Editing Aztec Code (section)

=== Laying out the message ===
[[File:Aztec-Code-With-Reference-Grid.png|alt=|thumb|9-layer (53×53) Aztec code with reference grid highlighted in red.]]
A full Aztec code symbol has, in addition to the core, a "reference grid" of alternating black and white pixels occupying every 16th row and column. A compact Aztec code does not contain this grid. These known pixels allow a reader to maintain alignment with the pixel grid over large symbols. For up to 4 layers (31×31 pixels), this consists only of single lines extending outward from the core, continuing the alternating pattern.  Inside the 5th layer, however, additional rows and columns of alternating pixels are inserted ±16 pixels from the center, so the 5th layer is located ±17 and ±18 pixels from the center, and a 5-layer symbol is 37×37 pixels.

Likewise, additional reference grid rows and columns are inserted ±32 pixels from the center, making a 12-layer symbol 67×67 pixels.  In this case, the 12th layer occupies rings ±31 and ±33 pixels from the center.  The pattern continues indefinitely outward, with 15-pixel blocks of data separated by rows and columns of the reference grid.

One way to construct the symbol is to delete the reference grid entirely and begin with a 14×14-pixel core centered on a 2×2 pixel-white square.  Then break it into 15×15 pixel blocks and insert the reference grid between them.

The mode message begins at the top-left corner of the core and wraps around it clockwise in a 1-bit thick layer.  It begins with the most significant bit of the number of layers and ends with the check words.  For a compact Aztec code, it is broken into four 7-bit pieces to leave room for the orientation marks.  For a full Aztec code, it is broken into four 10-bit pieces, and those pieces are each divided in half by the reference grid.

In some cases, the total capacity of the matrix does not divide evenly by full code words. In such cases, the main message is padded with 0 bits in the beginning. These bits are not included in the check word calculation and should be skipped during decoding. The total matrix capacity for a full symbol can be calculated as (112+16*L)*L for a full Aztec code and (88+16*L)*L for a compact Aztec code, where L is the symbol size in layers.<ref>{{cite web |url=http://recog.ru/blog/standarts/6.html |title=Спецификация Aztec Code (без Small Aztec) |language=ru |trans-title=Aztec Code Specification (without Small Aztec) |url-status=dead |archive-url=https://web.archive.org/web/20200225193653/http://recog.ru/blog/standarts/6.html |archive-date=2020-02-25}}</ref>  As an example, the total matrix capacity of a compact Aztec code with 1 layer is 104 bits.  Since code words are six bits, this gives 17 code words and two extra bits.  Two zero bits are prepended to the message as padding and must be skipped during decoding.

The padded main message begins at the outer top-left of the entire symbol and spirals around it ''counterclockwise'' in a 2-bit thick layer, ending directly above the top-left corner of the core.  This places the bit-stuffed data words, for which erasures can be detected, in the outermost layers of the symbol, which are most prone to erasures.  The check words are stored closer to the core. The last check word ends just above the top left corner of the bull's eye.

With the core in its standard orientation, the first bit of the first data word is placed in the upper-left corner, with additional bits placed in a 2-bit-wide column left-to-right and top-to-bottom.  This continues until 2 rows from the bottom of the symbol when the pattern rotates 90 degrees counterclockwise and continues in a 2-bit high row, bottom-to-top and left-to-right.  After 4 equal-sized quarter layers, the spiral continues with the top-left corner of the next-inner layer, finally ending one pixel above the top-left corner of the core.

Finally, 1 bit are printed as black squares, and 0 bits are printed as white squares.