FT_DATATYPE_SEGMENTATION describes the FieldTrip MATLAB structure for segmented
 voxel-based data and atlasses. A segmentation can either be indexed or probabilistic
 (see below).

 A segmentation is a volumetric description which is usually derived from an anatomical
 MRI, which describes for each voxel the tissue type. It for example distinguishes
 between white matter, grey matter, csf, skull and skin. It is mainly used for masking
 in visualization, construction of volume conduction models and for construction of
 cortical sheets. An volume-based atlas is basically a very detailed segmentation with
 an anatomical label for each voxel.

 For example, the AFNI TTatlas+tlrc segmented brain atlas (which can be created
 with FT_READ_ATLAS) looks like this

              dim: [161 191 141]        the size of the 3D volume in voxels
        transform: [4x4 double]         affine transformation matrix for mapping the voxel coordinates to head coordinate system
         coordsys: 'tal'                the transformation matrix maps the voxels into this (head) coordinate system
             unit: 'mm'                 the units in which the coordinate system is expressed
           brick0: [161x191x141 uint8]  integer values from 1 to N, the value 0 means unknown
           brick1: [161x191x141 uint8]  integer values from 1 to M, the value 0 means unknown
      brick0label: {Nx1 cell}
      brick1label: {Mx1 cell}


 An example segmentation with binary values that can be used for construction of a
 BEM volume conduction model of the head looks like this

           dim: [256 256 256]         the dimensionality of the 3D volume
     transform: [4x4 double]          affine transformation matrix for mapping the voxel coordinates to head coordinate system
      coordsys: 'ctf'                 the transformation matrix maps the voxels into this (head) coordinate system
          unit: 'mm'                  the units in which the coordinate system is expressed
         brain: [256x256x256 logical] binary map representing the voxels which belong to the brain
         scalp: [256x256x256 logical] binary map representing the voxels which belong to the scalp
         skull: [256x256x256 logical] binary map representing the voxels which belong to the skull

 An example of a whole-brain anatomical MRI that was segmented using FT_VOLUMESEGMENT
 looks like this

         dim: [256 256 256]         the size of the 3D volume in voxels
   transform: [4x4 double]          affine transformation matrix for mapping the voxel coordinates to head coordinate system
    coordsys: 'ctf'                 the transformation matrix maps the voxels into this (head) coordinate system
        unit: 'mm'                  the units in which the coordinate system is expressed
        gray: [256x256x256 double]  probabilistic map of the gray matter
       white: [256x256x256 double]  probabilistic map of the white matter
         csf: [256x256x256 double]  probabilistic map of the cerebrospinal fluid

 The examples above demonstrate that a segmentation can be indexed, i.e. consisting of
 subsequent integer numbers (1, 2, ...) or probabilistic, consisting of real numbers
 ranging from 0 to 1 that represent probabilities between 0% and 100%. An extreme case
 is one where the probability is either 0 or 1, in which case the probability can be
 represented as a binary or logical array.

 The only difference to the volume data representation is that the segmentation
 structure contains the additional fields xxx and xxxlabel. See FT_DATATYPE_VOLUME for
 further details.

 Required fields:
   - dim, transform

 Optional fields:
   - coordsys, unit

 Deprecated fields:
   - none

 Obsoleted fields:
   - none

 Revision history:
 (2012/latest) The explicit distunction between the indexed and probabilistic
 representation was made. For the indexed representation the additional
 xxxlabel cell-array was introduced.

 (2005) The initial version was defined.

 See also FT_DATATYPE, FT_DATATYPE_VOLUME, FT_DATATYPE_PARCELLATION