Photoshop Image Resources
The format is presumably derived from Macintosh resource file format. Except in an actual Macintosh resource file, it is serialized as a contiguous sequence of blocks.
A resource block has a numeric Resource ID, indicating the type of data stored in the block. Each block also contains a name, though the name is often left empty.
Each block usually begins with the ASCII characters "8BIM", a signature that appears in several Photoshop formats. Other block signatures that have been observed are "MeSa" (apparently associated with ImageReady), "PHUT" (apparently associated with PhotoDeluxe), "AgHg", and "DCSR".
Not all Resource IDs are publicly documented.
Apparently Photoshop data (at least in Mac formats) also makes use of the ANPA-1312 format, a news-media industry markup format, for metadata storage.
Custom application data ("plug-in resources") can be stored using resource IDs 4000–4999 (0x0FA0–0x1387). There might be more information about this somewhere in the Photoshop SDK.
[TODO: How are these resources identified? 1000 ID numbers seems insufficient for global uniqueness. Some or all plug-in resources appear to begin with a FourCC.]
[TODO: Is there any documentation available about specific plug-in resource formats?]
- In a TIFF file, tag 34377 contains Photoshop Image Resources.
- In a JPEG file, an APP13 marker with an identifier of "Photoshop 3.0" contains Photoshop Image Resources.
- Resource ID 0x0404 contains IPTC data.
- Resource ID 0x040c may contain a thumbnail in JPEG/JFIF format.
- Resource ID 0x040F contains an ICC profile.
- Resource ID 0x0422 contains Exif data.
- Resource ID 0x0424 contains XMP data.
See also Photoshop.