Representing Images (Cambridge International IGCSE Computer Science)

Bitmap Images

Spec point — 1.2.3 · "3 Understand how and why a computer represents an image, including the effects of the resolution and colour depth"

Bitmap Images

What is a bitmap?

• A bitmap image is made up of squares called pixels
• A pixel is the smallest element of a bitmap image
• Each pixel is stored as a binary code
• Binary codes are unique to the colour in each pixel
• A typical example of a bitmap image is a photograph

• The more colours and more detail in the image, the higher the quality of the image and the more binary that needs to be stored

Resolution & Colour Depth

Spec point — 1.2.3 · "3 Understand how and why a computer represents an image, including the effects of the resolution and colour depth"

Resolution & Colour Depth

Exam Hint *Cambridge IGCSE 0478 expects you to define resolution and colour depth, explain how both affect file size, and use formulas like *width × height × colour depth to calculate storage needs. This page keeps everything focused on exam-ready phrasing and examples.

What is resolution?

• Resolution is the total number of pixels that make up a bitmap image

• It is calculated by multiplying the image's width and height in pixels

• In general, higher resolution means more detail and better image quality

• Display resolution is often described by the vertical pixel count, for example:

  • Computer monitors: 1440p has 1440 pixels vertically (2560×1440), while 4K has 2160 pixels vertically (3840×2160)

    • The "4K" name comes from its horizontal resolution of roughly 4,000 pixels

  • TVs: HD channels are 1080p (1080 pixels vertically), while UHD/4K channels are 2160p (2160 pixels vertically, or 3840×2160)

  • YouTube: the quality selector lets you choose playback resolution from 144p (144 pixels vertically) up to 4K (2160p)

What is colour depth?

• Colour depth is the number of bits stored per pixel in a bitmap image
• The colour depth is dependent on the number of colours needed in the image
• In general, the higher the colour depth the more detail in the image (higher quality)
• In a black & white image the colour depth would be 1, meaning 1 bit is enough to create a unique binary code for each colour in the image (1=white, 0=black)

• In an image with a colour depth of 2, you would have 00, 01, 10 & 11 available binary codes, so 4 colours

• As colour depth increases, so does the amount of colours available in an image
• The amount of colours can be calculated as 2ⁿ (n = colour depth)

What is the impact of resolution and colour depth?

• As the resolution and/or colour depth increases, the **bigger the size of the file **becomes on secondary storage
• The higher the resolution, the** more pixels** are in the image, the more bits are stored
• The higher the colour depth, the more bits per pixel are stored
• Striking a balance between quality and file size is always a consideration

Exam Hint In casual use, people say "higher resolution = better quality". But in the exam, focus on what it actually means:

“Resolution is the number of pixels in an image”

Avoid vague terms like “better” and give technical impact: “more bits needed = larger file size.”

Worked Example

1. Define the term Pixel [1]

2. If an image has a colour depth of 2 bits, how many colours can the image represent? [1]

3. Describe the impact of changing an images resolution from 500x500 to 1000x1000 [2]

Answers

1. The smallest element of a bitmap image (1 square)
2. 4
3. The image quality would be higher [1] the file size would be larger [1]