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What is Lossy compression?

A compression method that makes data far smaller than lossless compression can by discarding components that human eyes or ears hardly notice. The restored data do not match the original exactly. Examples include JPEG for images, MP3 and AAC for audio, and H.264 and H.265 for video, which commonly split the data into frequency components and quantize the fine components coarsely. The stronger the compression, the more visible the degradation, such as blocky distortion. Repeated compression and restoration compounds the degradation.

Japanese name: 非可逆圧縮

Lossy compression appears in 2 practice questions across 2 topics, including Information Theory and Coding Basics, Web Technology Basics.

Field: Information Theory and Coding Basics

Try it: questions about Lossy compression

When compressing data, which of the following must use lossless compression rather than lossy compression?

  1. ACompressing a program's executable file for distribution
  2. BShrinking video to match the speed of the connection in a video streaming service
  3. CMaking music files smaller to listen to on a portable player
Show answer and explanation

Answer: A. Compressing a program's executable file for distribution

Lossless compression restores the original data without a single bit of difference, while lossy compression discards parts that people hardly notice in return for much greater shrinkage. A program does not run correctly if even one bit differs, so lossless compression such as ZIP is used. Small differences in images and sound are hard to see or hear, so lossy compression such as JPEG, MP3 and video coding formats is widely used for them.

Lossy compression in the explanations

Questions that mention Lossy compression (2)

  1. When compressing data, which of the following must use lossless compression rather than lossy compression?
  2. Which of the following is an appropriate pairing of an image format for web pages with its typical use?

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Related terms

Lossless compression Average code length Binary symmetric channel Forward error correction (FEC) Hamming code Hamming distance Huffman coding Information entropy Kraft inequality Mutual information

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