1.1.5 Image Representation — Cambridge 9618

Bitmap Image Representation

A bitmap (also called a raster image) represents an image as a rectangular grid of pixels (picture elements). Each pixel stores a colour value. The image data is stored as a sequence of these values.

Key Terms and Calculations

Pixel

The smallest addressable element of a digital image. Each pixel has a colour value stored as one or more bits.

Colour Depth (Bit Depth)

Colour depth is the number of bits used to represent each pixel. It determines how many different colours can be represented:

Number of colours = 2^{colour depth}

Colour depth	Colours possible	Example
1 bit	2	Black and white
8 bits	256	Indexed colour, greyscale
16 bits	65,536	High colour
24 bits	16,777,216	True colour (24-bit RGB)

Resolution

Resolution refers to the number of pixels in an image, typically expressed as width × height (e.g. 1920 × 1080). Higher resolution means more detail but requires more storage.

Resolution can also refer to pixels per inch (PPI) — the density of pixels, determining print quality.

Image File Size Calculation

The size of an uncompressed bitmap file is:

File size (bits) = Width × Height × Colour depth

To convert to bytes: divide by 8. To convert to KB: divide by 8192.

Worked Example

An image is 800 × 600 pixels with 24-bit colour depth. Calculate the file size in megabytes:

Total pixels = 800 × 600 = 480,000
Bits per pixel = 24
Total bits = 480,000 × 24 = 11,520,000 bits
Total bytes = 11,520,000 ÷ 8 = 1,440,000 bytes
Total MB = 1,440,000 ÷ 1,048,576 ≈ 1.37 MB

Metadata

Image files also contain metadata — data about the image stored alongside the pixel data. Common metadata includes:

Image dimensions (width and height in pixels)
Colour depth
File format
Creation date and device information (EXIF data)

Metadata means the actual file size is slightly larger than the pixel data alone would suggest.

Factors Affecting Image Quality

Factor	Effect of increasing	Trade-off
Resolution (pixels)	More detail, smoother image	Larger file size
Colour depth	More colours, more realistic	Larger file size

Vector Graphics vs. Bitmaps

Property	Bitmap	Vector
Stored as	Grid of pixel values	Mathematical descriptions of shapes
Scaling	Loses quality (pixelated)	Scales perfectly at any size
File size	Depends on resolution × colour depth	Depends on complexity of image
Best for	Photos, complex images	Logos, diagrams, text
Editing	Can edit individual pixels	Can edit individual shapes/paths

Exam tip: Know the formula: File size (bits) = Width × Height × Colour depth. Always convert carefully: ÷8 for bytes, ÷1024 for KB (or ÷1,048,576 for MB). Cambridge papers frequently ask you to calculate file sizes and explain effects of changing resolution or colour depth. Remember to mention metadata as an additional factor in actual file size.

⚠️ Common Mistakes

Forgetting to divide by 8 to convert bits to bytes
Confusing resolution (pixel count) with PPI (pixel density)
Saying more colour depth means "more pixels" — it does not; it means more colours per pixel
Forgetting that actual file size includes metadata (slightly more than pixel data)
Confusing bitmap with vector — bitmaps lose quality when scaled up; vectors do not

✅ Notes completed!

▶

Video coming soon

Click slide or press arrow keys to navigate

Worksheet — 1.1.5 Image Representation

8 questions · instantly marked · Cambridge 9618 standard

Q1Define the term 'pixel' in the context of bitmap images.[1]

✅ Mark scheme

Mark scheme

The smallest addressable element/unit of a digital image; each pixel stores a colour value [1].

Q2An image has a colour depth of 8 bits. How many different colours can it represent? Show your working.[2]

✅ Mark scheme

Mark scheme

Number of colours = 2^colour depth = 2^8 [1]; = 256 colours [1].

Q3Calculate the file size in kilobytes of an uncompressed bitmap image that is 640 × 480 pixels with a colour depth of 16 bits. Show all working.[4]

✅ Mark scheme

Mark scheme

Total pixels = 640 × 480 = 307,200 [1]; Total bits = 307,200 × 16 = 4,915,200 bits [1]; Bytes = 4,915,200 ÷ 8 = 614,400 bytes [1]; KB = 614,400 ÷ 1024 = 600 KB [1].

Q4Explain how increasing the resolution of an image affects (a) image quality and (b) file size.[2]

✅ Mark scheme

Mark scheme

(a) Higher resolution = more pixels = more detail/sharper image [1]; (b) More pixels × same colour depth = larger file size [1].

Q5State what is meant by 'metadata' in the context of an image file and give two examples.[3]

✅ Mark scheme

Mark scheme

Data stored about the image (not part of the pixel data itself) [1]; Examples (any two): image dimensions/resolution [1]; colour depth [1]; file format [1]; creation date [1]; camera/device information [1].

Q6Give two differences between bitmap and vector graphics.[4]

✅ Mark scheme

Mark scheme

Any two differences, each worth 2 marks (1 for each side of the comparison): Bitmap stored as a grid of pixels vs. vector stored as mathematical descriptions of shapes [2]; Bitmap loses quality when scaled (pixelated) vs. vector scales perfectly [2]; Bitmap file size depends on resolution × colour depth vs. vector file size depends on image complexity [2].

Q7A student doubles both the width and height of an image and also doubles the colour depth. By what factor does the file size increase?[3]

✅ Mark scheme

Mark scheme

File size = W × H × colour depth [1]; doubling W: ×2, doubling H: ×2, doubling colour depth: ×2 [1]; total factor = 2 × 2 × 2 = 8× increase [1].

Q8True colour images use 24-bit colour depth. Explain why this is called 'true colour' and how many colours it supports.[2]

✅ Mark scheme

Mark scheme

2^24 = 16,777,216 (approximately 16.7 million) colours [1]; called true colour because it can represent more colours than the human eye can distinguish — producing photo-realistic images [1].

Topic Quiz

Question 1 of 15

You scored

out of 15

Card 1 of 10

Click to reveal definition

🎉

All cards reviewed!

Term	Definition

🔒

Pro Content

Subscribe to unlock all Cambridge International A Level 9618 lessons.

Upgrade to Pro

Bitmap Image Representation

Key Terms and Calculations

Pixel

Colour Depth (Bit Depth)

Resolution

Image File Size Calculation

Worked Example

Metadata

Factors Affecting Image Quality

Vector Graphics vs. Bitmaps

Worksheet — 1.1.5 Image Representation

Mini Test — 1.1.5 Image Representation