rfc798.txt

RFC 的详细文档！

   fact be many scan lines in a block.

   The 585 bit data  block  is composed  of a 24 bit sync  code which is
   used to recognize the beginning of a block, a 37 bit header, 512 bits
   of actual data, and a 12 bit CRC checksum:

      ------------------------------------------------------------------
      |  24-bit  |    37-bit   |         512-bit         |    12-bit   |
      |sync code |    header   |           data          |   checksum  |
      ------------------------------------------------------------------

   The number  of useful  data bits is variable and can be between 0 and
   512 (although there are always 512 bits there, some of them are to be
   ignored).  The number of data bits to be used is given in the header.



[page 6]                                                    Alan R. Katz



RFC 798                                          DECODING FACSIMILE DATA
                                 IV. The Setup Block and the Data Header



   The 37 bits of header is composed of:

      ------------------------------------------------------------------
      | 2-bit |5-bit|  10-bit  |   12-bit  |  3-bit   |   3-bit  |2-bit|
      |seq num|flags|data count| x position|black size|white size|state|
      ------------------------------------------------------------------

   An explanation of these fields follows:

      IMPORTANT  NOTE:   Most (but not all)  of these fields are sent by
      the fax machine  in REVERSE  ORDER.  The order of each n-bit field
      must be inverted.

      Sync code

         This is used to synchronize  on each block.   The value of this
         24 bit field is 30474730 octal (not reversed).

      Sequence number

         This number  cycles through 0, 1, 2, 3 for the data blocks.  It
         is 0 for the Set-Up block (not reversed).

      Flags

         Each of these flags are 1 bit wide:

            Run

               Purpose unknown, it always seems to be 1.

            Cofb

               Purpose unknown, it always seems to be 0.

            Rpt

               1 for Set-Up  blocks (which are repeated when coming from
               the fax machine  though only one of them is transfered by
               FAXIE  to TOPS20  and stored  in the file) and 0 for data
               blocks.

            Spare

               Purpose unknown, doesn't matter.




Alan R. Katz                                                    [page 7]


                                                                        
DECODING FACSIMILE DATA                                          RFC 798
IV. The Setup Block and the Data Header                                 



            Sub

               1 if this is a Set-Up block.

      Data Count

         Number of useful bits to use out of the 512 data bits.  NOT ALL
         of the 512 data bits are used,  only this number of them.  This
         number can be 0 (usually in one of the first data blocks) which
         means to throw away this block. (This field is reversed!)

      X Position

         Current  position on the scan line, a value between 0 and 1725.
         If this number  is greater  than where the previous  block left
         off,  the intervening  space should be filled with white (0's).
         If this number  is less than where the previous block left off,
         set the X position  to this value  and replace  the  overlapped
         data with the new data from this  block.   If  this  number  is
         greater  than 1726,  ignore  this field and continue from where
         the previous block left off. (This field is reversed!)

      Black Size

         The size of the black  run length  field, must be between 2 and
         7.   This is the correct  value  for the black  size.   It  may
         differ  from what was found  at the end of the previous  block.
         (This field is reversed!)

      White Size

         The size of the white  run length  field, must be between 2 and
         7.   It may differ  from  what  was found  at the  end  of  the
         previous block. (This field is reversed!)

      State

         The current  state.   This is the correct state.  It may differ
         from the state at the end of the previous block. (This field is
         not reversed.)

      Data

         512 bits of the actual  encoding  of the document.   NOT ALL of
         this data is used in general,  only the amount specified by the




[page 8]                                                    Alan R. Katz



RFC 798                                          DECODING FACSIMILE DATA
                                 IV. The Setup Block and the Data Header



         data count.   If this is a set  up  block,  the  data  contains
         information about the type of document (see below).

      Checksum

         CRC  checksum   on   the   entire   block.    Uses   polynomial
         x**12+x**8+x**7+x**5+x**3+1.

   In a setup block, the data portion of the data block consists of:

      -----------------------------------------------------------
      |   6-bit |    5-bit  |   1-bit  |  20-bits  |  480-bits
      |   flags |    spare  |multi page|  of zeros |  1's and 0's
      -----------------------------------------------------------

   Specifically these are:

      6 flags (each are 1 bit):

         Start bit

            Always 0.

         Speed

            Is 1 if express mode.

         Detail

            Is 1 if detail  mode.  (NOTE:  If the Detail and Speed flags
            are both 0, then data is in Quality mode).

         14 inch paper

            is 1 if 14 inch paper length.

         5.5 inch paper

            is 1 if 5.5 inch  paper length.  (NOTE: If the 14 inch and 5
            inch flags are both 0, then paper length is 11 inch).

         paper present

            is 1 if paper is present at scanner (should be always 1).





Alan R. Katz                                                    [page 9]


                                                                        
DECODING FACSIMILE DATA                                          RFC 798
IV. The Setup Block and the Data Header                                 



      Spare:

         These 5 bits can be any value.

      Multi-page:

         1 if multi page mode

      Rest of data of set-up block:

         The above  fields are followed by twenty 0 bits and the rest of
         the 512 bits of the block are alternating 0's and 1's.

   There  are a number of important points to be remembered in regard to
   the header  of a data block.   First  of all,  the  data  count,  the
   x-position, and the black and white run sizes must be read IN REVERSE
   ORDER.   The reason for this is that the fax machine sends these bits
   in reverse  order.  However, the sequence number and the state fields
   ARE NOT REVERSED.  In addition to this, each run field in the data IS
   REVERSED.   This reversing  of  the  bits  in  each  n-bit  field  is
   completely  separate  from the reversing  and complementing  of  each
   octet mentioned earlier.

   Second, only the first n bits, where n is the value of the data count
   field  (remember its reversed!), of the data is valid, the rest is to
   be ignored.  If n is zero, the whole block is to be ignored.

   Third,  if the x position  is beyond where the last block ended, fill
   the space  between  where  the last block  ended  and the  current  x
   postion  with white (0's).  If the x postition is less then where the
   last block ended,  replace  the overlapped  data with the data in the
   new block.   If the x postition  is greater  than 1726, ignore it and
   continue from where the previous block left off.

   Fourth,  the black  size,  white  size  (reversed),  and  state  (not
   reversed!)  given in the header  are the correct  values even if they
   disagree with the end of the previous block.

   Finally,  the sequence  number  (not reversed)  should  count through
   0,1,2,3.  If it does not, a block is missing.









[page 10]                                                   Alan R. Katz



RFC 798                                          DECODING FACSIMILE DATA
                                               V. The Decoding Algorithm



V.   The Decoding Algorithm

   Upon first  glance  at the finite  state  diagram in Figure 1, it may
   seem that it would be difficult  to create a decoding procedure.  For
   example,  if you are in the WW state, and the next bit is a 1, how do
   you know  whether to do a transition to WB or BW?  The answer to this
   is to recognize  that every arc out of the BW state begins with 0 and
   every arc out of WB begins with 1.  Thus, if you are in the WW state,
   and the next  bit is 1,  followed  by a 0,  you know  to go to the BW
   state.   If the next bit is 1, followed by a 1, you know to go to the
   WB state.

   In writing  the decoding program it was necessary to have two ways of
   reading the next bit in the data stream.  The first way reads the bit
   and "consumes"  it,  i.e.  increments the bit pointer to point at the
   next bit.   The other  way does not "consume"  it.   Below  are  four
   statements  which show how  to  decode  fax  data.   The  numbers  in
   parentheses  are not to be consumed, that is to say they will be read
   again in making the next transition.

      If I am in state BW (2) and the next bits are:
         0 (0):             go to BW
         0111:              go to BB
         010 (1):           go to WB
         0100:              go to WW

      If I am in state WB (1) and the next bits are:
         1 (1):             go to WB
         1000:              go to WW
         101 (0):           go to BW
         1011:              go to BB

      If I am in state  WW (0),  then  first  go through  the run length
      algorithm, then if the next bits are:
         0:                 go to BB
         1 (0):             go to BW
         1 (1):             go to WB

      If I am in state  BB (3),  then  first  go through  the run length
      algorithm, then if the next bits are:
         0:                 go to WW
         1 (0):             go to BW
         1 (1):             go to  WB

      For the run length  algorithm,  remember, look at the next n bits,
      where  n is the length  of either  the black  or white  run length



Alan R. Katz                                                   [page 11]


                                                                        
DECODING FACSIMILE DATA                                          RFC 798
V. The Decoding Algorithm                                               



      word,  REVERSE  the bits,  and  output  that  many  BB's  or  WW's
      (depending  on whether black or white run).  If the field is full,
      increment  the size of the word, and get that many bits more, i.e.
      get the next n+1 bits,  etc.  Also, the run length word length can
      be decremented according to the rules given in section III.

      You always  go through the run length even if there is only one WW
      or BB, in this case, the run field will be 0.

      Let us look at the first example given in section III.  Suppose we
      want to decode the bits:  110111100010100100100101000...  (we have
      already reversed the run lengths to make things easier).