Nonogram

Nonogram Rules, Controls, and First Deductions

Goal and clues. Reconstruct the hidden picture by deciding which cells in every row and column are filled. The clues give the lengths of consecutive filled runs in their exact order. A clue of “3 2” means a run of three, at least one unfilled cell separating the runs, and then a run of two. A 0 means the entire line is unfilled. Column clues appear above the grid, and row clues appear to its left; both align toward the grid. A nonzero clue line dims when the lengths and order of its current filled runs match the complete clue. This visual cue does not check the crossing lines, and a 0 clue stays visible.

Desktop controls. Left-click or left-drag to fill cells. Right-click or right-drag to place X marks on cells established as unfilled. These mouse actions do not depend on the tool shown on screen. A drag decides whether to place or erase from its starting cell: starting on the same selected mark switches the drag to erase, clearing any filled or X-marked cell it crosses. With the keyboard, use the arrow keys to move, O or Y to fill, X or N to mark, and Delete or Backspace to clear; M switches the visible tool.

Touch and pen controls. Choose Fill or X with the floating tool, then tap or drag. The starting cell sets whether a drag places the selected mark or erases existing marks. Holding a cell for 420 ms applies the opposite tool to that cell once. On a pen, a barrel button or eraser tip also applies the opposite tool immediately.

Mistake-count and Free modes. In the default Mistake-count mode, filling a cell outside the picture changes it to X automatically and adds one mistake unless that cell is already marked X. Repeating the attempt while the X remains does not add another mistake; clearing the X and trying the same wrong fill again does. In Free mode, a wrong fill remains filled and there is no automatic correction or mistake count. Neither mode ends the puzzle because of one wrong fill. There is no separate red error-cell display, and an incorrect X is not marked as an error. Mistake-count mode finishes when every picture cell is filled; X marks on the background are not required. Free mode finishes when the set of filled cells exactly matches the answer. After either win, background X marks are cleared so the picture remains.

Overlap. For one line, consider every placement that obeys the clue order, the required gaps, and every fill or X already established by logic. Any cell covered in every legal placement is filled. For a single run, place it at its earliest and latest legal positions and fill the cells shared by both. With several clues, preserve their order and at least one separating cell while comparing the legal positions of each run. For an uninterrupted segment of length L assigned q clues, the arrangement is fixed when the clue lengths add up to L − (q − 1); the q − 1 intervening cells are the required gaps.

Anchoring at an edge. Anchor a run only when the first cell at a physical edge or within an X-bounded segment is established as filled and belongs to the first remaining clue. The same rule applies from the far edge to the last remaining clue. Once that anchored run reaches its clue length, mark the next cell inward X. A filled cell merely near an edge does not fix the run's starting point; use the overlap of its earliest and latest legal placements instead.

Closing completed runs with X. When a contiguous filled run has reached the length of its assigned clue and its place in the clue order is certain, the cells immediately before and after it, when present, are X. Those marks keep separate runs from joining and narrow the crossing clues. When every required run in a line has been placed, every other cell in that line can be marked X. Recheck rows after solving columns and columns after solving rows, because each established fill or X removes placements from the crossing line.

A video-game example. Nintendo's official page describes Mario's Picross as a 1995 Game Boy puzzle and explains how numbers at the edges guide which squares to chisel in each row and column until a picture appears.