APP0

APP0, or JPEG Application Segment 0, is an application-specific marker used in JPEG files to carry metadata for the JFIF standard. It is the first application marker in many JPEGs and is commonly present in JFIF-compliant images, though its inclusion is not mandatory for all JPEGs. Decoders may ignore APP0 if it is unrecognized or not present.

The APP0 segment begins with a marker (0xFFE0) and a two-byte length field, followed by an identifier field that contains the ASCII characters “JFIF” and a zero byte. This is followed by a two-byte version (major and minor), a one-byte density units field, two two-byte fields for X and Y density, and two one-byte fields for thumbnail width and height. If a thumbnail is present, it follows these fields as a 24-bit RGB image of size width by height; if either dimension is zero, no thumbnail is included.

Purpose and use: The APP0 data conveys display and printing-related metadata. The density units and density values indicate the image’s intended resolution (for example, dots per inch or dots per centimeter), aiding proper rendering and printing on different devices. The optional embedded thumbnail provides a small preview image that some applications display without loading the full JPEG.

Variants and compatibility: While APP0 commonly carries JFIF metadata, other data can be stored in APP0 variants (such as JFXX). Exif metadata is placed in APP1. Because APP0 is optional and not all decoders require it, some JPEGs may omit it entirely, and some software may ignore unknown APP0 contents.