A position library, as the name implies, is a collection of chess positions. At startup, Sigma Chess will load the standard "Sigma Library" which mainly contains opening lines/positions. However, a position library can also contain non-opening positions. In Sigma Chess 4 and earlier versions, the libraries were restricted to opening lines (hence they were called opening libraries), because they contained moves rather than positions.
When it's Sigma Chess's turn to move and it can play a move leading to a "good" position stored in the library, it will bypass its normal search algorithm and play the move right away. If more than one move leads to a "good" library position, Sigma Chess will pick one randomly. For instance, in the initial position Sigma Chess will choose randomly among the standard opening moves e2-e4, d2-d4, Ng1-f3 etc., because the resulting positions are all stored in the standard "Sigma Library".
So what constitutes a "good" position (as far as Sigma Chess is concerned)? A new feature in Sigma Chess 6 is that it supports position classification (+/=, +/-, ++/--, etc.). Additionally, the new Sigma Chess Access sub menu in the Library menu, defines the subset of positions/opening lines in currently loaded position library that Sigma Chess will play.
The status bar at the bottom of the move list shows the library classification, ECO code and variation name (if any).
Sigma Chess includes a powerful built-in Library Editor which lets you create your own opening libraries as well as edit existing ones.
The Library Editor is opened from the Library Menu, or by clicking the Edit Library tab in the right side of the game window. Like the Annotation Editor, the Library Editor is displayed beneath the move list as shown below:
The Position Library Editor
From the library toolbar you can classify (or unclassify) positions, and enter ECO codes and variation names:
ECO codes and Variation Names
The list below the toolbar shows the stored continuations, including classifications, ECO codes and opening names of the resulting positions. Clicking on a move will automatically play that move on the board.
You can create a new empty position library from scratch via the New Library... command. New moves can then be added as described above.
If you want to add thousands of positions, it will naturally be very time consuming to do it manually one move at a time as above. But fear not! Sigma Chess 6 now includes a powerful Library Import facility, which can import positions from a game collection to the currently loaded position library. Typically, you start out with a PGN file (for example, that you have downloaded from the Internet) containing games for a particular opening. You then open this PGN file into a collection (either by opening it directly or importing into an existing collection). Then you invoke the Import Collection command from the Library menu, which opens the following Library Import Dialog:
The Library Import Dialog
Once you have set the various options as required, you finally click the "OK" button. This will import the specified subset of positions into the currently loaded position library.
It's important to note that Sigma Chess imports the selected games and positions blindly, i.e. without analyzing or evaluating the positions. Therefore it is entirely up to you to make sure that the selected games are of a "good enough" quality (e.g. GM games only). A good approach is for instance to skip moves by the losing side. A more advanced approach is to first apply a filter to the collection on, for instance, player ELO (so only openings by GMs are included) and/or date (so "too old" games are ignored). Another reason for setting up a filter first is that all the new positions are classified the same way when you click the "OK" button in the Library Import Dialog. Therefore you might want to import the games in groups determined by ECO code and opening line, for instance.
If you want to build a new library from scratch it is recommended to start off from the "ECO Base Library" in the "Libraries" folder. If you start with a new, completely empty library, you will later have to manually add all the ECO codes and opening variation names. The "ECO Base Library" is locked so you don't accidentally modify it. Instead you should first copy it and rename it.
Sigma Chess 6.2.1 User's Manual - Copyright (C) 2011, Ole K. Christensen