3.2c Representing images — Edexcel 4CP0

Pixels

A digital image is made up of a grid of tiny squares called pixels (picture elements). Each pixel stores a single colour value as a binary number. The quality and detail of an image depends on how many pixels it has and how many colours each pixel can represent.

Resolution

Resolution is the number of pixels in an image, typically expressed as width × height (e.g. 1920×1080). The total number of pixels = width × height.

Higher resolution = more pixels = more detail = larger file size
Lower resolution = fewer pixels = less detail = smaller file size

Colour Depth

Colour depth (bit depth) is the number of bits used to represent the colour of each pixel. More bits = more possible colours.

Colour depth	Colours possible	Example
1 bit	2¹ = 2	Black and white only
4 bits	2⁴ = 16	Early computer graphics
8 bits	2⁸ = 256	GIF images
24 bits	2²⁴ = 16,777,216	True colour (24-bit RGB)

Calculating Image File Size

The (uncompressed) file size of an image in bits is calculated as:

File size (bits) = width (pixels) × height (pixels) × colour depth (bits per pixel)

File size (bytes) = File size (bits) ÷ 8

Example: A 640 × 480 image with 24-bit colour depth:

File size = 640 × 480 × 24 = 7,372,800 bits
= 7,372,800 ÷ 8 = 921,600 bytes ≈ 900 KiB

Factors Affecting File Size

Factor	Effect on file size
Increase resolution (more pixels)	File size increases
Increase colour depth (more bits per pixel)	File size increases
Use compression	File size decreases (may affect quality)

📝 Exam Tip: Always show your working when calculating image file size. The formula is: width × height × colour depth = bits. Divide by 8 for bytes. Divide again by 1024 for KiB. Common exam mistakes include forgetting to divide by 8 to convert bits to bytes.

⚠️ Common Mistakes

Forgetting to divide by 8 to convert bits to bytes
Confusing resolution with pixel dimensions — resolution = total pixels = width × height
Thinking higher colour depth means better resolution — they are independent properties