Kuromasu is a free online line-of-sight shading puzzle, also known as Kurodoko. Some squares of the grid hold a number, and you decide which of the other squares to shade. Each number counts the open squares it can see, itself included, looking straight up, down, left and right until the view is stopped by a shaded square or the edge of the grid. Three more rules hold at once: numbered squares are never shaded, shaded squares never share an edge (they may touch at a corner), and all the open squares stay connected in one piece. Tap a square to shade it, tap again to mark it with a dot as definitely open, and drag across squares to paint several at once. Select a number and every square it can currently see lights up, so you can read its count straight off the board; its ring turns green once its view is settled and right. Shaded squares that touch, a number that already sees too much or can no longer see enough, and open squares that have been walled off all turn red straight away. Every board is built by the game itself, and before you see it, it is proved to have exactly one answer and to be solvable without guessing. Eight levels grow from a 6×6 to a 10×10: the first two need only the numbers and the spacing rule, levels 3 and 4 also lean on keeping the open squares in one piece, and from level 5 every board needs at least one "what if" followed through to a contradiction. No download, no account.
How to play Kuromasu
- Some squares hold a number. Each number counts the open squares it can see — itself included — looking straight up, down, left and right. The view in each direction runs until it reaches a shaded square or the edge of the grid.
- Numbered squares are always open. You can never shade one.
- Two shaded squares may never share an edge. Touching at a corner is fine.
- All the squares you leave open must form one connected area, joined edge to edge. A shaded square can never be part of a ring that walls some open squares off.
- Tap a square to shade it. Tap again to put a dot on it, a note that it stays open, and a third time to clear it. Right-click puts a dot on straight away.
- Drag across several squares to paint them all the same way. A drag only changes squares that looked like the one it started on, so it never overwrites something you decided.
- Tap or select a number to light up every square it can see right now. When everything it sees is dotted and each direction ends at a shaded square or the edge, adding up to its number, its ring turns green.
- On a keyboard, arrow keys move between squares, S shades, D or a full stop dots, Backspace clears, and Space or Enter cycles a square.
- Shaded squares that touch, a number that already sees too many squares or can no longer see enough, and dotted squares walled off from the rest all turn red.
- The board is solved the moment every rule holds. You do not need to dot the open squares — undecided squares count as open.
- Undo steps back one change, Clear wipes the board, and New Puzzle deals a different board on the same level. The ? button settles one square, fixing a mistake first. Three per board.
Tips
- Start with the numbers that leave no room. A 2 in a corner sees itself and exactly one neighbour, so one of its two neighbours is shaded. A big number near the middle can often only be reached by seeing a long way in every direction, which makes those squares open.
- Count the most a number could still see: every square up to the first shaded one in each direction. If that is exactly its number, all of those squares are open and each direction ends in a shade.
- Every square next to a shaded square is open. Dot them as you go: dots stretch the sight lines they sit on, and that is what settles the numbers around them.
- Another number is always open, so it never stops a view. Two numbers in the same row or column with nothing shaded between them are each part of the other's count.
- A square that holds the open area together — the only link between two groups of open squares — can never be shaded. Watch the corners and edges, where two shaded squares on a diagonal can box a square in.
- On the later levels, try a square in your head: shade it, follow what it forces, and if a number breaks, it was open.
FAQ
Is Kuromasu free to play?
Yes. It runs in the browser with no download, no sign-up and nothing to buy. All eight levels are included and unlock as you solve them.
What are the rules of Kuromasu?
Shade some squares so that every number sees exactly that many open squares, counting itself, along its row and column until a shaded square or the edge stops the view. Numbered squares are never shaded, shaded squares never share an edge, and all the open squares stay connected.
Is Kuromasu the same as Kurodoko?
Yes. Kuromasu was the puzzle's first name, and Kurodoko is the name it is published under today. The rules are the same: numbers count the squares they can see, shaded squares never touch, and the open squares stay in one piece.
Does every puzzle have exactly one answer?
Yes, and it is checked rather than assumed. Each board is built from a real answer, and a program that only ever makes forced moves has to be able to finish it before it is shown; a board it can finish that way has only one answer. The tests also confirm every level with an exhaustive search that works a different way.
Will I ever have to guess?
No. On levels 1 and 2 every move follows from the numbers and the spacing rule. Levels 3 and 4 also need the rule that the open squares stay in one piece. From level 5 on, each board needs at least one "what if": shade a square in your head, follow it, and watch a number break.
How many levels are there?
Eight, from a 6×6 grid to a 10×10. The reasoning gets harder as the grids grow: level 5 needs one to three "what if" steps, level 6 two to five, level 7 six to eleven, and level 8 twelve or more. New Puzzle deals a fresh board on any level you have reached.
Why does a number count itself?
That is the standard rule: a number reports how many open squares are visible from its own square, and it is one of them. So a 1 would mean it sees nothing else at all, which can never happen on a board where the open squares are joined, and the smallest number you will meet is a 2.
What do the dots mean?
They are notes for you: a dot marks a square you know stays open. The game never requires them — undecided squares count as open when it checks your answer — but dotted squares are what it uses to spot a number that already sees too much, or open squares that have been cut off.
Does it work on a phone?
Yes. Tap to shade and slide a finger across squares to paint several at once. On a phone held upright the game uses a taller frame, so the board spans the full width of the screen.