I've found the "Adobe Photoshop File Formats Specification" in the documentation (http://www.adobe.com/devnet-apps/photoshop/fileformatashtml/).
But unfortunately it does not contain the spec for .abr brush file format.
Any ideas where to find this?
I don’t believe the current format is documented. It has a bunch of serialized ActionDescriptors inside, so documenting it fully and explicitly would probably be pretty difficult. If you use Photoshop’s code for parsing the ActionDescriptors though, you should be able to reverse engineer the format without impossible levels of effort – might take a couple days or something.
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