Dimension variable attributes

The attributes associated with either dimension or dimension width variables are given in this section.

• spacing - String with value ``regular__'' for a regularly spaced grid or ``irregular'' for irregular spacing of samples. If the value of this attribute is ``irregular, then the dimension variable must be a vector varying with the dimension of the same name. If this attribute is absent, a value of regular__ should be assumed. Can be used for both dimension and dimension width variables.

• step - Number indicating the step size between samples for regular spacing. This value may be negative to reverse orientation (to specify that xspace runs from patient right to left, for example). If the spacing is irregular, then the value can be an average step size. If this attribute is absent, a value of 1.0 should be assumed. Applies only to dimension variables.

• start - Number specifying the coordinate of the index 0 of the dimension. If this attribute is absent, a value of 0.0 should be assumed. Applies only to dimension variables.

• spacetype - String identifying the type of coordinate space. Currently one of ``native___'' (the coordinate system of the scanner), ``talairach_'' (a standardized coordinate system for brain images), or ``callosal__'' (another standardized coordinate system). If this attribute is absent, the spacetype should be assumed to be native. Applies only to dimension variables.

• alignment - String indicating the position of the coordinates relative to each sample, one of ``start_'', ``centre'', or ``end___''. If this attribute is absent, it is assumed to be start_ for the time dimension or centre for spatial dimensions. Applies only to dimension variables.

• direction_cosines - Numeric vector with 3 elements giving the direction cosines of the axes. Although axes are labeled x, y and z, they may in fact have a significantly different orientation - this attribute allows the direction relative to the true axes to be specified exactly. The vectors should be normalized unit vectors.

  If these attributes are not present, they are assumed to have the following default values:

  For xspace: [1, 0, 0], for yspace: [0, 1, 0], and for zspace: [0, 0, 1]

  The direction_cosines attributes apply only to dimension variables.

• width - Numeric attribute which gives the full-width half-maximum width of all samples for regularly sampled dimensions. It can be used for irregular widths to specify the average width. If this attribute is absent, a value of 1.0 should be assumed. Applies only to dimension width variables.

• filtertype - String specifying the shape of the convolving filter. Currently, can be one of ``square____'', ``gaussian__'' or ``triangular''. If this attribute is absent, a value of square____ should be assumed. Applies only to dimension width variables.