Pearson Edexcel · GCSE · Computer Science
Course version: 1CP2 · Version from Sept 2020
Understand how bitmap images are represented in binary (pixels,…
Full specification wording
Data representation: 2.2.2 understand how bitmap images are represented in binary (pixels, resolution, colour depth)
A bitmap image is stored as a grid of pixels, and each pixel is given a binary code for its colour.
Checked against the Pearson Edexcel 1CP2 specification (opens in a new tab) on .
- Point 2 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A bitmap image is stored as a grid of pixels, and each pixel is given a binary code for its colour. The resolution is the number of pixels in the image, often written as width × height, such as 1920 × 1080, which is 2 073 600 pixels. The colour depth is the number of bits used for each pixel; a colour depth of n bits gives 2ⁿ possible colours, so 1 bit gives 2 colours, 3 bits give 8 colours and 8 bits give 256 colours. The file size in bits is the number of pixels multiplied by the colour depth, so a 1920 × 1080 image at 8-bit colour depth needs 2 073 600 × 8 = 16 588 800 bits, which is 16 588 800 ÷ 8 = 2 073 600 bytes. Increasing resolution or colour depth stores more detail or more colours but increases the file size.
What earns the marks
- A bitmap image is a grid of pixels, and each pixel is stored as a binary code for its colour.
- Resolution is the number of pixels in the image, for example 1920 × 1080 = 2 073 600 pixels.
- Colour depth is the number of bits used per pixel, and n bits give 2ⁿ possible colours.
- File size in bits = number of pixels × colour depth; divide by 8 to convert bits to bytes.
- A 1920 × 1080 image at 8-bit colour depth needs 2 073 600 × 8 = 16 588 800 bits = 2 073 600 bytes.
- Increasing resolution or colour depth increases the file size and the amount of detail or colour stored.
Common mistakes and how to put them right
Confusing resolution with colour depth.
Put it right: Resolution is the number of pixels, while colour depth is the bits per pixel.
Using 2ⁿ for the number of pixels instead of the number of colours.
Put it right: n bits of colour depth give 2ⁿ colours.
Forgetting to divide by 8 when converting a file size in bits to bytes.
Adding resolution and colour depth instead of multiplying them when calculating file size.
Exam tips
- Write the formula file size in bits = pixels × colour depth before substituting numbers.
- Convert bits to bytes by dividing by 8, and to kilobytes by dividing by 1024 if required.
- Check whether the question asks for file size in bits, bytes or kilobytes before giving the final answer.
Try the lesson
The start of the Data representation: 2.2.2 understand how bitmap images are represented in binary (pixels, resolution, colour depth) lesson, made from checked teaching notes for this specification point.
Have a go first
A bitmap image has a resolution of 4 pixels across and 3 pixels down. How many pixels make up the whole image?
Listen
A bitmap image is stored as a grid of pixels, and each pixel is given a binary code for its colour. The resolution is the number of pixels in the image, often written as width × height, such as 1920 × 1080, which is 2 073 600 pixels. The colour depth is the number of bits used for each pixel; a colour depth of n bits gives 2ⁿ possible colours, so 1 bit gives 2 colours, 3 bits give 8 colours and 8 bits give 256 colours. The file size in bits is the number of pixels multiplied by the colour depth, so a 1920 × 1080 image at 8-bit colour depth needs 2 073 600 × 8 = 16 588 800 bits, which is 16 588 800 ÷ 8 = 2 073 600 bytes. Increasing resolution or colour depth stores more detail or more colours but increases the file size. A strong answer earns marks for these points. A bitmap image is a grid of pixels, and each pixel is stored as a binary code for its colour. Resolution is the number of pixels in the image, for example 1920 × 1080 = 2 073 600 pixels. Colour depth is the number of bits used per pixel, and n bits give 2ⁿ possible colours. File size in bits = number of pixels × colour depth; divide by 8 to convert bits to bytes. Exam tip: Write the formula file size in bits = pixels × colour depth before substituting numbers.
Key words
Say what each word means before you turn the card.
Bitmap images
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Resolution
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Colour depth
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File size calculation
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See every definition
- Bitmap images
- A grid of pixels, where each pixel is stored as a binary code for its colour.
- Resolution
- The number of pixels in the image, often written as width × height.
- Colour depth
- The number of bits used for each pixel, where n bits give 2ⁿ possible colours.
- File size calculation
- The number of pixels multiplied by the colour depth gives the file size in bits; divide by 8 to convert bits to bytes.
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