Representing images

What you need to know

• ✓A bitmap image is composed of a grid of individual pixels (picture elements).
• ✓In a 1-bit monochrome image, each pixel is stored as either 0 (white) or 1 (black).
• ✓Colour depth is the number of bits allocated per pixel; an n-bit colour depth gives 2ⁿ available colours.
• ✓File size calculation formula: File size (in bits) = width × height × colour depth.
• ✓To convert bits to bytes, divide by 8; to convert bytes to kilobytes (kB), divide by 1,000, and by 1,000 again for megabytes (MB).
• ✓Metadata is data about data stored in the file header (e.g. width, height, colour depth, resolution) allowing software to correctly reconstruct and display the continuous bitstream.

What you don’t need

• Vector graphics and SVG paths (AQA 8525 focuses on bitmap images).
• Compression algorithms like RLE or Huffman (covered in 3.3.8).
• Direct RGB colour palettes larger than 24-bit True Colour.

Worked examples

1. Calculate the storage size in bytes for a 200 × 100 pixel monochrome bitmap image.

   1. Monochrome uses 1 bit per pixel (colour depth = 1).
   2. Total pixels = 200 × 100 = 20,000 pixels.
   3. Total bits = 20,000 × 1 = 20,000 bits.
   4. Convert to bytes by dividing by 8: 20,000 ÷ 8 = 2,500 bytes.

   Answer: 2500 bytes (or 2.5 kB)

2. An icon has dimensions 32 × 32 pixels and is drawn using 16 colours. Calculate its file size in bytes.

   1. Calculate bit depth from colours: 16 colours = 2⁴, so colour depth is 4 bits per pixel.
   2. Total pixels = 32 × 32 = 1,024 pixels.
   3. Total bits = 1,024 × 4 = 4,096 bits.
   4. Convert to bytes: 4,096 ÷ 8 = 512 bytes.

   Answer: 512 bytes

3. Explain why metadata is included in a bitmap image file, and state two examples of metadata stored.

   1. State why metadata is needed: Raw bitmap pixels are stored as a continuous 1D bitstream without line breaks; the computer needs header metadata to know where each row wraps and how many bits represent each pixel.
   2. State two specific examples of metadata: image width/height (dimensions) and colour depth (bits per pixel).

   Answer: Metadata provides the necessary dimensions and colour depth so software can correctly decode and arrange the continuous bitstream on screen. Examples include image width/height and colour depth.

Common mistakes

• ✕Forgetting to divide by 8 when an exam question asks for file size in bytes rather than bits.
• ✕Multiplying by the number of colours instead of the colour depth (e.g. multiplying by 256 instead of 8 bits).
• ✕Claiming metadata makes an image sharper or improves quality — metadata merely describes file properties so the operating system can decode the pixels.