Representing images: image size

The idea

Image size is the amount of storage a bitmap needs, measured in bits or bytes. A bitmap is a grid of pixels, and each pixel stores a binary code describing its colour. The number of bits used per pixel is the colour depth. To calculate image size, multiply the number of pixels by the colour depth: image size in bits = width in pixels × height in pixels × colour depth in bits per pixel. For example, a 100 × 50 pixel image at a colour depth of 8 bits has 100 × 50 = 5000 pixels, so 5000 × 8 = 40 000 bits, which is 40 000 ÷ 8 = 5000 bytes. Dividing by 1024 converts bytes to kibibytes where that convention is used. Increasing width, height or colour depth increases file size.

What earns the marks

• Image size in bits = width in pixels × height in pixels × colour depth in bits per pixel.
• The number of pixels is found by multiplying the image width by its height.
• Colour depth is the number of bits stored per pixel, so it multiplies the pixel count.
• A larger image or greater colour depth produces a larger file size.
• Converting bits to bytes divides by 8; converting bytes to kibibytes divides by 1024.

Common mistakes and how to put them right

• Adding width and height instead of multiplying them.

  Put it right: Pixel count is width × height.

• Forgetting to multiply by colour depth.

  Put it right: Each pixel needs its bits, so pixel count × colour depth gives bits.

• Confusing bits and bytes.

  Put it right: The formula gives bits, and dividing by 8 gives bytes.

Exam tips

• Write the formula first, then substitute the numbers, so method marks are visible.
• Keep units consistent: state whether your answer is in bits, bytes or kibibytes.
• Check whether the question gives colour depth in bits per pixel before multiplying.