2. 2
Projection Theory
A projection is a mapping of a 3D space onto a
2D subspace
The goal in engineering graphics, whether it is
freehand sketching or CAD (computer aided
drawing) is to represent a physical object.
2D space is called the projection plane
3. Objects can be shown as 3-D projections or Multi-
view projections. 3-D projections are useful in that
they provide an image that is similar to the image in
the designer’s mind’s eye.
But 3-D projections are often weak in providing
adequate details of the object, and there is often
some distortion of the object.
For instance, a circular hole becomes an ellipse in an
isometric 3-D projection.
CONT…
4. Multi-view projections are used to overcome the
weaknesses of 3-D projections. Multi-view projections
are a collection of flat 2-D drawings of the different sides
of an object.
CONT…
6. 6
Projection means “To throw Forward”.
In this Object are being thrown (projected) forward
in the form of Projection.
Torch Ball Shadow Wall
Torch Observer
Ball Object
Shadow Projection
Wall Plane of Projection
CONT…
7. Engineering Drawing (MEng-
1011)
7
Common Elements of a Projection
System
• Center of Projection OR
Direction of Projection
• Projection Plane
• Object to be
projected
• Projectors
8. 8
Some Terminologies
• Center of Projection = Station Point = View point of
observer
• Projection Plane = View plane = Picture Plane
• Projectors = Sight lines = Line of Sight
• Bounding Box = Principal Enclosing Box (PEB)
13. 13
Perspective Projection
* Observer is at finite distance.
* Rays or Projectors are converging at observer’s eye.
* It does not provides exact size and shape of object.
Object
Station point
14. 14
Types of Perspective Projection
- Three Point Perspective
- Two Point Perspective
- One Point Perspective
15. 15
Parallel projection
• Preserves true relationship between
features
– Lines that are parallel on the
object are parallel on the
drawing
• There are three types of parallel
projections:
a. Multi-view/orthographic
projection
b. Oblique Projection
c. Axonometric Projection
16. 16
Orthographic Projection
•Orthographic projection is a two
dimensional projection method.
* FV : Length and height of Object
* As projectors are Parallel to each other, the size of
Orthographic View of an object is equal to the actual
size of an object.
17. 17
Oblique Projection
* Observer is at infinite distance.
* Rays or Projectors are Parallel to each other.
* Rays or Projectors are not Perpendicular to the
Plane of projection. (i.e. projectors are inclined to
the plane of projection i.e. oblique)
Φ ≠ 90o
Object
18. 18
Axonometric Projection
* Is three dimensional drawing named alternatively
as Pictorial Drawing.
* Observer is at infinite distance and Rays or
Projectors are parallel to each other &
perpendicular to the plane of projection.
* There are three types.
* All faces of the object are visible in a single view.
a
b c
If a=b=c Isometric
If a≠b=c Dimetric
If a≠b≠c Trimetric
