nonlossless

Nonlossless refers to data compression methods that do not preserve every bit of the original data. In nonlossless (often called lossy) compression, some information is discarded during encoding with the goal of reducing file size. After decompression, the result differs from the original, and some details are irretrievably lost. The degree of loss is usually adjustable via a quality setting or bit rate.

Most nonlossless compression relies on modeling human perception and removing information that is considered less important to the viewer or listener. Techniques commonly involve transforming data, quantizing coefficients, and applying entropy coding. Because some data is discarded, higher compression ratios are achievable compared with lossless methods, but artifacts such as blurring, ringing, or audible distortion can appear, especially at lower quality levels.

Nonlossless is widely used for multimedia, where bandwidth or storage limitations make small file sizes desirable. Audio, image, and video formats frequently employ lossy compression, including MP3 and AAC for audio, JPEG for images, and MPEG or H.264 for video. Some formats offer both lossless and lossy modes, but in general the term nonlossless is synonymous with lossy.

Trade-offs are central to nonlossless use: smaller files and faster transmission versus potential degradation of fidelity. It is typically unsuitable for applications requiring exact reconstruction, such as archival storage or certain scientific or medical imaging. For those needs, lossless compression or uncompressed formats are preferred.