Meaning of lossy compression

Lossy compression is a data compression method in which some of the original information is permanently discarded, so the decompressed data is not identical to the original but is close enough for the intended use. It gives much higher compression ratios than lossless methods.

How it works

• It removes information that humans barely notice: fine colour detail in images (we are less sensitive to colour than brightness), high-frequency sounds hidden by louder ones, or small differences between video frames.
• The main lossy step is quantization: values are rounded to fewer levels, so many small values become zero and compress well.
• The loss is irreversible; the removed detail cannot be restored.

Examples

• JPEG for images (DCT, then quantization).
• MP3 and AAC for audio (psychoacoustic model).
• MPEG and H.264 for video.

Advantages: very small files, fast transmission, suitable for streaming. Disadvantages: quality drops, especially at high compression, and repeated compression adds artefacts such as blocking in JPEG. It is unsuitable for text, program code and data where exactness is required.