Photoshop brush (ABR) is a file format used by Adobe Photoshop for brushes (graphics, and information about brush behavior).
There are at least two different ABR formats, which we'll call "old format" and "new format". The first two bytes of the file are the major version number.
Known major version numbers: 1, 2
This format is documented in old versions of the Photoshop file formats specification.
Known major version numbers: 6, 7
This format is not publicly documented by Adobe.
Some guesses about the format: Minor version number is at offset 2. This signature "8BIM" appears at offset 4; it and the rest of the file constitute a series of tagged blocks (the basic structure of which is publicly documented). The known block types are 'samp', 'patt', and 'desc'. The format of 'samp' is unknown (v6.1 appears to be different from later versions). The format of 'patt' is the same as that of 'Patt' (which is publicly documented). The 'desc' block contains a descriptor.
- Photoshop File Formats Specification V6.0 Release 2
- Photoshop Brushes (ABR) Specification (from archive.org)
- Adobe Photoshop File Formats Specification (Defines some data structures used in ABR format, but does not describe the format itself.)