Mosaic (Fill-a-Pix) Puzzle

Mosaic is a shading puzzle, also called Fill-a-Pix. Some cells show a number. You shade cells so each number equals the shaded cells in its neighbourhood, including the numbered cell itself. You can play grids from 5×5 up to 10×10, each in easy, medium, and hard. Every puzzle here has exactly one shaded set.

Rules

The board is an n×n grid. Thin lines divide the cells. A thick frame sits around the outside.

A number counts every shaded cell in the block of cells around it, including its own cell. That block is the neighbourhood. A corner number looks at four cells. An edge number, away from a corner, looks at six. A number in the middle looks at nine. The count includes the numbered cell when that cell is shaded.

A numbered cell may be shaded. The digit stays visible on the shade. A blank cell has no number and can still be shaded or dotted.

A dot is a mark for a cell you already know is empty. Dots are optional. The puzzle does not ask you to dot every empty cell. A numbered cell can take a dot, or a shade.

The puzzle is solved when the shaded cells are that unique set and nothing on the board breaks a rule. Empty cells may still be blank. You do not have to place a dot on every empty cell.

A number that already has too many shaded neighbours is a mistake. A number that cannot reach its count, because the cells still open are too few, is a mistake as well. A board that is filled in and still does not match the solution is a mistake. An unfinished board that still has room for every number is not a mistake yet.

Controls

Choose Shade or Dot under the board. Tap a cell, or drag across cells, to place that mark. Tap a cell that already has the mark to clear it. A cell that has the other mark takes the new one when you tap it. A drag follows the first cell: it places the mark on empty cells along the path, or it clears when that cell already has the mark. With a mouse, right-click places a dot, or clears a dot that is already there.

On a keyboard, the arrow keys move around the grid. Space or Enter places the selected mark, or clears it when the cell already has that mark. S or 1 shades the cell. X, D, E, or 2 places a dot. M switches Shade and Dot. Backspace, Delete, or 0 clears the cell.

A cell turns coral when a number already has too many shaded neighbours, or when a number can no longer reach its count. Press Hint for one step the rules already force, with a sentence saying why. Press Check when you want the board checked against the rules. Undo and Redo step through your moves, and Reset clears the board. When the shaded cells are the unique set, the board turns teal.

Techniques

Easy grids follow from one number at a time:

• Zero: a 0 clears its whole neighbourhood, so every cell around it stays empty.
• Full: a number that matches the size of its neighbourhood means every cell in it is shaded, including the numbered cell.
• Already enough: the shaded cells already meet this number, so the rest of the neighbourhood is empty.
• Only one way: the open cells left are exactly what the number still needs, so those cells are shaded.

Medium grids compare two overlapping neighbourhoods and keep the cells that one count forces and the other does not. Hard grids try shading one cell, or leaving it empty, and keep the choice that does not break a number.

Beginner tips

Start with the 0s and with numbers that match the whole neighbourhood. A 0 clears every cell around it. A 4 in a corner, a 6 on an edge, or a 9 in the middle shades the whole neighbourhood, including the number.

When a number already has its count, dot the rest of that neighbourhood before you look at the next number. A numbered cell can be part of the shade. Leave the digit where you can still read it.

If you are new to Mosaic, start with a 5×5 easy grid, or the daily 8×8 medium grid, and move up once the neighbourhoods feel natural. Play runs from Easy → Hard on every size from 5×5 to 10×10. 12×12 and 15×15 are print only, also Easy → Hard.