This document describes the file format for Blokus games as used by the program Pentobi. The most recent version of this document can be found in the source distribution of Pentobi in the folder pentobi/doc/blksgf.
The file format is a derivative of the Smart Game Format (SGF). The current SGF version 4 does not define standard properties for Blokus. Therefore, a number of game-specific properties and value types had to be defined. The definitions follow the recommendations of SGF 4 and the proposals for multi-player games from the discussions about the future SGF version 5.
The file extension .blksgf and the MIME type application/x-blokus-sgf are used for Blokus SGF files.
Although not specific to Blokus, it is recommended to use UTF-8 as the character set. Pentobi always writes files in UTF-8 and indicates that with the CA property in the file. Pentobi can read SGF files encoded in UTF-8 or Latin1.
Since there is no number for Blokus defined in SGF 4, a string instead of a number is used as the value for the GM property. Currently, the following strings are used:
In game variants with two players and two colors, B denotes the first player or color, W the second player or color. In game variants with three or four players and one color per player, 1, 2, 3, 4 denote the first, second, third, and fourth player or color. In game variants with two players and four colors, B denotes the first player, W the second player, and 1, 2, 3, 4 denote the first, second, third, and fourth color. This applies to move properties and properties related to a player or a color.
Example 1: in the game variant Blokus Two-Player PW is the name of the first player, and 1 is a move of the first color.
Example 2: in the game variant Blokus Two-Player, one could either use the BL, WL properties to indicate the time left for a player, if the game is played with a time limit for each player, or one could use the 1L, 2L, 3L, 4L properties to indicate the time left for a color, if the game is played with a time limit for each color. (This is only an example how the properties should be interpreted. Pentobi currently has no support for game clocks).
Note
Old versions of Pentobi (before version 0.2) used the properties
BLUE, YELLOW, RED, GREEN
in the four-color game variants, which did not reflect the current state
of discussion for SGF 5.
Current versions of Pentobi can still read games containing the old
properties but they are deprecated and should no longer be used.
Fields on the board (called points in SGF) are identified by a case-insensitive string with a letter for the column followed by a number for the row. The letters start with 'a', the numbers start with '1'. The lower left corner of the board is 'a1'. The strings are not allowed to contain whitespaces. Note that, unlike the common convention in the game of Go, the letter 'j' is used.
If there are more than 26 columns, the columns continue with 'aa', 'ab', ..., 'ba', 'bb', ... More than 26 columns are presently required for the game variant Trigon and may be required for supporting future game variants on rectangular boards larger than 20×20.
For Trigon, hexagonal boards are mapped to rectangular coordinates as in the following example of a hexagon with edge size 3:
6 / \ / \ / \ / \
5 / \ / \ / \ / \ / \
4 / \ / \ / \ / \ / \ / \
3 \ / \ / \ / \ / \ / \ /
2 \ / \ / \ / \ / \ /
1 \ / \ / \ / \ /
a b c d e f g h i j k
The value of a move property is a string with the coordinates of the played piece on the board separated by commas. No whitespace characters are allowed before, after, or in-between the coordinates.
Example: B[f9,e10,f10,g10,f11]
Note
Old versions of Pentobi (before version 0.3) used to represent moves by a
list of points, which did not follow the convention used by other games
in SGF to use single-value properties for moves.
Current versions of Pentobi can still read games containing the old
move property values but they are deprecated and should no longer be used.
Setup properties are currently not used but they might be used in the future. Therefore, a program that reads Blokus SGF files should already check nodes for the setup properties AB, AW, A1, A2, A3, A4, AE and display an error message if they are found. Setup properties cannot be silently ignored because the program would misinterpret the board state if it reads a file written by a future version of Pentobi that uses setup properties.