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.
Since this is a non-standard MIME type, links to Blokus SGF files on web servers will not automatically open the file with Pentobi even if Pentobi is installed locally and registered as a handler for Blokus SGF files. To make this work, you can put a file named .htaccess on the web server in the same directory that contains the .blksgf files or in one of its parent directories. This file needs to contain the line:
AddType application/x-blokus-sgf blksgf
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. Pentobi can read SGF files encoded in UTF-8 or ISO-8859-1 (Latin1). Other character sets are currently not supported. As specified by the SGF standard, ISO-8859-1 is assumed for files without CA property.
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).
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 'i' 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.
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.
The setup properties AB, AW, A1, A2, A3, A4 can be used to place pieces in one step on the board. The setup property AE can be used to remove a piece from the board. All these properties can have multiple values, each value represents a piece by its coordinates as in the move properties. The PL can be used to set the color to play in a setup position.
Older versions of Pentobi (before version 2.0) did not support setup properties, you need a newer version of Pentobi to read such files.