The ANHD chunk consists of the following data structure:

   UBYTE operation  The compression method:
                    =0 set directly (normal ILBM BODY),
                    =1 XOR ILBM mode,
                    =2 Long Delta mode,
                    =3 Short Delta mode,
                    =4 Generalized short/long Delta mode,
                    =5 Byte Vertical Delta mode
                    =6 Stereo op 5 (third party)
                    =74 (ascii 'J') reserved for Eric Graham's
                       compression technique (details to be
                       released later).

   UBYTE mask      (XOR mode only - plane mask where each
                    bit is set =1 if there is data and =0
                    if not.)
   UWORD w,h       (XOR mode only - width and height of the
                    area represented by the BODY to eliminate
                    unnecessary un-changed data)
   WORD  x,y       (XOR mode only - position of rectangular
                    area representd by the BODY)
   ULONG abstime   (currently unused - timing for a frame
                    relative to the time the first frame
                    was displayed - in jiffies (1/60 sec))
   ULONG reltime   (timing for frame relative to time
                    previous frame was displayed - in
                    jiffies (1/60 sec))
   UBYTE interleave (unused so far - indicates how may frames
                     back this data is to modify.  =0 defaults
                     to indicate two frames back (for double
                     buffering). =n indicates n frames back.
                     The main intent here is to allow values
                     of =1 for special applications where
                     frame data would modify the immediately
                     previous frame)
   UBYTE pad0        Pad byte, not used at present.
   ULONG bits        32 option bits used by options=4 and 5.
                     At present only 6 are identified, but the
                     rest are set =0 so they can be used to
                     implement future ideas.  These are defined
                     for option 4 only at this point.  It is
                     recommended that all bits be set =0 for
                     option 5 and that any bit settings used in
                     the future (such as for XOR mode) be compatible
                     with the option 4 bit settings.   Player code
                     should check undefined bits in options 4 and 5
                     to assure they are zero.

                     The six bits for current use are:

                     bit #              set =0               set =1
                     ===============================================
                     0              short data           long data
                     1                 set                  XOR
                     2             separate info        one info list
                                   for each plane       for all planes
                     3               not RLC        RLC (run length coded)
                     4              horizontal           vertical
                     5           short info offsets   long info offsets

   UBYTE pad[16]     This is a pad for future use for future
                     compression modes.