---
title: "Storing geometric and attribute information"
subject: "Computer Graphics"
chapter: 6
key: "semester_5/computer_graphics:geometric-attribute-info-storage"
source: https://www.bctengineer.com/q/semester_5/computer_graphics/geometric-attribute-info-storage
site: BCT Engineering
---

# Storing geometric and attribute information

Computer Graphics · Chapter 6: Surface Modeling

- Asked in: 2074 Chaitra, 5 marks; 2072 Chaitra, 4 marks; 2068 Chaitra, 5 marks; 2073 Chaitra, 5 marks; 2078 Kartik, 3 marks; 2071 Shrawan, 4 marks; 2070 Asar, 8 marks; 2070 Chaitra, 8 marks (asked 8 times)
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/computer_graphics/geometric-attribute-info-storage
- Whole chapter: https://www.bctengineer.com/md/semester_5/computer_graphics/chapter-6

## Question

Explain how the geometric and attribute information of a 3D object are stored for the object representation (polygon table: vertex, edge and surface tables) with examples.

## Short answer

Geometric data: vertex (coordinates), edge (vertex pairs), and surface (edge lists) tables. Attribute data (color, transparency, reflectivity) stored separately per surface. Example: two adjacent triangles share an edge; each surface references edges and has its own color. Plane coefficients (A,B,C,D) also stored.

## Answer

A 3D object is commonly represented by a set of polygon surfaces (boundary representation). The data are stored in **polygon tables**, which are of two types.

1. **Geometric data tables:** coordinates and shape of the polygons (vertex table, edge table, surface/polygon table).
2. **Attribute tables:** surface properties such as colour, transparency, reflectivity, texture, and so on.

#### Geometric tables
- **Vertex table:** coordinates (x, y, z) of each vertex.
- **Edge table:** each edge points to its two end vertices.
- **Surface (polygon) table:** each polygon points to the edges that bound it.

Example: an object made of two adjacent triangles S1 = (V1, V2, V3) and S2 = (V3, V2, V4) that share the edge V2V3:

```text
Vertex table        Edge table          Surface table
V1: x1 y1 z1        E1: V1 V2           S1: E1 E2 E3
V2: x2 y2 z2        E2: V2 V3           S2: E2 E4 E5
V3: x3 y3 z3        E3: V3 V1
V4: x4 y4 z4        E4: V2 V4
                    E5: V4 V3
```

Additional pointers, such as the surfaces that share each edge (for example, E2: S1, S2), make error checking and traversal fast.

#### Attribute data
Attributes are kept in a separate table or as fields attached to each surface, for example S1: colour = red, reflectivity = 0.5, transparency = 0, and so on. The plane equation coefficients (A, B, C, D) are also stored for each surface, for use in visibility tests and shading.

Advantages: shared vertices are stored only once, the structure is compact, and edges and surfaces can be processed quickly during transformation and rendering.

## Check yourself

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

1. Polygon tables are divided into:
   A. Geometric data tables and attribute tables
   B. Vertex and pixel tables
   C. Edge and colour tables only
   D. Input and output tables
   Answer: A. Geometric tables hold shape; attribute tables hold colour, transparency, etc. (Key point: Two table types)

2. What does the edge table store?
   A. Coordinates of each edge
   B. Colour of each edge
   C. The two end vertices of each edge
   D. The faces' plane equations
   Answer: C. Each edge points to its two vertices. (Key point: Edge table)

3. What does the surface table store for each polygon?
   A. The vertex coordinates directly
   B. Only its colour
   C. The edges that bound it
   D. The normal of its neighbours
   Answer: C. Each polygon lists its bounding edges. (Key point: Surface table)

4. Why are shared vertices stored in a vertex table?
   A. To increase redundancy
   B. To avoid storing edges
   C. Each vertex is stored once, giving compact storage and fast transformation
   D. To store colours
   Answer: C. Sharing avoids duplication. (Key point: Advantage)

## Related in Computer Graphics

- [Geometric tables for 3D object](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Ageometric-tables-3d-error-free) — Computer Graphics, chapter 6
- [Polygon tables to model Dharahara](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Apolygon-vertex-edge-table-dharahara) — Computer Graphics, chapter 6
- [Spatial orientation and polygon tables](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Aspatial-orientation-surface-significance) — Computer Graphics, chapter 6
- [Polygon table and normal of S1](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Apolygon-table-surface-normal-s1) — Computer Graphics, chapter 6
- [Polygon meshes for 3D objects](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Apolygon-meshes-3d-objects) — Computer Graphics, chapter 6
- [Conditions for error-free polygon table](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Aerror-free-polygon-table-conditions) — Computer Graphics, chapter 6, asked 3×

## Related in other subjects

- [Edge triggered JK flip-flop with excitation table](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Aedge-jk-excitation) — Digital Logic, chapter 6