Representing images: H = image height

The idea

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

What earns the marks

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

Common mistakes and how to put them right

• Treating H as the image width.

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

• Measuring height in centimetres or bytes.

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

• Using H 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: height and width 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.