daytar's radonge (© 2006 T. Hoffmann and N. Kutz)
Fields are either black or not. Aim of the game is to mark the
correct fields black.
There are two shades of gray to denote non-black fields. This is so you can mark fields as "not black for sure". To mark a field black click on it. More clicks (or rith mouse click) will step through the three states. you can drag the red circles. This selects a "line" of fields on the board (in red). The number on the left labeled sum is the number of fields in the current line that should be black. The number labeled fill is the number of fields in the current line that you have marked black. The goal is to have these numbers coincide on all possible lines. The game tells you if you are done. Then (or in fact at any time) typing a number over the baord will restart the game with a board of the size of the i-th prime (0->2, 1->3, 2->5, 3->7 and so on).
If you found a systematic way to solve the puzzle, you understood the principle of the inverse Radon transform (at least in a finite field setting...)