---
title: "Types of projections"
subject: "Computer Graphics"
chapter: 4
key: "semester_5/computer_graphics:types-of-projections"
source: https://www.bctengineer.com/q/semester_5/computer_graphics/types-of-projections
site: BCT Engineering
---

# Types of projections

Computer Graphics · Chapter 4: Three-Dimensional Graphics

- Asked in: 2073 Chaitra, 3 marks; 2069 Asar, 4 marks; 2066 Magh (old course) (asked 3 times)
- Video: [Projection in Computer Graphics | CG | Lec-37 | Bhanu Priya](https://www.youtube.com/watch?v=Tp7OSptUETg) — Education 4u
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/computer_graphics/types-of-projections
- Whole chapter: https://www.bctengineer.com/md/semester_5/computer_graphics/chapter-4

## Question

Briefly explain various types of projections (significance of projection and its types).

## Short answer

Projections map 3D objects to 2D. Parallel projections (orthographic—multiview, axonometric; oblique—cavalier, cabinet) preserve parallelism and scale. Perspective projections (one-, two-, three-point) use converging projectors for realistic foreshortening, but lose parallelism and distances.

## Answer

**Projection** converts 3D objects into a 2D view on a view plane. It is needed because display devices and paper are two-dimensional. A projection is defined by projectors (lines from each point of the object) and a projection plane.

```text
Projection
 |-- Parallel (projectors are parallel)
 |     |-- Orthographic (projectors perpendicular to plane)
 |     |     |-- Multiview (front, top, side)
 |     |     |-- Axonometric (isometric, dimetric, trimetric)
 |     |-- Oblique (cavalier, cabinet)
 |
 |-- Perspective (projectors meet at a point, COP)
       |-- One-point, two-point, three-point
```

**1. Parallel projection:** projectors are parallel, so the centre of projection is at infinity. Parallel lines stay parallel and relative dimensions are kept, so it is used in engineering drawing.
- *Orthographic:* projectors are perpendicular to the view plane. Front, top and side views show true shape of one face. *Axonometric* views (isometric, dimetric, trimetric) rotate the object so that several faces are visible.
- *Oblique:* projectors make an angle other than 90° with the plane. Faces parallel to the plane keep true shape. Types are cavalier (receding lines at full length) and cabinet (half length).

**2. Perspective projection:** projectors converge at the centre of projection. Distant objects look smaller, so the view looks realistic, but parallel lines (not parallel to the view plane) meet at vanishing points and distances are not preserved. Types depend on the number of principal vanishing points: one-point, two-point and three-point.

| Point | Parallel | Perspective |
|---|---|---|
| Projectors | Parallel | Meet at COP |
| Size with distance | Constant | Smaller when farther |
| Realism | Less | More |
| Measurement | Preserved | Not preserved |
| Use | CAD, drawings | Games, animation, architecture |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why is projection needed?
   A. To make objects smaller in memory
   B. To add colour and lighting
   C. Display devices and paper are 2D, so 3D objects must be converted to a 2D view
   D. To remove hidden lines automatically
   Answer: C. Projection maps 3D points onto a 2D view plane. (Key point: Need for projection)

2. Which pair correctly describes projectors?
   A. Parallel: projectors parallel; perspective: projectors meet at the COP
   B. Parallel: projectors meet at a point; perspective: projectors parallel
   C. Both have projectors meeting at a point
   D. Both have parallel projectors
   Answer: A. Parallel projection has the COP at infinity; perspective projectors converge at a finite COP. (Key point: Parallel vs perspective)

3. Oblique projection differs from orthographic in that:
   A. Its projectors are perpendicular to the plane
   B. Its projectors make an angle other than 90° with the plane
   C. It has vanishing points
   D. It has the COP at a finite distance
   Answer: B. Oblique is parallel projection with slanted projectors; faces parallel to the plane keep true shape. (Key point: Oblique projection)

4. One-, two- and three-point perspective are classified by:
   A. The angle of the projectors
   B. The number of objects in the scene
   C. The number of principal vanishing points
   D. The size of the view plane
   Answer: C. The count of axes cutting the view plane gives the number of principal vanishing points. (Key point: Perspective types)