JPEG Compression
Definition
A lossy codec that divides an image into 8×8 pixel blocks, applies the discrete cosine transform, quantises the coefficients, and entropy-codes the result. Every save round at a given quality setting introduces a new quantisation layer whose artefacts are detectable.
- Compression type
- Lossy (information is discarded)
- Block size used
- 8 by 8 pixels
- Forensic suitability
- Not suitable for archival or evidence preservation
Common questions
Why can't you use JPEG for archival forensic images?+
JPEG is a lossy format that discards fine-detail information permanently every time you save. Once that detail is gone, it's irrecoverable, making JPEG unsuitable for evidence that needs to be preserved exactly as captured.
What happens each time you re-save a JPEG image?+
Every save introduces a new quantisation layer. The artefacts from this quantisation are detectable and represent data loss from the previous version. If you save a JPEG multiple times, each round adds another layer of degradation.
How does JPEG compression actually work?+
JPEG divides an image into 8x8 pixel blocks, applies a discrete cosine transform to convert the pixel data, quantises the resulting coefficients to reduce them, and then entropy-codes the result. This process discards information at the quantisation step.
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Explained in these topics
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