Image Representation

This lesson covers how computers represent digital images, including bitmaps, pixels, resolution, colour depth, file size calculations, and the difference between bitmap and vector graphics. This is required for the OCR H446 specification.

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Bitmap Images

A bitmap (raster) image is made up of a grid of tiny coloured squares called pixels (picture elements). Each pixel has a single colour, and when viewed together, the pixels form a complete image.

Key Properties

Property	Definition
Pixel	The smallest individual element of a bitmap image
Resolution	The number of pixels in the image (width x height)
Colour depth	The number of bits used to represent the colour of each pixel
Metadata	Information stored alongside the image data (e.g., resolution, colour depth, file format)

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Resolution

Resolution refers to the number of pixels per unit area, often measured in pixels per inch (PPI) or dots per inch (DPI), or simply as total pixel dimensions (e.g., 1920 x 1080).

Aspect	Higher Resolution	Lower Resolution
Image quality	Sharper, more detail	Blurred, less detail
File size	Larger	Smaller
Display size	More pixels to render	Fewer pixels

A 1920 x 1080 image has 1920 x 1080 = 2,073,600 pixels (approximately 2 megapixels).

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Colour Depth

Colour depth (or bit depth) is the number of bits used to store the colour of each pixel. More bits = more possible colours.

Colour Depth	Number of Colours	Description
1 bit	2^1 = 2	Black and white
2 bits	2^2 = 4	4 colours
4 bits	2^4 = 16	16 colours
8 bits	2^8 = 256	256 colours (e.g., GIF)
16 bits	2^16 = 65,536	High colour
24 bits	2^24 = 16,777,216	True colour (RGB, 8 bits per channel)
32 bits	2^32 = ~4.3 billion	True colour + alpha (transparency)

RGB Colour Model

In the RGB model, each pixel's colour is defined by three channels:

• Red: 0-255 (8 bits)
• Green: 0-255 (8 bits)
• Blue: 0-255 (8 bits)

Total: 8 + 8 + 8 = 24 bits per pixel (true colour).

Colour	R	G	B	Hex Code
Red	255	0	0	#FF0000
Green	0	255	0	#00FF00
Blue	0	0	255	#0000FF
White	255	255	255	#FFFFFF
Black	0	0	0	#000000
Yellow	255	255	0	#FFFF00

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Calculating Bitmap File Size

File size (bits) = width (pixels) x height (pixels) x colour depth (bits per pixel)

File size (bytes) = file size (bits) / 8

Worked Example

An image is 800 x 600 pixels with 24-bit colour depth.

• File size = 800 x 600 x 24 = 11,520,000 bits
• File size = 11,520,000 / 8 = 1,440,000 bytes
• File size = 1,440,000 / 1024 = 1,406.25 KB
• File size approximately 1.37 MB

Note: This is the raw pixel data size. Actual file size includes metadata (header information) and may be reduced by compression.

Worked Example: Effect of Changing Colour Depth

Same image (800 x 600) with different colour depths: