Bitmap Images: Pixels, Resolution and Colour Depth
A picture stored as a grid of pixels, each one a binary colour value. Resolution and colour depth decide both how it looks and how big the file is.
What a learner can do afterwards
- Explain what resolution controls and what colour depth controls
- Work out the size of an uncompressed image from its dimensions and colour depth
- Predict what happens to a picture blown up past its resolution
1 · Read
A digital picture is a grid of pixels. Each pixel is one small square with a stored colour value. In a two-colour bitmap, one bit can mark one colour as 0 and the other as 1.
Each square is one pixel.
For an uncompressed image, multiply width by height by colour depth. An 80 by 50 image with 4 bits per pixel takes 80 times 50 times 4, or 16,000 bits. Doubling the depth to 8 bits doubles the bit count.
A 16 by 16 two-colour icon has 256 pixels. With one bit per pixel, its uncompressed data takes 256 bits.
A 30 by 20 image uses 2 bits per pixel. It has 600 pixels, so it takes 1,200 bits before compression.
Resolution describes the pixel dimensions of an image, such as 100 by 100. More pixels packed into the same space can show finer detail.
Colour depth is the number of bits stored for each pixel. More bits allow more possible colours and make the uncompressed file larger.
When a low-resolution picture is enlarged, each stored pixel must cover more space. The computer may guess values between pixels, so the picture can look blurry or blocky.
Resolution describes pixel dimensions, colour depth describes bits per pixel, and width times height times depth gives uncompressed size.
Original lesson wording, examples, assessment items, and worked steps authored for the P12 computing curriculum.
Enrichment facts are adapted from the cited source records; the replacement readings were authored for the P12 computing curriculum.
2 · Watch
Take it off screen
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.