Representing images: W = image width

The idea

In the AQA 8525 image-representation topic, a bitmap is a grid of pixels, and the symbol W stands for the image width: the number of pixels across the image from left to right. Width is one of the two dimensions that fix the total pixel count, because total pixels = W × H, where H is the image height. For example, an image 800 pixels wide and 600 pixels high contains 800 × 600 = 480 000 pixels. Width is measured in pixels, not in centimetres or bytes. File size in bits is then W × H × colour depth, so doubling W doubles the number of pixels and the file size if H and colour depth stay the same. In questions, identify W as the horizontal pixel count and use it correctly in the pixel-count and file-size calculations.

What earns the marks

• W is the image width, the number of pixels measured horizontally across the image.
• Total pixel count equals W × H, so width is one of the two dimensions giving the grid size.
• Width is counted in pixels, not in physical units such as centimetres or in bytes.
• File size in bits = W × H × colour depth, so W contributes directly to storage size.
• Doubling W while keeping H and colour depth constant doubles the pixel count and the file size.

Common mistakes and how to put them right

• Treating W as the image height.

  Put it right by remembering W is the horizontal width and H is the vertical height.

• Measuring width in centimetres or bytes.

  Put it right by stating width is a count of pixels.

• Using W alone as the pixel count.

  Put it right by multiplying W by H to get total pixels.

• Forgetting colour depth in file-size work.

  Put it right by using W × H × colour depth for size in bits.

Exam tips

• Write the formula total pixels = W × H before substituting numbers, so each symbol is used correctly.
• Check units: width and height are in pixels, colour depth in bits per pixel, giving file size in bits.
• If a question gives width and height, calculate the pixel count first, then multiply by colour depth.