1. 3-D GEOMETRIC TRANSFORMATION AND PROJECTION
LECTURE 3:
3-D GEOMETRIC TRANSFORMATION AND PROJECTION
PTT 105/3: Engineering Graphics

2. 3D PROJECTION
Three different types of 3-D projections are available in most CAD software:
isometric,
trimetric, and
perspective.
These three views of a cube are shown in figure below.
PTT 105/3: Engineering Graphics

3. 1. ISOMETRIC PROJECTION
The isometric projection has a standard orientation that makes it the typical projection used in CAD.
In an isometric projection, the width and depth dimensions are sketched at 30° above horizontal.
This results in the three angles at the upper front corner of the cube being equal to 120°.
The three sides of the cube are also equal, leading to the term iso (equal) -metric (measure).
PTT 105/3: Engineering Graphics

4. Isometric drawings work quite well for objects of limited depth.
However, an isometric drawing distorts the object when the depth is significant.
In this case, a pictorial perspective drawing is better.
PTT 105/3: Engineering Graphics

5. 2. TRIMETRIC PROJECTION
In general, the trimetric projection offers more flexibility in orienting the object in space.
The width and depth dimensions are at arbitrary angles to the horizontal, and the three angles at the upper front corner of the cube are unequal.
PTT 105/3: Engineering Graphics

6. This makes the three sides of the cube each have a different length as measured in the plane of the drawing; hence the name tri-metric.
In most CAD software, the trimetric projection fixes one side along a horizontal line and tips the cube forward.
Generally, small manufactured objects are adequately represented by isometric or trimetric views.
PTT 105/3: Engineering Graphics

7. 3. PERSPECTIVE PROJECTION
A simply perspective (pictorial perspective) projection is drawn so that parallel lines converge towards a single point, unlike isometric or trimetric projections where parallel lines remain parallel.
This has the effect that distant objects appear smaller than nearer objects.
A perspective projection is quite useful in providing a realistic image of an object when the object spans a long distance such as the view of a bridge, railways or aircraft from one end.
PTT 105/3: Engineering Graphics

8. But the oblique projection is usually even easier to sketch.
Two types of pictorial sketches are used frequently in freehand sketching:
isometric and
oblique.
The isometric projection is often used in freehand sketching because it is relatively easy to create a realistic sketch of an object.
But the oblique projection is usually even easier to sketch.
The figure compares an isometric and an oblique projection of a cube with a hole in it.
oblique freehand sketching easier than isometric sketching.
PTT 105/3: Engineering Graphics

9. Differences between 3D and 2D
Criteria 3D 2D
1. Viewing Direction
- Look at your object from an angle
- Look straight down on the drawing plane
2. Object Created
- have depth
- No depth
3. Command related
- 80 AutoCAD commands related primarily to 3D
- however, are also useful in 2D
4. Constructing object
- Yes
No.
Drawing picture from different viewpoints
PTT 105/3: Engineering Graphics

10. 3D Term Frequently Used PTT 105/3: Engineering Graphics Model
3D objects made in AutoCAD called models
2D work referred to as drawing/drafting
PTT 105/3: Engineering Graphics

11. 4 types of model used in 3D PTT 105/3: Engineering Graphics Model
Wireframe Model
Surface Model
Solid Model
Rendering
PTT 105/3: Engineering Graphics

12. 1. Wireframe Model Object by its edges only
Wireframe cannot hide object that are behind them
Hole has no meaning in a wireframe model because there is nothing in which to make a hole
PTT 105/3: Engineering Graphics

13. 2. Surface Model
Surface model often use wireframe models as a frame for their surfaces
Surface model: part wireframe + part surface
PTT 105/3: Engineering Graphics

14. 3. Solid Model Wireframe, surface model & computer-calculated mass;
eg: volume, centre of gravity, mass moment of inertia
Solid models look like wireframe unless a hidden line removal command is in effect
PTT 105/3: Engineering Graphics

15. 4. Rendering
Shaded, realistic-looking picture of a surface solid model is called a rendering
Two type:
Grayscale rendering
Fully capable of colour rendering
PTT 105/3: Engineering Graphics

16. Closer to representing real object than a 2D Show design more clearly
Reasons For Using 3D
Closer to representing real object than a 2D
Show design more clearly
Good for verifying design as well as for use in presentation and documentation as well.
PTT 105/3: Engineering Graphics

17. 3D Capabilities of AutoCAD
1. AutoCAD has complete 3D coordinate system for specifying points and drawing objects anywhere in space
2. To assist in point input and for working in local area, AutoCAD has a movable user coordinate system
3. It can set viewpoints from any location in space that can look in any directions
4. The computer screen can be divided into multiple viewports for simultaneously viewing 3D space from different viewpoints and different direction
PTT 105/3: Engineering Graphics

18. PTT 105/3: Engineering Graphics
5. AutoCAD has a good assortment of surface entities for making surface models that have a variety of shapes
6. Solid models of most objects typically manufactured in machine shop can be made within AutoCAD
7. 3D models can be transformed into standard multi-view, dimensioned production drawing
8. AutoCAD has a built-in renderer with lights and surface materials, capable of making realistic-looking shaded picture from 3D models
PTT 105/3: Engineering Graphics

19. EPT 112/3: Introduction to Engineering