Image Representation: Pixels, Colour Depth, Resolution, and File Size

Computers represent images as grids of tiny coloured squares called pixels. This lesson covers how images are stored digitally, including the concepts of colour depth, resolution, metadata, and file size calculation. This is part of OCR J277 Section 2.6.

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What Is a Pixel?

A pixel (picture element) is the smallest individual element of a digital image. Each pixel stores a single colour value as a binary number. When millions of pixels are displayed together, they form a recognisable image.

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Colour Depth

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

Colour depth	Number of colours	Use
1 bit	2^1 = 2	Black and white
2 bits	2^2 = 4	Simple graphics
4 bits	2^4 = 16	Basic colour icons
8 bits	2^8 = 256	GIF images
16 bits	2^16 = 65,536	High colour
24 bits	2^24 = 16,777,216	True colour (photos)

24-bit colour is the standard for photographs. It uses 8 bits each for red, green, and blue (RGB):

• Red: 0-255 (8 bits)
• Green: 0-255 (8 bits)
• Blue: 0-255 (8 bits)

This is why web colour codes like #FF0000 use hex pairs — each pair is one 8-bit channel.

OCR Exam Tip: The number of colours available is 2^(colour depth). If the colour depth is 8 bits, there are 2^8 = 256 colours. You must be able to calculate this in the exam.

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Resolution

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

Term	Meaning
Image resolution	Total number of pixels (width x height)
Screen resolution	Number of pixels a display can show
Print resolution	Pixels per inch (PPI) or dots per inch (DPI)

Higher resolution means more pixels, which means:

• More detail in the image
• Larger file size (more pixels to store)
• Longer processing time

flowchart LR
    A[Real-world scene] --> B[Camera sensor]
    B --> C[Grid of pixels - W x H]
    C --> D[Each pixel: RGB colour value]
    D --> E[Colour depth = bits per pixel]
    E --> F[Pixel data + metadata]
    F --> G[File size bits = W x H x colour depth]
    G --> H[Optional compression - JPEG / PNG]
    H --> I[Stored image file]

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File Size Calculation

The formula for calculating the file size of an uncompressed image is:

File size (bits) = width (pixels) x height (pixels) x colour depth (bits)

To convert to bytes: divide by 8 To convert to kilobytes: divide by 1024 To convert to megabytes: divide by 1024 again

Worked Example

An image is 800 x 600 pixels with 24-bit colour depth. Calculate the file size.

Step	Calculation	Result
Total pixels	800 x 600	480,000
Total bits	480,000 x 24	11,520,000 bits
Total bytes	11,520,000 / 8	1,440,000 bytes
Kilobytes	1,440,000 / 1024	1,406.25 KB
Megabytes	1,406.25 / 1024	1.37 MB

OCR Exam Tip: Always show every step of your file size calculation clearly. Write the formula first, then substitute the values. This guarantees method marks even if you make an arithmetic error.

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Metadata

Metadata is data about data. Image files contain metadata that describes properties of the image, stored alongside the pixel data. Metadata is essential for correctly displaying the image.

Metadata	Purpose
Width and height	Tells the software how to arrange the pixels
Colour depth	Tells the software how many bits represent each pixel
File format	Identifies how the data is encoded (e.g., BMP, PNG, JPEG)
Date/time	When the image was created
Camera settings	Aperture, shutter speed, ISO (for photos)
GPS location	Where the photo was taken

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Effect of Changing Image Properties

Change	Effect on quality	Effect on file size
Increase resolution	Higher quality (more detail)	Larger file
Decrease resolution	Lower quality (less detail)	Smaller file
Increase colour depth	More colours, smoother gradients	Larger file
Decrease colour depth	Fewer colours, banding/posterisation	Smaller file

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Comparing Image Properties

Property	Low value	High value
Resolution	Blocky, pixelated image	Sharp, detailed image
Colour depth	Limited colours, flat appearance	Rich, realistic colours
File size	Small, quick to transfer	Large, slow to transfer

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Practice

1. An image is 1024 x 768 pixels with 8-bit colour depth. What is the file size in KB?

   • 1024 x 768 = 786,432 pixels
   • 786,432 x 8 = 6,291,456 bits
   • 6,291,456 / 8 = 786,432 bytes
   • 786,432 / 1024 = 768 KB

2. How many colours can be represented with a 4-bit colour depth?

   • 2^4 = 16 colours

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Worked Example: Comparing Two Versions of the Same Photograph

A wildlife photographer exports the same photograph at two quality settings and wants to know how much storage each version requires.

Version A — web preview: 1280 x 720 pixels, 16-bit colour depth.

• Total pixels: 1280 x 720 = 921,600.
• Total bits: 921,600 x 16 = 14,745,600 bits.
• Bytes: 14,745,600 / 8 = 1,843,200 bytes.
• KB: 1,843,200 / 1024 = 1,800 KB.
• MB: 1,800 / 1024 = 1.76 MB.

Version B — print master: 4032 x 3024 pixels, 24-bit colour depth.

• Total pixels: 4032 x 3024 = 12,192,768.
• Total bits: 12,192,768 x 24 = 292,626,432 bits.
• Bytes: 292,626,432 / 8 = 36,578,304 bytes.
• KB: 36,578,304 / 1024 = 35,721.00 KB.
• MB: 35,721.00 / 1024 = 34.88 MB.

Comparison.