Resolution and the file header

By the end of this lesson

By the end you can define image resolution, screen resolution and file header, and explain why software needs the header to decode a bitmap.

A pile of bits is useless on its own

Here are the pixel data of a tiny image: 0001101111100100.

Is it 4 pixels wide or 8? Is each pixel 2 bits or 4? You can't tell, and neither can the computer.

So a bitmap file carries two things: facts about the image first, then the pixels themselves. Let's meet the facts.

Definition

Image resolution

The number of pixels that make up an image, usually given as width × height in pixels (for example 1920 × 1080). Total number of pixels = width × height.

Definition

Screen resolution

The number of pixels a screen can display, given as the number of pixels horizontally × the number vertically (for example 1366 × 768).

Same word, different owners

	Image resolution	Screen resolution
Belongs to	the image	the display hardware
Set by	whatever created the image (camera, scanner, software)	the monitor and its settings
Example	a 4000 × 3000 photo: 12 000 000 pixels	a 1920 × 1080 laptop screen

When they don't match:

Image bigger than the screen: you can't see every pixel at full size. The image is scaled down for display (detail is dropped), or you scroll around it.
Image smaller than the screen, stretched to fill it: each image pixel is spread across several screen pixels, so it looks blocky (pixelated).

Quick check

Multiple choiceQuestion 1 · Recall

CalculateQuestion 2 · Recall

CalculateQuestion 3 · Apply

Answer the 3 questions above to continue.