Chapter 2 Projection Theory in Engineering Drawing

The theory of projection is the fundamental principle of engineering drawing used to represent three-dimensional (3D) objects on two-dimensional (2D) planes (such as paper or screen) accurately and systematically.

1. Engineering Drawing
Chapter 2
Theory of Projection
1

2. 2
Introduction
A projection is a graphical representation of a 3-D object on
a 2-D media (paper, screen)

3. Concept
A projection theory is based on 2 variables:
1) Line of sight
2) Plane of projection (image or picture plane)
projection
line
LOS
A projection involves four components
1.The actual object that the drawing or
projection represents
2.The eye of the viewer looking at the object
(Centre of Projection)
3.The imaginary projection plane
4.Imaginary lines of sight called Projectors

4. Line of sightLine of sight is an imaginary ray of light between an
observer’s eye and an object.
Line of sight
Parallel projectionParallel projection
Line of sight
Perspective projectionPerspective projection
There are 2 types of LOS :parallel convergeand

5. Plane of projectionPlane of projection is an imaginary flat plane which
the image is created.
The image is produced by connecting the points where
the LOS pierce the projection plane.
Parallel projectionParallel projection Perspective projectionPerspective projection
Plane of projection Plane of projection

6. The two broad types of projections are parallel and
perspective projection.
Perspective Projection - the
line of sights are converging at
observer's eye.
notnot used by engineer for
manufacturing of parts, because
1) It is difficult to create.
2) It does not reveal exact shape
and size.

7. Perspective Projection
Width is distorted

8. parallel projection
Parallel Projection - the line of sights are parallel to the picture planes.
Multi-view Drawing:2-D drawing of
an object
Axonometric Projections: 3-D
drawings (Pictorial Drawing),

9. Line of sight
Lines of sight can be parallel or converge.
Converge projection Parallel projection

10. The parallel projection lines can be normal (orthogonal)
or oblique to the plane of projection.
ObliqueOrthogonal

11. As projection lines are parallel to each other, the size of
orthographic view of an object is equal to its actual size.
ObliqueOrthogonal
Oblique projection: the projection lines are parallel to
each other, but not perpendicular to the projection plane

12. Orthographic Projection
Orthographic projection is a technique that is used to
create multi-view drawings.
It is a system of drawing views of an object using
perpendicular projectors from the object to a plane
of projection.
Engineering Drawing 12

13. Image on a projection plane.
Rotate
Tilt
Multiview drawing
shows a 2D view of
an object.
Axonometric drawing
shows a virtual 3D
view of an object.
View
View depends on a relative orientation between an
object and a plane.

14. Summary : Types of views
Projections
ConvergeParallel
Orthogonal Oblique
AxonometricMultiview
Pictorial drawing Perspective
drawing
Multiview drawing

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

16. Thank you!
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