Students should be able to explain how bitmaps are made from pixels.

The idea

A bitmap is created by dividing the image area into a rectangular grid of pixels. The resolution fixes how many columns and rows there are, so a 6 × 4 grid contains 24 pixels. Each pixel is sampled and assigned a binary code for its colour, using the chosen colour depth. With 1 bit per pixel only two codes exist, so each pixel is black or white; with 4 bits, 2⁴ = 16 colours are available. The codes are stored row by row, so the file is a sequence of binary values, one per pixel. More pixels capture finer detail, and greater colour depth captures more shades, but both raise the number of bits stored: total bits = pixels × colour depth. The computer rebuilds the image by reading each code and drawing the corresponding coloured square.

What earns the marks

• The image is divided into a grid of pixels.
• The number of pixels across and down is set by the resolution.
• Each pixel is assigned a binary code representing its colour.
• The code length per pixel is the colour depth, giving 2 raised to the power of the depth possible colours.
• The pixel codes are stored in order, one value per pixel, forming the bitmap file.
• Total storage in bits equals number of pixels multiplied by colour depth.
• The image is displayed by drawing each pixel in its stored colour.

Common mistakes and how to put them right

• Saying the computer stores shapes or outlines.

  Put it right: Is that only per-pixel colour codes are stored.

• Treating colour depth as the number of pixels.

  Put it right: Is that colour depth is bits per pixel.

• Assuming a higher resolution always means a smaller file.

  Put it right: Is that more pixels increase storage.

• Mixing up rows and columns when counting pixels in a grid.

Exam tips

• Use a small grid example, such as 6 × 4 = 24 pixels, to make your explanation concrete.
• Link each stage: grid, sampling to a binary code, storage, then display.
• State the storage formula pixels × colour depth and give a worked value in bits or bytes.