Perspective Drawing: A Comprehensive Guide for Engineering Students, Study notes of Engineering Drawing and Graphics

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AXONOMETRIC DRAWING

PERSPECTIVE DRAWING

1.1 Introduction In engineering practice, an engineer has to build or construct various objects such as buildings, machines, etc. It is often desired to show how the object would appear when constructed. This pictorial view of the object that most nearly approaches the picture as seen by the eye is known as perspective projection or perspective view. Perspective view not only indicates the appearance of the object before hand but they also afford an opportunity to make the necessary changes in them. Perspective views are generally made by architects and have attained great importance in the engineering world. 1.2 Perspective Projection The view of an object as it actually appears to an observer when viewed from a fixed position relative to the object is called perspective projection or view. An ordinary photograph of an object is a perspective. In perspective projection, the eye is assumed to be situated at a definite position relative to the object. The picture plane (vertical plane) is placed between the object and the eye. Visual rays from the eye to the object pierce the picture plane and form an image on it. This image is known as perspective of the object. Thus the image or perspective obtained will depend on the relative position of the object, picture plane and point of sight. Perspective projection is sometimes called scenographic projection or central projection since the lines of sight converge to a single point or centre. 1.3 General Terms Used In perspective: The following are the various terms used in obtaining perspective view ( Fig1-2 )

1. Station point ( SP ) – is an assumed point representing the position of the observers’ eye. The elevation of the SP is on the horizon line and determines whether the object is viewed from above, on center or from below.
2. Vanishing point ( VP ) – are points in space where, by definition all parallel lines, which are not parallel to the picture plane meet. For horizontal parallel lines the vanishing point ( VP ) are always located on the horizon line. On the drawing the vanishing point is the perspective of the meeting point of lines in space. Every set of parallel lines not parallel to the picture plane has a vanishing point.
3. Visual rays – are lines of sight from the object to the station point. They represent the light rays that produce an image in the eye.
4. Picture plane ( PP ) – is the vertical transparent plane located between the station point and the object. It is the plane on which the perspective of an object is formed.
5. Horizontal line ( HL ) – is a line of intersection of the horizon plane with the picture plane. It is parallel to the ground line.
6. Ground line ( GL ) – is the line of intersection of the picture plane with ground plane.

(i) For large objects (i.e. buildings), the station point is usually taken at a normal standing height of a person, i.e. about 1.8m. (ii) For small objects, the station point should be fixed at such a height so as to give a good view of the top surface as well as side surfaces. (iii) For objects having heights and widths more or less equal, the station point may be fixed in such a way that the angle between the visual rays from the station point to the outermost boundaries of the object make approximately 30^0 angle (best picture is obtained at angles between 20^0 and 30^0 ; greater angles give unpleasing perspective). (iv) The distance of the station point from the picture plane should be equal to twice the greatest dimension of the object. (v) The station point should be so situated in front of the object that the center of the vision is somewhat near the picture’s center. 1.5 Location of the Picture Plane. The location of the picture plane ( Fig 2 ) relative to the object is of great importance in determining the size of perspective view. In general, the picture plane is located; (i) In front of the object (perspective shows enlarged size of the object). (ii) Behind the object (perspective shows reduced size of the object). (iii) Behind the station point (perspective shows object is reversed). Note : When the picture plane coincides with the object, the perspective of the object will be of the same size. 1.6 Position of the object with respect to the horizon The figure below ( Fig 3 ) shows the different positions of the object when placed ABOVE or BELOW the horizon. When the object is placed above the horizon it is above the level of the eye and will appear as seen from below. Similarly, when the object is below the horizon, it will appear as seen from above. Enlarged Normal perspective perspective Reversed Lines of perspective Object sight SP PP Picture plane (PP) PP Fig 2 : Location of Picture Plane

1.7 Types of Perspective There are three types of perspectives depending on the number of vanishing points required in the drawing: parallel or one point perspective , angular or two point perspective and oblique or three point perspective. (a) Parallel or one point perspective When one face of the object is parallel to the picture plane and the other perpendicular to it, the resulting picture is called parallel or one point perspective. The term one point refers to the fact that such perspectives have only one principal vanishing point as illustrated in Fig. 1.. The one or point perspective is often used to represent the interiors of buildings, and machine parts. (b) Angular or two point perspective When two faces of the object are inclined to the picture plane and the other parallel to it, the resulting picture is called angular or two point perspective , as shown in Fig. 1.5. In this method there are two principal vanishing points. The angular or two point perspective is used for representing buildings in architectural drawings or large structures in civil engineering such as bridges and dams. (c) Oblique or three point perspective PP A- view from the left with horizon below Above HL A B- View from the right with horizon behind object Horizon C- View from the centre VPR and above B On HL C Below HL Fig 3 : Effect of Variation of Horizon Line and Station Point on Perspective Drawings

(ii) Mark the station point S , the top view, 70mm below PP on the vertical line through O , the center of top view abcd. Join S with corners a, b, c , and d intersecting PP at points a 1 , b 1 , c 1 , and d 1. (iii) Draw the ground line GL at any convenient distance from PP , and the horizontal line HL , 45mm above GL. (iv)Through S , draw lines parallel to ad and ab cutting PP at the points v 1 and v 2 on the line PP. project v 1 to V 1 and v 2 to V 2 on HL line_._ Then V 1 and V 2 are the vanishing points. (v) Draw the front view of the rectangle block on GL with 20mm as height. From e, or a, draw a vertical line to a point E′ on GL. Mark E′A′ = 20mm = AE. Join A′E′ to V 1 and V 2. Through b 1 , draw a vertical line to cut A′V 2 at B′ and E′V 2 at F′. Similarly, draw vertical lines from c 1 and d 1 and obtain points C′-G′ and D′-H′ respectively. Join C′-G′ V 2 and D′ and H′ to V 1. (vi)Finally, join all the points as shown in figure, to obtain the required perspective of the rectangular block.
c g 40 40 f b O d h TOP VIEW e 300 v 1 a v 2 PP d a 1 c 1 b 1 S V1 V2 HL C′ D′ B′ F′ 50 H′ A′ E′ PERSPECTIVE VIEW FRONT VIEW FIG 6 45 70 20

QUESTIONS

Fig 1 shows the plan and front elevation of an object. If the position of the stationary point is as shown in Fig 2 , construct a two-point perspective of the object using the setup provided Fig 1