Multimedia – Graphics, Sound

Pixel: the smallest individual element (a single dot) of a bitmap image; each pixel stores a binary code for its colour.

Image resolution: the number of pixels in the image, usually given as width × height (e.g. 1920 × 1080); more pixels means greater detail.

Number of pixels = 500 × 400 = 200,000. Size in bits = 200,000 × 8 = 1,600,000 bits. Size in bytes = 1,600,000 ÷ 8 = 200,000 bytes. Size in KiB = 200,000 ÷ 1,024 ≈ 195.3 KiB.

A vector graphic is built from drawing objects — basic geometric shapes such as lines, rectangles, circles and curves.

Each drawing object has a set of properties that describe it, for example its coordinates/position, dimensions, fill colour, line thickness and line colour.

All the objects and their properties are stored together in a drawing list — the complete set of instructions for the image.

To display or resize the image, the software reads the drawing list and recalculates the objects from their properties, which is why a vector graphic can be scaled to any size without losing quality.

Sound is naturally analogue — a continuously varying wave. To store it digitally, the amplitude of the wave is measured at regular time intervals; this measuring is called sampling.

The sampling rate is the number of samples taken per second (measured in Hz). A higher rate takes measurements more often, so the digital version follows the shape of the original wave more closely.

The sampling resolution (bit depth) is the number of bits used to store each sample. More bits allow each amplitude to be recorded with greater precision, reducing rounding error.

Each sample's measured value is stored as a binary number, producing a stream of binary data (digital) that approximates the original analogue wave. Higher rate and resolution give a more accurate recording but a larger file.

(i) Photo-realistic poster — bitmap. A photograph contains millions of subtly different colours and fine, irregular detail that cannot be described as simple geometric shapes. A bitmap stores an individual colour for every pixel, so it can capture this realistic, continuous-tone detail. (A vector graphic would be impractical, needing an enormous number of objects to approximate a photo.)

(ii) Scalable app icon — vector. An icon must look sharp at many sizes (from a tiny menu icon to a large display). A vector graphic stores the image as objects and properties that are recalculated when resized, so it scales to any size with no loss of quality and no pixelation. A simple icon also stores compactly as a few objects. A bitmap icon would pixelate when enlarged.

Effect on file size (the goal): File size = sampling rate × sampling resolution × time × channels. Reducing the sampling rate takes fewer samples per second, and reducing the resolution stores fewer bits per sample, so both reduce the file size proportionally — making it faster to stream over a slow connection and using less bandwidth/storage.

Effect on quality (the cost): A lower sampling rate means the waveform is measured less often, so higher-frequency detail is lost and the sound becomes less faithful to the original. A lower resolution means each sample's amplitude is stored less precisely, increasing rounding and adding 'noise', so the sound is less clear.

Balanced judgement: There is a trade-off between size and quality. For speech or background audio the reduction may be acceptable, but for music the loss of clarity may be noticeable, so the service might offer different quality levels depending on the user's connection.