Topics from the Cambridge IGCSE (9-1) Computer Science syllabus.

1.2.3 | IMAGE REPRESENTATION

Topics from the Cambridge IGCSE (9-1) Computer Science 0984 syllabus 2023 - 2025.

OBJECTIVES
1.2.3 Understand how and why a computer represents an image, including the effects of the resolution and colour depth

ALSO IN THIS TOPIC
1.1.1 NUMBER SYSTEMS
1.1.2 NUMBER SYSTEMS
1.1.3 NUMBER SYSTEMS
1.1.4 NUMBER SYSTEMS
1.1.5 NUMBER SYSTEMS
1.1.6 NUMBER SYSTEMS
1.2.1 TEXT, SOUND AND IMAGES
1.2.2 TEXT, SOUND AND IMAGES
YOU ARE HERE | 1.2.3 TEXT, SOUND AND IMAGES
1.3.1 STORAGE AND COMPRESSION
1.3.2 STORAGE AND COMPRESSION
1.3.3 STORAGE AND COMPRESSION
1.3.4 STORAGE AND COMPRESSION
TOPIC 1 KEY TERMINOLOGY
TOPIC 1 ANSWERS
TOPIC 1 TEACHER RESOURCES (CIE)

WHAT IS IMAGE REPRESENTATION?

Just as numbers and letters are represented with Binary, so are images. To a computer an image is nothing more than a series of ones and zeros which allow the display of pixels in various colours.

To aid human reading of different colour codes, colours are commonly represented in Hexadecimal using the RGB Red Green Blue system. For example the chart below shows how pure blue is represented using HEX, the HEX code for pure blue is #00 00 FF, meaning it has zero red components, zero green components and the maximum amount of Blue components.

A brief colour chart for some of the other colours represented in hexadecimal.
Most software that allows you to manipulate images or colours will allow you to select colours by their HEX or RGB values.

Remember: Hex is used for our benefit to make it easy for use to see and work with, but the computer still deals with colour in binary format.
Image quality
The quality of an image depends predominantly of two factors, the amount of Pixels Per Inch (PPi) and the colour depth. The colour depth is determined by how many bits are used to represent each pixel, if 4 bits were used, most images would be grey scale, represented with only 2^4 (16) different shades possible to represent. HEX colour systems are commonly used and allow for 255 possible representations for each colour, Red, Green and Blue which equates to 8 bits used for each of the 3 colour, meaning 24 bit colour depth per pixel (2^24 = 16777216), giving over 16 million individual colours.
The two images below illustrate the difference in image quality where the same image is represented on an 8 x 8 PPi scale and a 16 x 16 PPi scale, both images are the same physical size however the 16 x 16 image is better quality because more resolution is greater.

WHAT IS METADATA

Metadata is extra information that is within a picture file, as we learn from above the picture is actually just strings of binary, and within these string there is often extra data stored. This data often includes details of the device that took the picture and information such as the date the picture was taken. The amount of meta data has an impact on the actual file size of the image so calculating the file size from the image dimensions and bit depth might not be 100% accurate.
For more info on metadata CLICK HERE - BBC BITESIZE

IMAGE FILE TYPES

There are many different type of images and each format has its unique attributes as follows:

JPEG or JPG : Joint Photographic Expert Group, a jpeg image has been compressed from its original size. The compression type is lossy, meaning it does loose some of its quality during compression. JPGs are good for photographs that require good quality images but a small data storage size, such as a photograph being used on a website.

Some other popular image file types include.
PNG: PORTABLE NETWORK GRAPHICS - Supports good quality graphics with 24BIT representation and designed to work well online through web-browsers.

GIF: GRAPHICS INTERCHANGE FORMAT - Normally not compressed and can retain features such as background transparency. The File can be large due to lack of compression. Compatible with many platforms.

BMP: BITMAP - Normally not compressed and therefore often maintain good quality but can take up more file space.

1: Explain what is meant by the term ‘pixel’?
2: What is meant by the term ‘megapixel’?
3: What is meant by the term 'colour depth'?
4: Suggest two things that happen when you increase the colour depth.
5: What is meant by the term ‘image resolution’?
6: Suggest two things that happen when you increase the image resolution.
7: What is a Vector Graphic?
8: What is Meta Data
9: Give an example of information that could be stored in Meta data

ALSO IN THIS TOPIC

1.1.1 NUMBER SYSTEMS | Why Computers use Binary
1.1.2 NUMBER SYSTEMS | Binary, Hex and converting between number systems
1.1.3 NUMBER SYSTEMS | Benefits of Hex
1.1.4 NUMBER SYSTEMS | Binary Addition and Overflow
1.1.5 NUMBER SYSTEMS | Binary Shifts
1.1.6 NUMBER SYSTEMS | Negative Number Representation, Two's Complement
1.2.1 TEXT, SOUND AND IMAGES | How Computers Represent Text
1.2.2 TEXT, SOUND AND IMAGES | How Computers Represent Sound
1.2.3 TEXT, SOUND AND IMAGES | How Computers Represent Images
1.3.1 STORAGE AND COMPRESSION | How Data Storage is Measured
1.3.2 STORAGE AND COMPRESSION | Calculating Image and Sound File Size
1.3.3 STORAGE AND COMPRESSION | Purpose of Data Compression
1.3.4 STORAGE AND COMPRESSION | Lossy and Lossless, how files are compressed
TOPIC 1 KEY TERMINOLOGY
TOPIC 1 ANSWERS
TOPIC 1 TEACHER RESOURCES (CIE)

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