PRINCIPLE OF PROJECTION:-
If
straight lines are drawn from various point on the contour of an object to meet
a plane. The object is said to be projected on that plan. The figure formed by
joining in correct sequence the points at which these lines meet the plan is
called the projection of the object. The line from the object to the plan are
called projectors.
TYPE OF PROJECTION SYSTEMS:-
The
projection are classified according to the method of talking the projection on
the plan
(1) PERSPECTIVE PROJECTION: -The
perspective projection is the representation of an object on a plan surface
called the picture plane as it would appear to the eyes when viewed from a
fixed position.
There are three types of
perspective projections-
(a)
ONE POINT PERSPECTIVE: -A drawing has one-point
perspective when it contains only one vanishing point on the horizon line. This
type of perspective is typically used for images of roads railway tracks
hallways or buildings viewed so that the front is directly facing the viewer.
(b)TWO
POINT PERSPECTIVE: - A drawing has
two-point perspective when it contains two vanishing points on the horizon
line. In an illustration, these vanishing points can be placed arbitrarily
along the horizon. Two point perspective can be used to draw the same objects as
one-point perspective rotated looking at the corner of a house or at two forked
roads shrinking into the distance
(c)THREE POINT PERSPECTIVE: - Three-point perspective is often used for buildings seen
from above or below. In addition to the two vanishing points from before one
for each wall there is now one for how the vertical lines of the walls recede.
For an object seen from above this third vanishing point is below the ground.
For an object seen from below as when the viewer looks up at a tall building
the third vanishing point is high in space.
(2) 
Parallel Projection: - Parallel
projection discards z-coordinate and parallel lines from each vertex on the
object are extended until they intersect the view plane. In parallel projection,
we specify a direction of projection instead of centre of projection.
There are three types of parallel
projections
•
Oblique Projection: -Oblique projection is a simple type of technical drawing of graphical projection used
for producing two-dimensional imagesof three-dimensional objects.Oblique projection is
commonly used in technical drawings.
There are four types of oblique
projections −
(a) Cavalier:
- Cavalier projection makes 45° angle
with the projection plane. The projection of a line perpendicular to the view
plane has the same length as the line itself in Cavalier projection. In a
cavalier projection the foreshortening factors for all three principal
directions are equal.
(b) Cabinet: -
The Cabinet projection makes 63.4°
angle with the projection plane. In Cabinet projection, lines perpendicular to
the viewing surface are projected at ½ their actual length. Both the
projections are shown in the following figure –
(c)
Clinoghraphi
(d) Shades and
shadows
•
Orthographic Projection
Orthographic projectionis
a means of representing three-dimensional objects in two dimensions. It is a form of parallel projectionin which all the projection
lines are orthogonalto
theprojection plane resulting in every plane of
the scene appearing in affine transformationon the viewing
surface.. There are four types of orthographic
projections –
FIRST ANGLE: -
First Angle Projection is commonly used
in all countries other than United States. The Indian Standard Institution recommends
the use of First Angle Projection method now in all the institutions.
SECOND ANGLE:
- Now after drawing this projection, the
horizontal plane is to be rotated 90 degree clockwise. In second and fourth
angle projection due to this process horizontal and vertical plane will overlap
with each otherthat’s why we can't draw second and fourth angle projection.
THIRD ANGLE:-However America and Australia use a method known as Third
Angle ProjectionIn First Angle Projection we place our object in the First
Quadrant see above figure. This means that the Vertical Plane is behind the
object and the Horizontal Plane is underneath the object In Third Angle
Projection the Object is placed in the Third Quadrant. This means that the
Vertical Plane is in front of the object and the Horizontal Plane is above the
object. To see the effects of this watch the animation below.
•
AXONOMETRIC
(a)ISOMETRIC:
-Orthographic projections that show more
than one side of an object are called axonometric orthographic projections. The
most common axonometric projection is an isometric projection where the
projection plane intersects each coordinate axis in the model coordinate system
at an equal distance. In this projection parallelism of lines are preserved but
angles are not preserved.
(b) DIMETRIC:-the direction of viewing is such that two of the three axes
of space appear equally foreshortened of which the attendant scale and angles
of presentation are determined according to the angle of viewing; the scale of
the third direction (vertical) is determined separately. Approximations are
common in diametric drawings.
(c) TRIMETRIC:
- direction of viewing is such that all
of the three axes of space appear unequally foreshortened. The scale along each
of the three axes and the angles among them are determined separately as
dictated by the angle of viewing. Approximations in trimetric drawings are
common and trimetric perspective is seldom used.
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