Richard,
It seems silly to ask this, but did you do the arbitrary rotation while in, for example, the grayscale mode? Or did you simply try converting to another mode and then returning to Bitmap mode, followed by the attempted rotation? If you take a 1-bit image, convert it to grayscale, rotate arbitrary, then convert back to Bitmap, that should be an easy Action to create and seemingly solve your dilemma. I tend to agree with Pierre’s speculation that arbitrary rotation in 1-bit mode isn’t supported due to resampling issues…if you select a high zoom level of a non-rotated 1-bit image, you’ll see that it is cleanly composed of 2 colors, white & black. If you then convert to grayscale, those 2 colors are intact. But, once you arbitrarilyi rotate the canvas to any non-rectangular angle, additional colors are introduced into the grayscale image. So, it seems to me that Photoshop is effectively saying "I need some additional color values to work with, if I’m going to arbitrarily rotate this image, and you’ve not given them to me by staying in Bitmap mode." You’d think that an automatic mode conversion could be performed as need be to accomplish this objective; why it isn’t, we’d need one of the software developers to explain I suspect.
Regards,
Daryl
We don’t do automatic depth conversion because we don’t like lying to customers or ruining their image data. If they want to do it and they find the results acceptable – then they can do it themselves. (some other products do automatic conversions, and people don’t realize that they’re doing damage to their images until too late)