Chapter 2
Orthographic
projection
CONTENTS
Principle of Orthographic projection
Multi view representation
First angle and Third angle method projection
Missing views and lines
2.1 Introduction
Principle of Orthographic projection
Projection: is the image of an object represented /
drawn on a plane of projection /picture plane as it
would appear to the observer stationed at a point and
viewing along the direction of line of projection .
Projectors / lines of projection: are an imaginary
lines that emerges from observer’s eyes and moves to
contour of an object.
Station point: is the location of an observer.
Plane of projection / picture plane: are an
imaginary plane on which the image of an
objected is represent / drawn.
The plane on which principal view of an object
is represented / drawn is called principal plane.
The planes are assumed to expand infinitely
and transparent.
Cont…
The three principal plane are vertical plane used to represent front /
rear view; horizontal plane used represent top / bottom view; and
profile plane used to represent right / left side view of an object.
Projectors or line of Projectors or line of sights are
sights are not parallel parallel to each other
to each other
Orthographic Projection
The word orthographic means to draw at right
angles and is derived from the Greek words:
ORTHOS - straight, rectangular, upright and
GRAPHOS - written, drawn.
Orthographic projection is a parallel projection. It is
used as an unambiguous and accurate way of
providing information, primarily for manufacturing
and detail design.
Purpose
To graphically represent a 3-D object on 2-D media
(paper, screen etc.).
Object (3D) placing Transparent plate is placed A view of an object on
on the paper (2D) between object and 2D media
observer’s eyes.
Inconvenient to Object’s features are transferred Convenient to
communicate through projection. communicate
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
LOS
between an observer’s eye project
i on
line
and an object.
Plane of projection
is an imaginary flat plane upon
which the image created by the
LOS is projected.
Line of sight
Lines of sight can be parallel or converge.
Parallel projection Converge projection
Play Play
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
Play Play
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 Tilt
an object.
Axonometric drawing
shows a virtual 3D
view of an object.
Summary : Types of views
Projections
Parallel Converge
Orthogonal Oblique
Multiview Axonometric
Multiview drawing Pictorial drawing Perspective
(This chapter) (Later chapter) drawing
View comparison
Type Advantage Disadvantage
Multiview drawing Accurately presents Require training
object’s details, i.e. to visualization.
size and shape.
Pictorial drawing Easy to visualize. Shape and angle distortion
Circular hole
becomes ellipse
Right angle becomes
obtuse angle.
Perspective drawing Object looks more Difficult to create
like what our eyes Size and shape
perceive. distortion
Distorted
width
Multiview
drawing
Contents
Definition
Multi view drawing is a set of related images that are created
by viewing the object from a different direction.
Adjacent view(s)
Depth
is needed to
Height fulfill the object
description.
Width Depth
Height
Width Depth
Methods
1. Revolve the object with respect to observer
2. The observer moves around the object.
Glass box concept
Top view
Top view
Front
Right side
view
Front view Right side view play view
play
Glass box : Revolution of the planes of projection
Rear view
Left side view
Bottom view
Relative orientation of views
Top view
Depth
Front view
Width
Height
Rear view Left side view Right side view
Bottom view
Summary : Problem solving steps
Given
1 2
3 4
Alignment
of views
Contents
First Angle Projection
When the object is situated in First Quadrant;
that is in front of V.P and above H.P, the
projections obtained on these planes is called
First angle projection.
The object lies in between the observer and the
plane of projection.
Third Angle Projection
The object is situated in Third Quadrant. The
Planes of projection lie between the object and
the observer. The front view comes below the
XY line and the top view about it.
Projection systems
1. First angle system
- European countries First
- ISO standard quadrant
2. Third angle system
- Canada, USA,
Japan, Thailand
Transparent
Opaque
planes
planes
Third
quadrant
Orthographic views
1st angle system 3rd angle system
(Opaque planes) (transparent planes/glass box)
Orthographic views
1st angle system 3rd angle system
Folding
line
Folding
line
Folding
line
Folding
line
Views arrangement
1st angle system 3rd angle system
Top View
Right Side View Front View Front View Right Side View
Top View
Projection symbols
1st angle system 3rd angle system
d 1.7d
2.2d
2.7 Missing views and lines
The lines missed to complete the drawing of an object are
called missing lines.
The true and correct shape and size of an object can be
rightly understood by drawing multi-view orthographic
projections.
In general practice, three views are sufficient to clear the
shape of an object.
Incase of simple objects, two views are enough.
Missing Lines
Missing lines are a common problem in drawings,
especially sketches done by hand.
Missing lines can be visible, hidden, or centerlines.
Missing Lines are found by lining up views and comparing
features (similar to point, edge, surface tracking)
Missing Views
Missing views are a challenge for 3D visualization.
Using information in orthographic views, the missing 3 rd
view can be determined.
