Glossary
Chunk
A chunk is a self-describing section of a file, carrying its own type label and length.
Updated
Most modern image containers are sequences of chunks under one name or another — chunks in PNG and RIFF, boxes or atoms in the ISO base media formats, marker segments in JPEG. The shared idea is that each section declares what it is and how long it is.
That property is what makes lossless metadata removal possible at all. A parser can walk the file, identify each section by its label, drop the ones carrying metadata, and copy the rest through without ever decoding the compressed image data.
The alternative — decoding the picture and re-encoding it without the metadata — is far simpler to implement and works on every format at once, which is why so many tools do it. It is also lossy on every JPEG it touches.
Related terms
- PNG text chunk — A PNG text chunk is a named section of a PNG file holding arbitrary text metadata, optionally compressed.
- ISO BMFF box — A box, sometimes called an atom, is the nested container unit that HEIC, MP4 and MOV files are built from.
- Container format — A container format is the file structure that wraps compressed image or video data together with the metadata describing it.