magic

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nifti1.h header documentation

   NIFTI-1 FLAG (MAGIC STRINGS):
   ----------------------------
   To flag such a struct as being conformant to the NIFTI-1 spec, the last 4
   bytes of the header must be either the C String "ni1" or "n+1";
   in hexadecimal, the 4 bytes
     6E 69 31 00   or   6E 2B 31 00
   (in any future version of this format, the 1 will be upgraded to 2,
   etc.).  Normally, such a "magic number" or flag goes at the start of the
   file, but trying to avoid clobbering widely-used ANALYZE 7.5 fields led to
   putting this marker last.  However, recall that "the last shall be first"
   (Matthew 20:16).
   If a NIFTI-aware program reads a header file that is NOT marked with a
   NIFTI magic string, then it should treat the header as an ANALYZE 7.5
   structure.
   NIFTI-1 FILE STORAGE:
   --------------------
   "ni1" means that the image data is stored in the ".img" file corresponding
   to the header file (starting at file offset 0).
   "n+1" means that the image data is stored in the same file as the header
   information.  We recommend that the combined header+data filename suffix
   be ".nii".  When the dataset is stored in one file, the first byte of image
   data is stored at byte location (int)vox_offset in this combined file.
   The minimum allowed value of vox_offset is 352; for compatibility with
   some software, vox_offset should be an integral multiple of 16.

/* DATA STORAGE:
   ------------
   If the magic field is "n+1", then the voxel data is stored in the
   same file as the header.  In this case, the voxel data starts at offset
   (int)vox_offset into the header file.  Thus, vox_offset=352.0 means that
   the data starts immediately after the NIFTI-1 header.  If vox_offset is
   greater than 352, the NIFTI-1 format does not say much about the
   contents of the dataset file between the end of the header and the
   start of the data.
   FILES:
   -----
   If the magic field is "ni1", then the voxel data is stored in the
   associated ".img" file, starting at offset 0 (i.e., vox_offset is not
   used in this case, and should be set to 0.0).
   When storing NIFTI-1 datasets in pairs of files, it is customary to name
   the files in the pattern "name.hdr" and "name.img", as in ANALYZE 7.5.
   When storing in a single file ("n+1"), the file name should be in
   the form "name.nii" (the ".nft" and ".nif" suffixes are already taken;
   cf. http://www.icdatamaster.com/n.html ).

MISCELLANEOUS C MACROS

/! Given a nifti_1_header struct, check if it has a good magic number.
    Returns NIFTI version number (1..9) if magic is good, 0 if it is not. /

#define NIFTI_VERSION(h)                               \
 ( ( (h).magic[0]==n && (h).magic[3]==\0    &&     \
     ( (h).magic[1]==i || (h).magic[1]==+ ) &&     \
     ( (h).magic[2]>=1 && (h).magic[2]<=9 )   )    \
 ? (h).magic[2]-0 : 0 )
/! Check if a nifti_1_header struct says if the data is stored in the
    same file or in a separate file.  Returns 1 if the data is in the same
    file as the header, 0 if it is not.                                   /
#define NIFTI_ONEFILE(h) ( (h).magic[1] == + )