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I guess this This is happening because your images are stored as integers and when you add them together, they go over the integer overflow and are wrapped round to the bottom of the number line (-ve). if you specify fslmaths -dt float im1 -add im2 imsum -odt float It sould force everything to work in floating points. T On 11 Jul 2008, at 22:03, Matt Glasser wrote: > What commandline are you using to do the averaging, and what > datatype are > the volumes you are inputting? > > Peace, > > Matt. > > -----Original Message----- > From: FSL - FMRIB's Software Library [mailto:[log in to unmask]] On > Behalf > Of Anastasia Ford > Sent: Friday, July 11, 2008 2:56 PM > To: [log in to unmask] > Subject: Re: [FSL] Averaging using fslmaths > > Hello, > > I am having some trouble with fslmaths again. I am averaging two > data sets > that have strictly positive values, however, when I add the two > datasets > together the image has negative values. Why would this be happening? > > > Sincerely, > > Anastasia >