APP13

APP13, short for Application Marker 13, is a JPEG marker segment (marker 0xED) used to carry application-specific data inside JPEG files. It is one of several APP markers defined by the JPEG standard and is not required for every image. APP13 is historically associated with Adobe Photoshop, which used this marker to embed Image Resource Blocks (IRBs) that store metadata and other resources alongside the image data.

Within APP13, the data begins with a signature such as "Photoshop 3.0" to identify the resource set, followed by a sequence of resource entries identified by numeric IDs, a name, and a payload. Common resource blocks include those that store a thumbnail image, resolution information, and various metadata blocks. IPTC data has been stored in APP13 as part of the Photoshop IRBs in some workflows, particularly in older workflows, though metadata practices have evolved over time.

The usage of APP13 is software-dependent and optional. Many JPEGs do not include APP13 at all. Modern metadata conventions more often place IPTC, XMP, or other metadata in APP1 or other markers, and some save operations may strip APP13 data, leading to inconsistent retention of the embedded resources. Support for APP13 varies among image viewers and editors; some can extract and display Photoshop IRBs, while others ignore them.

Overall, APP13 is a legacy yet valid component of the JPEG format used primarily for storing Photoshop-related resources and metadata. Its presence can aid compatibility with older software, but it is less central to contemporary metadata workflows.