Lecture 6 (06.08.12):

Theory of Multi-view Orthographic

Projections

Dr. Sharad Gokhale

Civil Engineering Department, IIT Guwahati

208, M-Block, Academic Complex

Email: sharadbg@iitg.ernet.in

Telephone #: 2419

Multi-view Orthographic

ProjectionsProjections

Terms and definition

• Projection – image or the act of obtaining an

image of an object

• In technical drawing – we call it a view

• Method – we use projection method to obtain • Method – we use projection method to obtain

a view of an object

Orthographic Projections

• It is a technical drawing in which different views of

an object are projected on different reference

planes

• Different Reference planes (principal planes):

– Horizontal Plane (HP),– Horizontal Plane (HP),

– Vertical Frontal Plane (VP)

– Side Or Profile Plane (PP)

• Different views:

– Front View (FV),

– Top View (TV),

– Side View (SV)

Projection System

Perspective

Projection System

Projection lines

Plane of Projections (POP)

Perspective

Parallel

Projection System

Projection lines

Plane of Projections (POP)

Perspective (Convergent

projection)

Parallel Projection

Three basic elements:

i. Object

ii. Observer

iii. POP

Projection of an ObjectProjection of an ObjectThe outline on the plane of projection shows how the object appears to the

observer. In orthographic projection, projections from all points of the

object extend parallel to each other and perpendicular to the plane ofprojection.

X

Y

X

X

Y

1st Quad.2nd Quad.

X Y

VP

HP

Observer

X

3rd Quad. 4th Quad.

This quadrant pattern,

If observed along x-y line ( in red arrow direction) will exactly appear as shown on

right side and hence, It is further used to understand illustration properly.

Methods of Drawing Orthographic Projections

First Angle Projections Method

Here views are drawn

by placing object

in 1st Quadrant

Third Angle Projections Method

Here views are drawn

by placing object

in 3rd Quadrant. in 1st Quadrant( FV above X-Y, TV below X-Y )

in 3rd Quadrant.

( TV above X-Y, FV below X-Y )

FV

TV

X Y X Y

G L

TV

FV

Symbolic presentation of both methods

with an object standing on HP (ground) on it’s base.

NOTE:-

HP term is used in 1st angle method

&

ground term is used

in 3rd angle method of projections

Planes

PRINCIPAL PLANES

HP AND VP

AUXILIARY PLANES

Profile Plane (P.P.)

A.V.P.

⊥ to HP & ∠ to VP

Auxiliary Vertical Plane

(A.V.P.)Auxiliary Inclined Plane

(A.I.P.)

This is a pictorial set-up of all three planes. Arrow direction is a

normal way of observing the object. But in this direction only VP and a

view on it (FV) can be seen. The other planes and views on those can

not be seen.

Procedure to solve above problem:-

To make those planes also visible from the arrow direction,

i) HP is rotated 900 downward, ii) PP, 900 in right side direction.

This way both planes are brought in the same plane containing VP.

Planes & Views (first angle method)

HP IS ROTATED DOWNWARD 900

AND

BROUGHT IN THE PLANE OF VP.

PP IS ROTATED AWAY IN RIGHT SIDE 900

AND

BROUGHT IN THE PLANE OF VP.

X

Y

X Y

VP

HP

PP

FV

ACTUAL PATTERN OF PLANES & VIEWS

OF ORTHOGRAPHIC PROJECTIONS

DRAWN IN

FIRST ANGLE METHOD OF PROJECTIONS

LSV

TV

FOR T.V.First angle projection

IN THIS METHOD,

THE OBJECT IS ASSUMED TO BE

SITUATED IN FIRST QUADRANT

MEANS

ABOVE HP & INFRONT OF VP.

OBJECT IS INBETWEEN

OBSERVER & PLANE.

PP

ACTUAL PATTERN OF

PLANES & VIEWS

IN

FIRST ANGLE METHOD

OF PROJECTIONS

X Y

VP

HP

PP

FV LSV

TV

FOR T.V.

IN THIS METHOD,

THE OBJECT IS ASSUMED TO BE

SITUATED IN THIRD QUADRANT

( BELOW HP & BEHIND OF VP. )

PLANES BEING TRANSPERENT

AND INBETWEEN

OBSERVER & OBJECT.

THIRD ANGLE

PROJECTION

ACTUAL PATTERN OF

PLANES & VIEWS

OF

THIRD ANGLE PROJECTIONS

X Y

TV

LSV FV

Orthographic projections- points, lines, planes, and solids

• To draw projections of any object, one must have the following information

– Object (with it’s description, well defined)

– Observer (always observing perpendicular to respective reference plane)reference plane)

– location of object (means it’s position with reference to HP & VP)

• Terms ‘above’ & ‘below’ with respective to HP and terms ‘infront’ & ‘behind’ with respective to VP form 4 quadrants.

• Objects can be placed in any one of these 4 quadrants

UNDERSTANDING PROJECTIONSUNDERSTANDING PROJECTIONS

To make and interpret drawings you need to know how to

create projections and understand the standard arrangement ofviews.

You need to be familiar with the geometry of objects and

be able to visualize a 3D object that is represented in a2D drawing.

Views of ObjectsViews of Objects

The system of views is called

multi-view projection. Each

view provides certain definite

information. e.g. a front view

shows the true shape and size

of surfaces that are parallel tothe front of the object.

Principal DimensionsPrincipal Dimensions

The three principal dimensions of an object are width, height, and depth.

The front view shows only the height and

width of the object and not the depth.

In fact, any principal view of a 3D object

shows only two of the threeshows only two of the three

principal dimensions; the third is found

in an adjacent view.

Height is shown in the rear, left-side,

front, and right-side views.

Width is shown in the rear, top, front, and

bottom views.

Depth is shown in the left-side, top, right-side, and bottom views.

Envision the object in a Glass BoxEnvision the object in a Glass BoxTo understand the standard arrangement of views on the sheet of paper

To draw the views on a sheet of paper, imagine thesix planes of the glass box being unfolded to lie flat.

Note the six standard

views (front, rear, top,

bottom, right side, leftside).

The Glass Box UnfoldedThe Glass Box UnfoldedLines extend around the glass box from one view to another on the planes of

projection. These are the projectors from a point in one view to the same point inanother view.

The Orthographic ProjectionThe Orthographic Projection

The front, top, and right-side views of the object shown now without the folding lines.

Necessary ViewsNecessary Views

The top, front, and right-side views, arranged

together, are called the three regular viewsbecause they are the views most frequently used.

A drawing should contain only the views needed to clearly and completely describe the object.completely describe the object.

View Selection

Select the most descriptive views

Use minimum number of views to

describe the object

Select the most descriptive views

How to project Side Views?

• Projecting across meter line

• Projecting through arcs

• Projecting through 45 degree projectors

Projecting across meter line

X1

FVLHSV

45oX Y

Y1

Meter line

TV

Projecting through 45o projectors

X1

FVLHSV

X Y

Y1

45o projectors

TV

Projecting through arcs

X1

FVLHSV

X Y

Y1

Arcs

TV

Top viewThree basic views (FV, TV, SV) will provide

complete information about the real object

Top ViewTop View

VIEWS OF SURFACESVIEWS OF SURFACES

The three orientations that a plane surface can have to

the plane of projection are normal, inclined, andoblique.

A plane surface that is

perpendicular to a plane of

projection appears onedge as a straight line

Standard Views of Primitive Solids

Ex. 1 & 2: Draw the three

principal views of the

objects shown In fig 9.12

(a) and 9.13 (b)

Ex. 3 & 4: From the pictorial

view of the objects shown

In fig 9.14 (a) and 9.15 (b)

Draw FV, TV, RHSV