TJ, You can't actually repair the problem completely since the template to .bum conversion adds rounding errors and loss of information that is not reversible. In other words, there is no way to exactly re-create the (bitwise identical) Mac .bum conversion out of the PC one (unless you still have the original template and convert from scratch). However, you can come pretty close by using this formula: if (G <= 254) G = 254 - G; else G = 0; So it's not really the bitwise invert (G ^= 0xFF or G = 255-G) but "one level lower". The "else" branch should not be necessary since the value can never be 255, however (with all those .bum hackers around) it's better to have it in. The Photoshop "invert" will end up with a value that is 0 to 2 levels wrong, while this formula above ends up with an average error of -1 to 1 levels, and is therefore much better.