Pictures drawn by logic
Nonograms turn a grid into a picture using only numbers along the edges. Each clue lists the lengths of the filled runs in that line, in order. It also goes by Picross, Griddlers and Hanjie. On this site the hidden image is a generated pattern rather than a drawing, so you cannot guess the shape from a theme. You have to read it from the clues, which makes every board a pure logic exercise.
The overlap trick
The most useful technique is the overlap. Imagine pushing a run as far left as it can go, then as far right. Any cells covered in both positions must be filled.
On the 10×10 Medium board, a lone clue of 7 can start no earlier than cell 1 and end no later than cell 10. Pushed left it covers cells 1 to 7; pushed right it covers 4 to 10. Cells 4 to 7 are in both, so fill them now. A quick way to work it out: subtract the clue from the line length, and that many cells at each end stay uncertain. Here 10 minus 7 is 3, so the middle four are certain.
The same idea works with several runs. For a clue of 3 4 on a 10-wide line, the packed-left layout uses 3, a gap, then 4, which is eight cells. That leaves two cells of slack. Any run longer than the slack gets an overlap: the 3 fills one middle cell and the 4 fills two.
Working the edges and the gaps
Once some cells are filled, the line's ends and gaps start to give more away.
- Edges anchor runs. If a filled cell sits in the first position of a row, the first run starts there. Fill the rest of that run and cross the cell right after it.
- Finished runs close off. When a run reaches its full length, the cells on either side must be empty. Cross them so you do not lose track.
- Small gaps die. A gap of two empty cells between crosses cannot hold a run of 3. If every remaining run is longer than a gap, cross the whole gap.
- Zero clues. A line whose clue is 0 is entirely empty. Cross it straight away; it then feeds every column it crosses.
Then switch direction. Every cell you settle in a row is new information for its column. Most boards solve by passing back and forth like this, and clues fade as each line is satisfied, so you can see which ones still need attention.
Mistakes to watch for
Filling a cell that should be empty adds to the mistake counter. The cell stays filled until you clear it, so a wrong fill can quietly break later deductions.
- Filling instead of crossing. Use cross mode, or right-click on a computer, for cells you know are empty. Crosses do not count toward the win, but they protect you from confusing "unknown" with "empty".
- Losing the order of runs. A clue of 2 5 means the 2 comes first. A run of 5 near the left edge cannot be the 5 if there is no room for the 2 before it.
- Overcounting in long lines. On 15×15 boards, count twice before you drag a long stripe. Drag-to-paint is fast, which also makes it easy to overshoot by one.
Practising across the three sizes
Easy is a 5×5 grid. It is small enough that most lines can be solved from overlaps alone, which makes it the right place to get the push-left, push-right habit into your hands.
Medium's 10×10 board is where gaps and edge anchors become necessary. Try solving each new board by first doing one full pass of rows, then one of columns, and noting how much each pass unlocks.
Hard is 15×15, with longer clues full of several runs. Here the slack method earns its keep: add up the runs plus one gap between each, compare with 15, and you will know at once which runs give an overlap.
The hint button fills one correct cell at random. Treat it as a last resort, and after using it, look at why that cell had to be filled. If you enjoy reading numbers along the edges, Tents and Trees uses row and column counts in a different way, and Sudoku shares the habit of checking every line twice.
