that are bounded by edges or surface limit. Surface can be plane or curve.
No edges!
Prism
Cylinder
Sphere
Note
These features will appear as lines in a multiview drawing.

18. Class activity : Object’s features
Identify name of the features
denoted by a NO. 1 to 9? 4 3
Edge
Surface
limit
Surface 5   1 7   2   3 2   1 4 8 6   5  6    7   8
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19. Projection of a normal line
Glass box concept
Multiview drawing BT BT AT B AT A BR AF AF BF AR BR BF AR
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20. Projection of a normal plane
Glass box concept
Multiview drawing
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21. Projection of an object
Glass box concept
Multiview drawing
Hidden line is used
to show existence of
a hidden edge.
Do the same procedures for
all remaining edges (or planes)
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22. Projection of an inclined line
Glass box concept
Multiview drawing BT BT B AT BR AT BF BF BR A AF AR AF AR
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23. Projection of an inclined plane
Glass box concept
Multiview drawing
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24. Projection of an obliqued line
Glass box concept
Multiview drawing BT BT AT B BR AT BF BR BF A AF AR AF AR
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25. Projection of an obliqued plane
Glass box concept
Multiview drawing
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26. Projection of a curve line
Glass box concept
Multiview drawing BT BT AT CT CT B AT A BR C AF BF CR AF BF CF AR CR BR AR CF
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27. Projection of a curve surface
Glass box concept
Multiview drawing
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28. Transferring a depth : Direct measurement1 2 3 27
Prefer space between
front and side views 1 2 3 27

29. Transferring a depth : miter line
45o
Prefer space between
front and side views

30. Projection of an object having curved surface and plane
Curved surface can either tangent or intersect with an adjacent plane or curve surface.
In the case of intersection, an edge exists and becomes a line in a multiview drawing
In the case of tangential, there is no edge and line in a multiview drawing T I T I I T

31. Examples 1 2
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Play 3 4
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32. Examples 5 6
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33. Examples 7 8
No line exists
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Play 9 10
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34. Suggestion for practicing 1
Try to relate the object’s features to the lines in a multiview drawing, i.e. interpret the meaning of lines.
Given
Multiview
drawing
3D representation
of an object 1
Choose
features 2
Represent
each features
by a line 2
Match
each lines
(or areas)
to features 1
Choose lines
(or areas) 3
Identify line
(or plane) types,
i.e. normal,
inclined, and
obliqued.

35. Suggestion for practicing 2
Modify the object’s features of a given object, then observe what’s changed on a multiview drawing.
Given
Modified object
3D representation of a simple object

36. Self study : Type of planes
Click on any area
of a given object.
You will get the
type of plane.
Normal plane
Inclined plane
Curve surface
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37. Line
convention
Contents

38. Line convention Precedence of coincide lines Hidden line drawing
Center line drawing

39. Precedence of line When lines coincide with each other, the more
important lines cover up
the other lines.
Order of importance
(highest to lowest)
visible line
hidden line
center line
construction line.
Play
Line convention

40. Class activity : Precedence of lines4
What is an appropriate line type to replace the lines NO. 1-4? V H C   1   2 1   2 3 3  4 
“V” denotes visible line
“H” denotes hidden line
“C” denotes center line
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41. Hidden line drawing
Hidden line should join a visible line, except it extends from a visible line.
space
join
space
join
space
space

42. Hidden line drawing
Intersection between hidden lines should form L, T, V or Y corner.
inside L T L T Y V

43. Hidden line drawing A curve hidden line should start on a center line.
Line convention

44. Center line drawing
Center line should always start and end with long dash.
In a circular view, short dash of a center line should cross at the center of the circle or arc.
Center line should not extend between views.
3~4 mm
Leave space
Leave space
3~4 mm
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45. Center line drawing
For a small hole, a center line is presented as a thin continuous line.
Leave a gap when centerline forms a continuation with a visible or a hidden line.
Leave space
Leave space
Leave space
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Line convention

46. Class activity : Hidden line drawing
correct
wrong
correct
wrong              
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