Working out image and sound file sizes

In this lesson

Calculate the file size of a bitmap image from its width, height and colour depth, and of a sound file from its sample rate, duration and sample resolution, convert the result into sensible units for your own board, work out the storage needed for a set of such files, and show the working that carries most of the marks.

What it covers

• IMAGE file size in bits = colour depth x width in pixels x height in pixels (OCR prints it; AQA prints Size (bits) = W x H x D)
• AQA ALSO PRINTS THE BYTES FORM: Size (bytes) = (W x H x D) / 8 - so on AQA the divide-by-8 is inside the printed formula, not an extra step someone remembered
• SOUND file size in bits = sample rate x duration in seconds x sample resolution (AQA prints rate x res x secs; OCR prints sample rate x duration x bit depth)
• Bits to bytes is DIVIDE BY 8, and the prefix step is separate - two divisions, never one. Owned by CS02-05, reused here
• GETTING THE COLOUR DEPTH FIRST when the question gives a number of COLOURS rather than a number of bits - the smallest n with 2 to the n at least as big as the colour count. Two AQA reports in different years name this as the commonest mistake
• AQA's own scope limit: students only need to use colour depth and number of pixels within their calculations
• OCR's own allowance: an allowance for metadata in calculations may be used
• STORAGE CAPACITY FOR A SET OF FILES: work out one, then multiply by how many - and the how-many step is the last thing you do, not the first thing you forget
• EDEXCEL'S DEMAND SHAPE: construct an EXPRESSION rather than always a number, in binary multiples - kibibytes and mebibytes - with the divisor applied to the WHOLE numerator rather than to one term of it
• SHOWING WORKING: AQA states that all but one mark on each of these calculations can be gained from the working shown

Key words

On the specification