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Lossy file compression makes a file smaller by discarding some of its information. When the compressed file is decoded, the result is not identical to the original. Some of the differences may be hard to notice, but a result that looks similar is not the same as a result that is exactly recovered. Lossless compression is the opposite: the decoded output matches the original exactly.
The formal definition
The clearest official wording comes from ITU-T Recommendation T.800, clause 3.62, in its November 2015 edition. It defines lossy as “a descriptive term for the effect of the overall encoding and decoding processes in which the output of the decoding process is not identical to the input to the encoding process.” The same clause adds two conditions: there is distortion, which is measured mathematically, and at least one step in encoding or decoding is irreversible.
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In plain terms, a lossy process is one that cannot be undone exactly. Once some information has been thrown away during encoding, decoding can rebuild only an approximation of what went in.
Lossy compared with lossless
The two categories are defined by the outcome of the encode-and-decode round trip, not by how small the file ends up. The table below sets them side by side.
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| Question | Lossy compression | Lossless compression |
|---|---|---|
| Is the decoded output identical to the input? | No. The output is not identical to the encoding input. | Yes. Decoded output is identical to the encoding input. |
| Does the process discard information? | Yes, at least one step is irreversible. | No irreversible step is needed for exact restoration. |
| Can the original be recovered exactly? | Not guaranteed. | Yes. |
| Is the difference measurable? | Yes, as mathematical distortion. | No distortion in the restored data. |
| Is the difference noticeable to a person? | Depends on the material, the amount of loss, and the viewer or listener. | Not applicable, because the output matches the original. |
The table shows the key point: “lossy” is a statement about exact recoverability. Whether a person notices the change is a separate question, and it is the one that determines whether a particular lossy result is acceptable.
Why lossy is a tradeoff, not automatically a defect
Lossy compression is a deliberate design choice. It accepts that some source information will not survive in exchange for a smaller representation. Whether that exchange is worthwhile depends on what the file is for. A reference copy that will be edited, measured, or archived usually needs exact recovery. A file meant only for viewing or listening may tolerate differences that the audience cannot perceive.
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Because the measurable criterion is non-identity, it is more accurate to say that a lossy file has lost information than to say it has “lower quality.” Quality is a judgment about the result, and it varies by use. Loss is a fact about the process.
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A familiar format name does not always tell you whether a given file is lossy. The JPEG 2000 core standard covers both lossless and lossy still-image compression. Both appear in ITU-T T.800 (version 4, July 2024) and in ISO/IEC 15444-1:2024. A file that carries the JPEG 2000 name may therefore use either mode, so the mode should be checked in the file or in the encoder settings rather than assumed.
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The same caution applies to terminology used to describe quality. IETF RFC 8761 (2020) uses the phrase “visually lossless” for lossy compression in which the data lost is not detectable to the eye, and the decompressed output appears identical to the uncompressed data. The term describes perception, not exact recovery. A visually lossless file is still lossy: the discarded data is gone, even if no one can see the difference.
How to tell which kind of compression a file uses
- Check the format’s documentation or the encoder’s settings for a lossless option, a quality setting, or a “lossless” label.
- If the original is available, decode the compressed file and compare it with the source. Any difference in the decoded pixels or samples means the process was lossy.
- Treat “visually lossless” as a description of how the output looks, not as proof that the file is bit-for-bit identical to the source.
- Do not use a format name as a shortcut. The same family can include both lossless and lossy modes.
When lossy compression is the right choice
Lossy compression tends to fit when the audience will consume the result rather than edit or verify it, when storage or transfer size matters more than exact fidelity, and when the material is one where small losses are acceptable. It tends to be the wrong choice when the original must be preserved, when repeated edit-and-save cycles are expected, or when the output will be measured or compared against the source. Keep an unmodified lossless master in those cases, and create lossy copies from it when needed.
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Size savings and perceptible effects also depend on the content and the settings used, so no fixed amount of reduction can be assumed for every file. Test the result on the material you actually plan to use.
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In short, lossy file compression is defined by irreversible loss and by the fact that decoded output is not identical to the original. Its suitability depends on the use case, not on the definition itself.
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