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Hi > Ah, > > So vecreg isn't a "general purpose" transformation of vector fields? No, it’s for resampling an image made of unit vectors. If you just want to apply a matrix to a vector field, you can use the fslutils for that (fslmaths/fslroi) > It's specifically meant for transforming a unit "axis" field and ensuring the output is also a unit "axis" field? (I say "axis" here instead of vector since the direction does not matter). Yes. > > Just curious, could you have also done interpolation using a linear combination of the absolute values of the vectors and then normalize? I'm wondering why the outer product is necessary. You can flip them to all have the same sign of dot product with some axis but the eigenvector trick is better (e.g. more robust to outliers). Cheers Saad > > Thanks! > > -Justin