Pixels and Bitmaps

A digital image is made up of tiny squares called pixels (picture elements). Each pixel stores a single colour value. The collection of all pixels in an image is called a bitmap.

In a black-and-white image, each pixel is 0 (white) or 1 (black) — just 1 bit. In a grayscale or colour image, more bits are needed per pixel.

Colour Depth

Colour depth (also called bit depth) is the number of bits used to represent each pixel. More bits per pixel = more possible colours.

• 1 bit: 2 colours (black and white)
• 8 bits: 256 colours
• 16 bits: 65 536 colours
• 24 bits: 16 777 216 colours (True Colour / RGB)

Number of colours = 2^{colour depth}

Resolution

Resolution is the number of pixels in an image, usually expressed as width × height (e.g. 1920 × 1080) or as pixels per inch (ppi/dpi).

• Higher resolution → more pixels → more detail → larger file size
• Lower resolution → fewer pixels → less detail → smaller file size

File Size Calculation

Image file size (bits) = number of pixels × colour depth (bits per pixel)

Number of pixels = width (pixels) × height (pixels)

Example: A 200 × 150 pixel image with 24-bit colour depth:

• Total pixels = 200 × 150 = 30 000
• File size = 30 000 × 24 = 720 000 bits = 90 000 bytes = 87.9 KB

Metadata

Image files also contain metadata — additional data stored alongside the image itself. Examples include: image width and height, colour depth, file format, date taken, and for photos, GPS location or camera settings. Metadata increases the overall file size slightly beyond just the pixel data.