PROJECTION OF LINES

Projection Theory

• Projection:

• 3-d object represented on 2-d media

• Image is known as view

• Projection theory is based on

• Line of sight

• Plane of Projection

Projection Methods

• Perspective

• Observer at finite distance from object

• All lines of sight start at a single point

• Parallel

• Observer at infinite distance from object

• All lines of sight are parallel

Parallel Projection

• Orthographic

• Projectors are perpendicular to picture plane

• One of object’s face is parallel to picture plane

• Oblique

• Projectors make an angle other than 90 with Picture plane

• One of object’s face is parallel to picture plane

• Axonometric

• The object’s three principle planes are inclined to Picture plane

• Three Principle planes are represented in one view

• Type: Isometric, Diametric, Trimetric

SIX

PRINCIPAL

VIEWS

• Front View – Elevation

• Top View – Plan

Principle planes of projection

The symbol recommended by BIS is to draw the two sides of a

frustum of a cone placed with its axis horizontal.

Convention for Projection

Different positions of a line wrt principle planes

• Parallel to both the planes (HP and VP)

• Parallel to one plane and perpendicular to the other

• Parallel to one plane and inclined to the other

• Inclined to both the planes

Line parallel to both planes

Line perpendicular to HP

& parallel to VP

Line parallel to VP and

inclined to HP

Line inclined to HP and VP

• φ: True inclination to VP• θ: True inclination to HP• α: Apparent inclination with HP• β: Apparent inclination with VP