theory of projection

perspective and parallel projection

1. Theory of projection By Mr Leliso H. Perspective and Parallel Projections CHAPTER TWO

2. 2 Projection Theory A projection is a mapping of a 3D space onto a 2D subspace The goal in engineering graphics, whether it is freehand sketching or CAD (computer aided drawing) is to represent a physical object. 2D space is called the projection plane

3. Objects can be shown as 3-D projections or Multi-view projections. 3-D projections are useful in that they provide an image that is similar to the image in the designer’s mind’s eye. But 3-D projections are often weak in providing adequate details of the object, and there is often some distortion of the object. For instance , a circular hole becomes an ellipse in an isometric 3-D projection . CONT…

4. Multi-view projections are used to overcome the weaknesses of 3-D projections. Multi-view projections are a collection of flat 2-D drawings of the different sides of an object. CONT…

5. 5 Torch Light Rays Ball (Observer) (Projectors) (Object) (Plane of Projection) Shadow (Projection) Wall CONT…

6. 6 Projection means “ To throw Forward ”. In this Object are being thrown (projected) forward in the form of Projection . Torch Ball Shadow Wall Torch Observer Ball Object Shadow Projection Wall Plane of Projection CONT…

7. Engineering Drawing (MEng-1011) 7 Common Elements of a Projection System Center of Projection OR Direction of Projection Projection Plane Object to be projected Projectors

8. 8 Some Terminologies Center of Projection = Station Point = View point of observer Projection Plane = View plane = Picture Plane Projectors = Sight lines = Line of Sight Bounding Box = Principal Enclosing Box (PEB)

9. 9 Classification of Projections

10. 13 Perspective Projection * Observer is at finite distance . * Rays or Projectors are converging at observer’s eye . * It does not provides exact size and shape of object. Object Plane of projection Projection Station point

11. 14 Types of Perspective Projection - Three Point Perspective - Two Point Perspective - One Point Perspective

12. 15 Parallel projection Preserves true relationship between features Lines that are parallel on the object are parallel on the drawing There are three types of parallel projections: a. Multi-view/orthographic projection b. Oblique Projection c. Axonometric Projection

13. 16 Orthographic Projection Orthographic projection is a two dimensional projection method. * FV : Length and height of Object * As projectors are Parallel to each other, the size of Orthographic View of an object is equal to the actual size of an object. FOR F.V. 90 o 90 o

14. 17 Oblique Projection * Observer is at infinite distance . * Rays or Projectors are Parallel to each other . * Rays or Projectors are not Perpendicular to the Plane of projection . (i.e. projectors are inclined to the plane of projection i.e. oblique) Φ ≠ 90 o Object Projectors Plane of projection shadow

15. 18 Axonometric Projection * Is three dimensional drawing named alternatively as Pictorial Drawing. * Observer is at infinite distance and Rays or Projectors are parallel to each other & perpendicular to the plane of projection . * There are three types. * All faces of the object are visible in a single view . a b c If a=b=c Isometric If a≠b=c Dimetric If a≠b≠c Trimetric

16. 12/2/2018 Engineering Drawing (MEng-1011) 20 Parallel versus Perspective Projection Parallel Perspective Parallel Perspective

17. THANK YOU