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jhr |
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The ASCII lines in the header end with '\n'.
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First line should be the magic:
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NRRD0004
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Rest of header will be a sequence of plain text lines, terminated by
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an empty line ('\n' alone). Some header lines give per-array
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information, some give per-axis information.
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Note that the distinction between scalar and non-scalar images is
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given by the per-axis "kinds" field, so that is where further
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information is given about the constraints associated iwth non-scalar
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image data.
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============================================================
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The basic fields that should be there are:
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----------------
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type: <string>
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Gives the type of the scalar values in the array. Current values are:
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signed char
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unsigned char
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short
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unsigned short
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int
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unsigned int
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long long int
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unsigned long long int
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float
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double
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----------------
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dimension: <int>
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Gives the dimension of the array. Must be greater than 0. Current
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code has a #define that limits the value to <= 16. The per-axis fields
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will have this many pieces of information.
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----------------
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encoding: <string>
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How scalar values are encoded on disk. Currently, should only see
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"encoding: raw".
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----------------
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endian: <string>
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Endian-ness of raw values saved on disk. Possibilities are:
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little
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big
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----------------
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space dimension: <int>
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Gives the dimension of the world-space in which the array or oriented
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(For now we're not allowing the named spaces that are given with the
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"space: " field). The points and vectors given by "space origin"
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and "space directions" will have this many coefficients, so this
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field has to precede "space origin" and "space directions".
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----------------
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space origin: (<float1>,<float2>,...,<floatN>)
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Gives the location of the center of the first sample (with index
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space coordinates (0,0,0,...,0). The number of coefficients
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separated by commas and bounded by parens (with optional whitespace
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throughout) is given by the "space dimension" field. Each coefficient
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is a floating point value (of the sort produced by printf("%g", val))
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============================================================
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The per-axis fields that should be there are as follows. The ordering
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of these fields is always *fast* to *slow*, and the number of pieces
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of information is always equal to the image dimension.
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----------------
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sizes: <int1> <int2> ... <intD>
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The number of samples along each axis. Each integer value must be >= 0.
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----------------
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kinds: <string1> <string2> ... <stringD>
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The kind of information represented by the samples along each axis. The
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possible values are:
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2-vector
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3-vector
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4-vector
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2D-symmetric-matrix
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2D-matrix
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3D-symmetric-matrix
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3D-matrix
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space
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The "space" kind is for a typical axis of the image, with samples that
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subtend some region of the spatial domain of the image. All the other
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kinds are the sequence of scalar values that compose a non-scalar
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image value (vectors and tensors).
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We currently allow there to be *at* *most* *one* axis with a
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non-"space" kind. We can have non-scalar image values, but we don't
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have cartesian products of non-scalar image values. The non-"space"
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kind can be anywhere in the axis ordering. If all the axis kinds are
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"space", then then image data is for scalars, else it is for
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non-scalar data.
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In scalar data, the image "dimension" must equal the "space dimension".
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In non-scalar data, the image "dimension" must equal *one* plus
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the "space dimension". That is, the spatial axes have to form a
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basis, not necessarily orthonormal, for world space (the vectors
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are defined by the "space directions" field, below). Thus, a
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3-D scalar image can't reside in a 2-D or 4-D world space, nor can
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a 3-D vector image reside in a 3-D or 5-D world space.
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We are currently not allowing the "measurement frame" field that
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identifies the coordinate system in which these are measured relative
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to the image orientation, so nothing should be assumed about the space
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in which these coefficients are measured. The order of coefficients
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for these non-scalar kinds is:
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2D-symmetric-matrix: Mxx Mxy Myy
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2D-matrix: Mxx Mxy Myx Myy
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3D-symmetric-matrix: Mxx Mxy Mxz Myy Myz Mzz
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3D-matrix: Mxx Mxy Mxz Myx Myy Myz Mzx Mzy Mzz
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----------------
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space directions: (<x1>,<x2>,...,<xN>) (<x1>,<x2>,...,<xN>) ...
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(and at most one of these vectors is actually "none")
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These give the "direction" in world-space between successive samples
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along each axis: the columns of the index-to-world transform matrix.
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There is one per image axis, even for the axis of coefficients of
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non-scalar image values. Because such an axis really has no spatial
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extent, for this and only this axis, the space direction vector is
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instead the string "none".
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============================================================
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Field ordering:
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The fields should appear in this order:
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type
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dimension
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space dimension
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sizes
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space directions
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kinds
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endian
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encoding
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space origin
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