lossless

Lossless refers to data compression methods that allow exact reconstruction of the original data from the compressed form. In lossless compression, no information is discarded during encoding, so decompression yields a bit-for-bit identical copy of the input. This property distinguishes lossless techniques from lossy compression, which permanently removes data to achieve higher compression ratios and may degrade fidelity.

Lossless compression works by exploiting redundancy and patterns in the data. Common approaches include dictionary-based coding (for example, Lempel-Ziv variants), and entropy coding (such as Huffman or arithmetic coding). Many formats apply a sequence of steps, including preprocessing, prediction, and reversible transformations, followed by entropy coding to produce compact representations.

Practical applications span general data storage and transmission to media formats designed for fidelity. General-purpose archives use lossless codecs such as ZIP and gzip. Image formats like PNG and lossless Web formats retain full image fidelity, while GIF is limited to a small color palette. In audio, lossless formats such as FLAC and ALAC provide transparent reproduction of the original recordings. Some video and image pipelines offer lossless modes or codecs (e.g., FFV1, HuffYUV), though lossy options are more common for streaming and broadcasting.

Because lossless compression preserves all original information, file sizes may be larger than lossy equivalents for natural media. The effectiveness depends on the data’s inherent redundancy and structure. Lossless methods are preferred for archival storage, legal and scientific data, software distribution, and any situation requiring exact reproducibility.