Lossy compression

Lossy compression reduces data size by permanently discarding information judged less significant. The decoded result approximates the original rather than reproducing it exactly.

Quantization: A process that maps many possible values to a smaller set of representative values. Coarsening transformed values is a direct way codecs discard information.
JPEG: A family of image coding standards widely used for photographic images. JPEG is a canonical transform-based lossy format for still images.
Lossless compression: Compression that reduces data size while allowing exact reconstruction of the original data. It preserves every bit of information, unlike lossy compression.
Compression artifact: A visible or audible defect introduced by a compression process. Artifacts are the perceptible evidence of information discarded or approximated by a codec.
Discrete cosine transform: A mathematical transform that represents a signal as a sum of cosine waves at different frequencies. Image and video codecs use it to concentrate signal energy into coefficients they can quantize.
MP3: A digital audio coding format that reduces file size using perceptual audio coding. MP3 applies psychoacoustic masking to remove less audible audio information.
Uncompressed data: Data stored without applying a compression scheme. It avoids compression-induced distortion at the cost of larger storage or transmission size.
Psychoacoustics: The study of how people perceive sound, including loudness, pitch, and masking. Audio codecs rely on auditory limits to decide which signal details can be removed.
Perceptual coding: Encoding that uses models of human perception to allocate fewer bits to less noticeable signal components. It guides which details a codec can discard with less perceptual impact.
Advanced Audio Coding: An audio coding standard designed to deliver compressed audio at a given quality using fewer bits than MP3. AAC is a widely deployed lossy audio codec in streaming and mobile media.

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