1.2.4f Representing Images — OCR J277

Pixels and Bitmaps

Digital images are stored as bitmaps — grids of tiny squares called pixels (picture elements). Each pixel stores a single colour value as a binary number. The more pixels, the more detail the image can show.

A bitmap image stores the colour of every individual pixel
The arrangement is a rectangular grid: width × height in pixels
Each pixel's colour is represented by binary numbers

Image Resolution

Resolution refers to the number of pixels in an image, expressed as width × height (e.g. 1920 × 1080). A higher resolution means more pixels, so the image is sharper and shows more detail — but the file size is larger.

Resolution	Total pixels	Typical use
640 × 480	307,200	Old webcam, thumbnail
1920 × 1080	2,073,600 (~2MP)	HD TV, monitor
3840 × 2160	8,294,400 (~8MP)	4K video/display

Higher resolution → better quality → larger file size.

Colour Depth

Colour depth is the number of bits used to represent the colour of each pixel.

Colour depth	Bits per pixel	Number of colours	Example
1-bit	1	2 (black & white only)	Simple icons
4-bit	4	16	Early games graphics
8-bit	8	256	GIF images
24-bit (True Colour)	24	16,777,216 (~16.7 million)	JPEG, PNG photos

Number of colours = 2^(colour depth bits). e.g. 8-bit → 2⁸ = 256 colours.

24-bit colour uses 8 bits each for red, green and blue (RGB) channels: 3 × 8 = 24 bits. With 8 bits per channel, each channel has 256 levels, giving 256³ = 16,777,216 possible colours.

Calculating File Size

The uncompressed file size of a bitmap image is calculated using:

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

Then divide by 8 for bytes, by 1024 for kilobytes, etc.

Worked Example

Image size: 800 × 600 pixels, 24-bit colour depth.

Total pixels = 800 × 600 = 480,000
Bits per pixel = 24
Total bits = 480,000 × 24 = 11,520,000 bits
÷ 8 = 1,440,000 bytes
÷ 1024 = 1406.25 KB ≈ 1.37 MB

Second Example: 1-bit image

A 100 × 100, 1-bit (black & white) image:

100 × 100 × 1 = 10,000 bits = 1,250 bytes ≈ 1.22 KB

Effect on File Size

Factor	Increase causes	Why
Resolution	Larger file size	More pixels → more data to store
Colour depth	Larger file size	More bits per pixel → more data per pixel
Resolution	Better image quality	More detail captured
Colour depth	More realistic colours	More shades and hues available

Exam tip: The file size formula (width × height × colour depth) is a guaranteed exam question. Learn the formula and always show each step. Also know: 2^n colours for n-bit depth; 24-bit = RGB, 8 bits each. Know that higher resolution AND higher colour depth both increase file size — you'll be asked to explain why.

⚠️ Common Mistakes

Forgetting to convert bits to bytes — divide by 8 after multiplying
Confusing resolution (number of pixels) with image dimensions (physical print size)
Saying 24-bit colour = 24 colours — it's 2²⁴ = 16.7 million colours
Thinking higher colour depth always produces a visually better image — after 24-bit True Colour, most humans can't tell the difference
Forgetting to include units — always state bits, bytes, KB, MB

✅ Notes completed!

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What's in this video

• Pixels and bitmap images: what they are
• Resolution: width × height, more pixels = better quality + larger file
• Colour depth: bits per pixel, 2ⁿ colours, 24-bit True Colour RGB
• File size formula: width × height × colour depth ÷ 8 = bytes

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Worksheet — 1.2.4f Representing Images

8 questions · 22 marks · Show all calculations

Q1What is a pixel? What is a bitmap image?[2]

✅ Mark scheme

A pixel (picture element) is the smallest unit of a digital image — a single square of a fixed colour [1]; a bitmap image is a grid of pixels where each pixel's colour is stored as a binary number [1].

Q2What is colour depth? How many colours can a 4-bit colour depth represent? Show your calculation.[2]

✅ Mark scheme

Colour depth is the number of bits used to represent the colour of each pixel [1]; 2⁴ = 16 colours [1].

Q3Calculate the file size in bytes of a 200 × 150 pixel image with 8-bit colour depth.[3]

✅ Mark scheme

200 × 150 = 30,000 pixels [1]; 30,000 × 8 = 240,000 bits [1]; 240,000 ÷ 8 = 30,000 bytes [1].

Q4A 24-bit colour image uses 8 bits each for red, green and blue. How many different colours can this represent? Show working.[2]

✅ Mark scheme

2²⁴ = 16,777,216 colours [1]; or 256 × 256 × 256 = 16,777,216 [1].

Q5Calculate the file size in KB of a 1024 × 768 image with 24-bit colour. Show all steps.[4]

✅ Mark scheme

1024 × 768 = 786,432 pixels [1]; × 24 bits = 18,874,368 bits [1]; ÷ 8 = 2,359,296 bytes [1]; ÷ 1024 = 2,304 KB [1].

Q6State two factors that affect the file size of a bitmap image. For each, explain the effect.[4]

✅ Mark scheme

Resolution: higher resolution → more pixels → larger file size [1+1]; Colour depth: higher colour depth → more bits per pixel → larger file size [1+1].

Q7A 1-bit image and a 24-bit image are both 400 × 300 pixels. Calculate the difference in file size in bytes between the two images.[3]

✅ Mark scheme

400×300=120,000 pixels [1]; 1-bit: 120,000 bits ÷ 8 = 15,000 bytes; 24-bit: 120,000×24=2,880,000 bits ÷ 8 = 360,000 bytes; Difference = 360,000 − 15,000 = 345,000 bytes [1 each].

Q8Explain why increasing the colour depth of an image increases its quality but also increases its file size.[2]

✅ Mark scheme

More bits per pixel → more possible colour values → more shades and hues → more realistic/accurate colour reproduction (quality increase) [1]; but each pixel needs more bits to store → total file size = pixels × bits per pixel, so the file is larger [1].

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Pixels and Bitmaps

Image Resolution

Colour Depth

Calculating File Size

Worked Example

Second Example: 1-bit image

Effect on File Size

What's in this video

Worksheet — 1.2.4f Representing Images

Mini Test — 1.2.4f Representing Images