The GEDCOM Standard Release 5.5

Appendix E

Encoding and Decoding Algorithms for Multimedia Objects
Introduction:

Embedded multimedia objects in GEDCOM requires special handling. These objects are normally represented by binary files containing characters which interfere with data transmission protocols. This document describes how the binary characters are encoded for transmission and then decoded to rebuild the multimedia file.

The algorithm for encoding binary images, compatible with GEDCOM transmission, is similar to an encoding scheme that would be used in creating a hexadecimal representation, but it uses a base-64 number representation rather than base-16 number representation.

Encoding:

This algorithm is for converting multimedia images represented in binary numbers into a collection that does not contain any of the ASCII control characters. This conversion eliminates the occurrence of special characters such as the "@" which has special meaning to GEDCOM.

The encoding routine converts a binary multimedia file segment of from 1 to 54 bytes in length into an encoded GEDCOM line value of 2 to 64 bytes in length. This encoded value becomes the <ENCODED_MULTIMEDIA_LINE> used in the MULTIMEDIA_RECORD.

The algorithm accomplishes its goal using the following steps:

1.Each 3 bytes (24 bits) of the binary 1 to 54 character segment is divided into four (6-bit) values. Each of these (6-bit) values are converted into an (8-bit) character making a character whose hexadecimal representation is between 0x00 and 0x3F (0 to 63 decimal.)

2.Each of the 4 new characters represents an Encoding key which is used to obtain the new replacement character from an Encoding Table included in this appendix.

3.Exception processing may be required in processing the last 3 byte chunk of the 1 to 54 character segment, which may consist of 0, 1, or 2 bytes:



Retrieved Action
a. 0 bytesPad the last 3 characters with 0xFF. The conversion is complete.
b. 1 byte:Pad last two bytes with 0xFF then complete steps 1 and 2 above.
c. 2 bytes:Pad last byte with 0xFF then complete steps 1 and 2 above.

5.Repeat until all characters in the received line value has been substituted. The return value of new encoded characters should contain from 4 to 72 characters. The length of the return value will always be a multiple of 4.


Decoding:

The Decoding routine converts the encoded line value back into the original binary character multimedia file segment.

The decoding algorithm can be accomplished in the following steps:

1.Each encoded multimedia line segment is divided into sets of 4 (8-bit) characters.

2.Each of these characters becomes a decoding key used to look up a corresponding character from the Decoding Table. A new (24-bit) group is formed by concatenating the low-order 6 bits from each of the 4 characters obtained from the decoding table.

3.Divide this new 24 bit group created by step 2 into three (8-bit) characters and concatenate them into the stream of characters being built as the decoded results.

4.Processing ends when the 0xFF padded bytes are encountered.

Encoding Table 

    Encoding  Replacement
    
              Key   Character
    0x00      0x2E  .
    0x01      0x2F  /
    0x02      0x30  0
    0x03      0x31  1
    0x04      0x32  2
    0x05      0x33  3
    0x06      0x34  4
    0x07      0x35  5
    0x08      0x36  6
    0x09      0x37  7
    0x0A      0x38  8
    0x0B      0x39  9
              ----  --
    0x0C      0x41  A
    0x0D      0x42  B
    0x0E      0x43  C
    0x0F      0x44  D
    0x10      0x45  E
    0x11      0x46  F
    0x12      0x47  G
    0x13      0x48  H
    0x14      0x49  I
    0x15      0x4A  J
    0x16      0x4B  K
    0x17      0x4C  L
    0x18      0x4D  M
    0x19      0x4E  N
    0x1A      0x4F  O
    0x1B      0x50  P
    0x1C      0x51  Q
    0x1D      0x52  R
    0x1E      0x53  S
    0x1F      0x54  T
    0x20      0x55  U
    0x21      0x56  V
    0x22      0x57  W
    0x23      0x58  X
    0x24      0x59  Y
    0x25      0x5A  Z
              ----  --
    0x26      0x61  a
    0x27      0x62  b
    0x28      0x63  c
    0x29      0x64  d
    0x2A      0x65  e
    0x2B      0x66  f
    0x2C      0x67  g
    0x2D      0x68  h
    0x2E      0x69  i
    0x2F      0x6A  j
    0x30      0x6B  k
    0x31      0x6C  l
    0x32      0x6D  m
    0x33      0x6E  n
    0x34      0x6F  o
    0x35      0x70  p
    0x36      0x71  q
    0x37      0x72  r
    0x38      0x73  s
    0x39      0x74  t
    0x3A      0x75  u
    0x3B      0x76  v
    0x3C      0x77  w
    0x3D      0x78  x
    0x3E      0x79  y
    0x3F      0x7A  z
Decoding Table 

Decoding  Replacement
    
Key       Character

    0x2E  .    0x00
    0x2F  /    0x01
    0x30  0    0x02
    0x31  1    0x03
    0x32  2    0x04
    0x33  3    0x05
    0x34  4    0x06
    0x35  5    0x07
    0x36  6    0x08
    0x37  7    0x09
    0x38  8    0x0A
    0x39  9    0x0B
    0x3A - 0x40     not valid
    0x41  A    0x0C
    0x42  B    0x0D
    0x43  C    0x0E
    0x44  D    0x0F
    0x45  E    0x10
    0x46  F    0x11
    0x47  G    0x12
    0x48  H    0x13
    0x49  I    0x14
    0x4A  J    0x15
    0x4B  K    0x16
    0x4C  L    0x17
    0x4D  M    0x18
    0x4E  N    0x19
    0x4F  O    0x1A
    0x50  P    0x1B
    0x51  Q    0x1C
    0x52  R    0x1D
    0x53  S    0x1E
    0x54  T    0x1F
    0x55  U    0x20
    0x56  V    0x21
    0x57  W    0x22
    0x58  X    0x23
    0x59  Y    0x24
    0x5A  Z    0x25
    0x5B - 0x60  not valid
    0x61  a    0x26
    0x62  b    0x27
    0x63  c    0x28
    0x64  d    0x29
    0x65  e    0x2A
    0x66  f    0x2B
    0x67  g    0x2C
    0x68  h    0x2D
    0x69  i    0x2E
    0x6A  j    0x2F
    0x6B  k    0x30
    0x6C  l    0x31
    0x6D  m    0x32
    0x6E  n    0x33
    0x6F  o    0x34
    0x70  p    0x35
    0x71  q    0x36
    0x72  r    0x37
    0x73  s    0x38
    0x74  t    0x39
    0x75  u    0x3A
    0x76  v    0x3B
    0x77  w    0x3C
    0x78  x    0x3D
    0x79  y    0x3E
    0x7A  z    0x3F