Resolution, colour depth and quality

By the end of this lesson

By the end you can describe how changing the image resolution and the colour depth affects a bitmap's quality and file size, and work out the factor by which the size changes.

Two dials, one bill

A bitmap has two quality dials: image resolution (how many pixels) and colour depth (how many bits per pixel).

Turn either one up and the picture gets better. Turn either one up and the file gets bigger. No free lunch, because file size = width × height × colour depth.

Change	Effect on quality	Effect on file size
Increase image resolution	more pixels, so finer detail; smoother edges	larger: more pixels to store
Decrease image resolution	less detail; blocky (pixelated) at the same display size	smaller
Increase colour depth	more colours available, so colours and shading are more accurate	larger: more bits per pixel
Decrease colour depth	fewer colours; colours approximated, visible bands in gradients	smaller

More pixels, more detail, more bytes. Each step doubles both dimensions, so the file grows ×4.

Resolution counts twice

Image resolution has two numbers. Double the width and the height and you get $2 \times 2 = 4$ times the pixels, so 4 times the file size.

Halve both and the file shrinks to a quarter. Scale both by $k$ and the size scales by $k^2$ .

Quick check

Multiple choiceQuestion 1 · Recall

CalculateQuestion 2 · Apply

Answer the 2 questions above to continue.