From VDR Wiki
Sudoku is a VDR plugin to generate and solve Number Place puzzles, so called Sudokus.
A Sudoku puzzle consists of 9 x 9 cells subdivided into 9 regions with 3 x 3 cells. The rules are simple. There have to be the numbers from 1 to 9 in every row, column and region. In the beginning some numbers are given. These cells are painted with cyan background color. The aim of the puzzle is to find the missing numbers. There is only one solution of a Sudoku puzzle.
The Sudoku puzzles are generated on-the-fly. The number of givens can be set in the plugin's setup page down to a minimum of 26 givens. The generation of puzzles with less than 26 givens takes too long. By default the cells with givens are symmetrically ordered. But this can be changed in the setup.
To solve difficult Sudoku puzzles some hints can be used. Incorrect cells are red and cells with ambiguous numbers are magenta. These hints can be disabled in the setup. With the green key a cell can be marked. A marked cell has green background color. With the yellow key the cursor is moved to the next free cell with minimal possible numbers. The red key set the next possible number for the current cell.
Each time the plugin is started from the main menu the same puzzle is shown. A new puzzle is only generated on VDR startup or if it has been requested by pressing the blue key. This key has two functions. If no numbers are set a new puzzle is generated. Otherwise all numbers are reset so that only the givens are shown.