Data masking

To minimize DotCode problematic symbols, the data codewords are masked to create others visual sequences. The mask pattern is applied only to data sequence and does not affect error correction codewords. DotCode standard has 4 mask pattern which are codded into 2 bits and placed as the first 2 bits of symbol bits array.^[2]: 5.2.4

Error correction

DotCode uses Reed–Solomon error correction^[2]: 5.3 with prime power of 3 and finite field ${\displaystyle \mathbb {F} _{113}}$ or GF(113). The data codewords is represented with values from 0 to 112 and mask value is counted as leading data codeword from 0 to 3. In this way the data protected array length is (1 + ND). But amount of error correction codewords is calculated only from ND:
${\displaystyle NC=3+(ND/2)}$,
where ND is data codewords and NC - error correction codewords.

The resulting codewords NW with error correction codewords is:
${\displaystyle NW=(1+ND)+NC}$,
where NW is all encoding codewords: 1 mask codeword + data codewords(ND) + error correction codewords(NC).

Because Reed–Solomon error correction cannot correct amount of codewords which are more than polynomial, if NW happens to exceed 112, the data is split into error correction blocks:
${\displaystyle B=(NW+111)/112}$,
where B is block counts.

The data can be split into block in the following way, for each block ‘’’n’’’, for n equals 1 to B:

1. ${\displaystyle ND(block)=((1+ND)-(n-1)+(B-1))/B}$
2. ${\displaystyle NW(block)=(NW-(n-1)+(B-1))/B}$
3. ${\displaystyle NC(block)=NW(block)-ND(block)}$

The error correction data ${\displaystyle NC}$ is written after single data block ${\displaystyle ND}$ in scrambled mode:^[17]
(ND)(NC1_1)(NC2_1)(NC3_1)...(NC1_n)(NC2_m)(NC3_k)

Binary byte encoding

DotCode can encode full 8-bit charset in two ways:^{[2]: 5.2.1.1}

• With Upper Shift, which requires 2 codewords on one (128 to 255) symbol;
• With Binary Latch, which requires 1 Binary Latch symbol and 6 codewords on every 5 bytes.

Upper Shift modes can encode (128 to 255) extended ASCII characters in two codewords with returning to previous mode:

Binary Latch mode can encode 8-bit charset and ECI sequences from 1 to 5 symbols. It uses the following rules:

• The data is split into block of 5 symbols (byte or ECI mode identifier) or 6 codewords;
• 0 – 258 values are radix converted from five base 259 into six base 103 values;
• 0 – 255 values are byte values;
• 256, 257 or 258 values marks to encode ECI sequence in next 1, 2 or 3 bytes respectively;
• Any DotCode codewords in the encoded sequence above 102 (from 103 to 112) interrupt or change mode.

As we see in the following table, Binary Latch encodes data more effectively, starting from 3 bytes.

ECI encoding

DotCode can encode ECI indicator int two ways:^{[2]: 5.2.1.2}

• In Binary Latch mode (reviewed upper);
• With FNC2 character.

FNC2 in any position except at the end of data signals the insertion of an ECI sequence – "\nnnnnn", which represents values between 000000 and 811799. The values can be encoded in 1 or 3 codewords:

• In case of next codeword < 40 directly encodes ECI value 000000 to 000039;
• Otherwise, the next three codewords valued A, B, & C encode an ECI value of ${\displaystyle (A-40)*12769+B*113+C+40}$.

GS1 encoding

Any two digits in the position of the first codeword identify a symbol as GS1 encoded (opposite to Code 128). In case of symbol with two digits in the position of the first codeword must be decoded as ordinary data, the FNC1 (omitted in decoded message) must be inserted at the place of the first codeword.^{[2]: 5.2.1.2} FNC1 in the other than the first position works as GS1 Application Identifier splitter and decoded as GS (ASCII value 29) character.

Codeword 100 in Code Set C encodes application GS1 AI (17)^[5] the next 3 codewords is an expiration date and inserts GS1 AI (10) before decoding other codewords:
(100)(24)(12)(30)(56)(64) -> 17241230105664

Macros mode

Some data codewords 97 – 100 in the lead data position in Code Set B can encode “Macros”.^{[18][2]: 5.2.1.1} In any other position it encodes ASCII symbol:
(Latch B)(HT) -> [)>RS05GS … RSEoT
(Shift B)(HT) -> [)>RS05GS … RSEoT

Structured append

DotCode can create composite symbol, where data from multiple DotCode symbols can be logically united. This can be made with FNC2 symbol in last data position. When FNC2 is in the final data position,^{[2]: 5.2.1.2} then the preceding two message characters, digits and uppercase letters in order 1 to 9 then A to Z (for values 10 to 35) shall as "m" and "n" designate where this message belongs in a "m out of n" sequence. As an example, a symbol whose message ends "4 B FNC2" shall be the 4th symbol out of 11 that comprise the entire message.

Special modes encoding

FNC3 in the first codeword position indicates that the message^{[2]: 5.2.1.2} is the instructions for initialization or reprogramming of the bar code reader.

FNC3 in any other position than first indicates that encoded message must be logically separated into two distinct messages (before and after it);

Data padding

DotCode symbol codewords capacity is:
${\displaystyle NW=((H*W)/2-2){\pmod {9}})}$

DotCode symbol data codewords capacity is:
${\displaystyle ND=(NW-3)-(NW-3)/3}$

In this way we need to pad data codewords in case with have free space. There are two rules:^[2]: 5.2.3

• Binary mode must be terminated with Latch to Code Set A (codeword 109);
• In other modes codeword 106 (Latch to Code Set C / Latch to Code Set B) must be used.