1. Engineering Communications GL2 Geometric modelling Projection Systems
Lecture presentations available on WWW:

2. A graphic is a representation on a 2-D surface of a 3-D scene
An artist may attempt to create a ‘realistic’ image.
Note the use of perspective.
In fact, there are distortions in this picture, and it does not create the same projection on the retina as a real scene would.

3. Meaning may be communicated better by deliberate distortion

4. In engineering graphics:
a variety of types of distorted images are available to communicate meaning
strict rules apply to the construction and interpretation of these images
a universal language of graphic communication is thus achieved

5. Perspective projection
Projection plane
2-D projection
View point
3-D object
Projection rays
Perspective projection
Engineering graphics are obtained by projection from the 3-D object to the viewing surface (the projection plane)

6. Types of projection
Perspective projection is rarely used in manual drawing
Rather, we us a variety of orthographic projections, for which the projection rays are parallel

7. View point at 
Projection plane
2-D projection
Parallel projection rays
3-D object
In orthographic projection, the projection rays are parallel (view point at infinity)

8. Perspective projection is useful for ‘non technical’ communications
Perspective renderings for marketing, etc. are readily obtained with computer-aided drawing (CAD) systems

9. Projection techniques
Bertoline, et al. Fig. 9.2
Projection techniques
Orthogonal (multiview)
Axonometric
Oblique
Perspective

10. Categories of orthographic projection
Projectors
Principal plane of object

11. Third-angle orthogonal projection
Top view
Top horizontal plane
Glass projection box
First quadrant
Third quadrant
Left side view
Front vertical plane
Front view
Left profile plane

12. Third-angle orthogonal projection
horizontal plane
vertical plane
left profile plane
horizontal plane
width
depth
depth behind
vertical plane
top (plan) view
depth
depth behind
vertical plane
height below
horizontal plane
height
left side view
front view
left profile plane
vertical plane

13. Axonometric projection
Lines of sight perpendicular to projection plane
Principal axes all inclined to projection plane
TRIMETRIC
DIMETRIC
ISOMETRIC A C C A C B B B A y z x y z x y z x
Example:
A=120º B=130º C=110º
x:y:z = 1 : : 0.938
Example:
A=C=131.5º B=97º
x : y : z = 0.5 : 1 : 1
Always:
A = B = C = 120º
x : y : z = 1 : 1 : 1

14. Isometric projection X Y Z isometric projection A B projection plane 
0.816  b C
A = B = C = 120°
a = b = 30°
Scale ratios = (2/3) = 0.816
X : Y : Z = 1 : 1 : 1
For an isometric drawing, scale = FS on each axis

15. Oblique projection  Full scale Scale = cot Full scale
Principal object face parallel to projection plane

16. Varieties of oblique projection
Cavalier
Cabinet
General

17. Isometric sketch T-square Set square depth height width Top view
Front view
Side view
T-square
Set square

18. Projections of a cube compared ...
Full scale
Full scale
Oblique (Cabinet)
Half scale
Isometric
45º
Full scale
30º
60º
Full scale
radius = 1
semi-minor axis = (1/2)
semi-major axis = (3/2)

19. Introduction to Cartesio software (download from EngCom homepage)

20. Follow up Read Bertoline: Do problems from Bertoline:
§ 4.5: Introduction to Projections
§ 8.1: Projection Theory
§ 8.2: Multiview Projection Planes
§ 8.3:Advantages of Multiview Drawings
Do problems from Bertoline:
Probs 4.2(6)(47), 4.3(2)(6)
Check the EngCom web site