i'm using ZBrush 3.1. i thought i was familiar with 2.5D, i had spent maybe 40 hours just painting & doing RBG-grabs and Depth-grabs using "MRGBZGrabber".
and all of a sudden the brush or whatever - you - call - it goes BONKERS, and the nice sensuous ZBrush brush is gone, and all i have is this wierd brush.
i've had this happen before and i clicked on one of the brushes, to get back to the brush mode. but now it won't go back, if that makes any sense.
the second menu down on the left, labelled "dots" in the screen capture and "3D and 2.5D Sculpting & Poly-Painting strokes" if you open it up - i think that's how i got back to the normal Simple Brush.
it's wierd, if i go to 2.5D strokes on the right, and change it to "Snakehook", it'll do Snakehook properly. but when i go back to Simple Brush, it's stuck with the rectangle.
i closed & re-started the application (ZBrush) & also re-booted & re-started the application.
under "Brush" or preferences i don't find anything that affects it, yet.
anyway, either i got a HUUUUGE bug or it's just a simple little fix of some kind.