---
title: "Selection, manipulation and editing of entities"
subject: "Computer Aided Design"
chapter: 5
key: "semester_4/computer_aided_design:entity-selection-manipulation-editing"
source: https://www.bctengineer.com/q/semester_4/computer_aided_design/entity-selection-manipulation-editing
site: BCT Engineering
---

# Selection, manipulation and editing of entities

Computer Aided Design · Chapter 5: CAD Modeling Techniques

- Asked in: year not given, 5 marks
- Video: [Modify - Move, Rotate, Trim, Extend, Array, Mirror, Copy, Stretch, Scale, Offset](https://www.youtube.com/watch?v=0RA8nHtNyJA) — Arcs W
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/computer_aided_design/entity-selection-manipulation-editing
- Whole chapter: https://www.bctengineer.com/md/semester_4/computer_aided_design/chapter-5

## Question

Write short notes on the selection methods and the manipulation and editing commands used to modify entities in a CAD system.

## Short answer

Selection methods include picking, window, crossing window, fence/lasso, multiple-select, filters and grips. Manipulation commands are move, rotate, scale, mirror, copy, pattern and stretch; editing commands include trim, extend, break, join, offset, fillet, chamfer, delete, modify and suppress. These tools prevent errors and speed up modelling.

## Answer

Before an entity (line, arc, face, feature, part) can be edited, it must be **selected**. After selection, commands manipulate it.

#### Selection methods
- **Pick (click):** direct selection of one entity, using a pick box (aperture) around the cursor.
- **Window:** a rectangle drawn from left to right selects entities *completely inside* it.
- **Crossing window:** a rectangle drawn from right to left selects entities inside or *touching* it.
- **Fence / lasso:** a line or free-hand shape selects whatever it crosses or encloses.
- **Multiple (Ctrl/Shift) and select-all:** add or remove entities from a set.
- **Filter / by property:** select by layer, colour, type, name or feature.
- **Drag:** hold and move an entity or handle (grip) to change position or size.
- **Previous / last:** reuse the last selection.

#### Manipulation (transformation) commands
- **Translate / move, rotate, scale, mirror** (reflect), **copy**, **pattern** (linear, circular), **stretch**.
- These use homogeneous transformation matrices ($3\times3$ in 2D, $4\times4$ in 3D); for example, a 2D scaling by $S_x, S_y$ about the origin gives $x'=S_xx,\ y'=S_yy$.

#### Editing commands
- **Trim / extend** a curve to a boundary, **break**, **join**, **offset**, **fillet**, **chamfer**, **explode** (split a group).
- **Delete, undo/redo**, **modify** a dimension, **edit sketch**, **suppress** or **roll back** a feature.
- Grip editing: move control points of a curve or surface to change its shape.

Good selection tools reduce errors and speed up modelling: working with layers, groups and filters avoids selecting the wrong entity.

## Check yourself

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

1. How do a window and a crossing window differ?
   A. Window selects entities completely inside (left to right); crossing selects entities inside or touching (right to left)
   B. Window selects touching entities; crossing selects only inside ones
   C. They are identical
   D. Window selects by layer; crossing by colour
   Answer: A. A window needs full enclosure, while a crossing window also picks entities that merely touch the rectangle. (Key point: Window vs crossing)

2. Which set contains only manipulation (transformation) commands?
   A. Trim, extend, break, explode
   B. Translate, rotate, scale, mirror, copy, pattern
   C. Pick, fence, window, filter
   D. Undo, redo, delete
   Answer: B. Move, rotate, scale, mirror and pattern transform entities, typically with 4x4 homogeneous matrices. (Key point: Manipulation commands)

## Related in Computer Aided Design

- [Classification of CAD surface entities](https://www.bctengineer.com/question.md?key=semester_4%2Fcomputer_aided_design%3Asurface-entities-classification) — Computer Aided Design, chapter 4

## Related in other subjects

- [Distribution system used in INPS](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Ainps-distribution-system-type) — Transmission and Distribution System Design, chapter 5