Images, sound and file compression

Learning objectives

Describe how a bitmap image is stored as a grid of pixels, and calculate its file size in bytes from the image resolution and the colour depth, expressing the answer in sensible units

Explain the terms resolution, colour depth, screen resolution and image resolution, and describe the effect on picture quality and on file size of changing each one

Compare bitmap and vector graphics, describing how a vector drawing stores a drawing list of objects and their properties, and state when each format is the better choice

Describe how an analogue sound wave is sampled to produce a digital recording, and calculate the file size of a mono or stereo recording from the sampling rate, the sampling resolution and the duration

Distinguish lossy from lossless compression, give the reasons for compressing a file for storage and for transmission, and decide which type suits text, images, sound and video

Explain how run-length encoding compresses a bitmap image or a text file, and work out the encoded data for a short given example