Hi Joel,

AFNI stores the ORIGIN coordinates using the DICOM RAI
signing scheme, which is to say that right, anterior
and inferior are always stored as negative coordinates,
whereas L, P and S are positive. Some data formats are
different in this way (ANALYZE 7.5?).

> If the X direction of the file is in the R2L direction,
then ORIGIN[0] will presumably be less than 0.0 and DELTA[0]
must be greater than 0.0.

Even if the RL direction is truely right-to-left, ORIGIN[0]
does not have to be negative, the data could be entirely on
the left side, going negative to negative. Here, the DELTA
sign will indeed be positive.

However, if the RL direction is actually left-to-right, then
the DELTA sign will be negative (since the coordinates will
go from positive to negative, because left is positive).
Again, that doesn't imply the ORIGIN will be positive or
negative (but since most datasets include 0,0,0, the ORIGIN
here will generally be positive).

> Will I ever see an R2L file with DELTA[0]<0.0?

No, because right is considered negative, with respect to
left. But an L2R file will have DELTA[0] < 0.0.

Is this clear (at all :)?

- rick