Glossary
APP marker segment
An APP marker segment is a labelled block in a JPEG file where applications store metadata, ahead of the compressed image data.
Updated
Also written: APPn segment.
A JPEG is a sequence of marker segments followed by entropy-coded scan data. Sixteen application markers, APP0 through APP15, are reserved for metadata, and convention assigns them: EXIF and XMP in APP1, the ICC profile in APP2, C2PA in APP11, IPTC inside a Photoshop resource block in APP13. Free-text comments go in a COM segment.
Because all of this sits before the scan data, metadata can be removed by dropping the relevant segments and copying everything from the start-of-scan marker onward byte for byte. The image is never decoded, so it cannot lose quality.
Not every APP segment is metadata. APP0 carries the JFIF header and APP14 declares the Adobe colour transform, both of which are structural — dropping them is how naive tools break CMYK files.
Related terms
- PNG text chunk — A PNG text chunk is a named section of a PNG file holding arbitrary text metadata, optionally compressed.
- RIFF chunk — A RIFF chunk is a labelled, length-prefixed block in the container format WebP is built on.
- ICC profile — An ICC profile is a colour description embedded in an image so that viewers reproduce its colours consistently.