Thanks for your help! Unfortunately this doesn't seem to be what's affecting me.
The problem is certainly an 8bit vs 16 bit problem, but not as a result of a PPA: in Ubuntu 15.04 the only version of libmagickcore-6 available is a 16-bit version. I've even tried reinstalling everything with "magick" or "octave" in the name; all the downloads were from the official Ubuntu repository, but the problem persists.
Thanks for your help! Unfortunately this doesn't seem to be what's affecting me.
The problem is certainly an 8bit vs 16 bit problem, but not as a result of a PPA: in Ubuntu 15.04 the only version of libmagickcore-6 available is a 16-bit version. I've even tried reinstalling everything with "magick" or "octave" in the name; all the downloads were from the official Ubuntu repository, but the problem persists.