I suppose that would’ve been using
This is not 100% correct then: there will be negative values in there for sure.
mrview automatically deals with this by ignoring the sign (i.e. taking the absolute value) when using the RGB colour map. The RGB colour map itself is automatically selected when opening a 3-volume 4D image.
So apart from getting it recognised correctly as an RGB image by other software, you might also want to perform an
mrcalc -abs on it beforehand.
That must be something different altogether though. I don’t have experience with Trackvis, but maybe it simply doesn’t open 4D images in general? Apart from the intended RGB interpretation of the image, it’s “simply” a regular 4D image after all, so it should at least open in viewers that allow for 4D images. The only possible incompatibility I can think of there (again, based on zero experience, just guessing how other software might be picky about something particular), might be the “voxel size” for the 4th axis? Since it’s not a physical axis, some software might expect a special value there, or some software might not be able to deal with seeing a 4th number for the voxel size at all… Or finally, maybe an expected order of strides, maybe in particular for the 4th axis. Again, all just random guesses. But beyond all that, it’s simply a 4D image, with no other special properties beyond that.