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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