Nonogramspicross, logic

Goal of the game

Fill in the right cells to reveal the picture, following the numbers given at the start of each row and each column.

A correct grid always has a single solution, reachable by deduction alone.

What the numbers say

• The numbers of a row give the lengths of the blocks of filled cells, in the order they appear.
• “4 2” means: a block of four filled cells, then at least one empty cell, then a block of two.
• The numbers do not say where the blocks start, only their size and their order.
• A “0”, or no number at all, means the row is completely empty.
• Rows and columns constrain each other: it is the crossing of the two that solves the grid.

The overlap technique

• It is the fundamental deduction, the one that opens almost every grid.
• Take a row of 10 cells with a block of 8. Push it all the way left, then all the way right: the cells filled in both positions are necessarily filled.
• Here, 8 + 8 − 10 = 6 certain cells in the middle. The general rule: block length × 2 − row length.
• It also works with several blocks: work out the leftmost and rightmost position of each block, and keep the intersection.
• A block longer than half the row always gives certain cells. Always start with the biggest numbers.

Crosses are worth as much as filled cells

• Mark with a cross every cell you know to be empty. It is not a comfort: it is stored deduction.
• As soon as a block is complete, frame it with crosses: the cells on either side are necessarily empty.
• When a row has reached its total of filled cells, cross out the rest of the row.
• A grid without crosses becomes unreadable after ten minutes, and that is when you start guessing — and so making mistakes.

The mistakes that cost the most

• Guessing. A correct grid never requires a bet. A cell placed at random silently invalidates everything else.
• Forgetting the mandatory gap between two blocks. “2 2” on a row of 5 has only one possible layout — provided you count the gap.
• Only working on rows. After each deduction, read the affected columns again: that is where the next step is.
• Neglecting nearly full rows. A row whose numbers plus gaps equal its length fills up completely, no thinking needed.

Controls

Keyboard and mouse

• Left click to fill, right click to mark a cross.
• Click and drag to handle several cells in one stroke.
• Numbers grey out automatically when their block is satisfied.

Touch screen

• A button switches between fill mode and cross mode.
• Slide your finger to fill a series of cells.
• Pinch to zoom on large grids.

Frequently asked questions

Nonogram, picross, griddler: are they the same thing?

Yes, they are names for the same game. “Picross” comes from the Nintendo versions; “griddlers” and “paint by numbers” are the names used in puzzle magazines.

What does “3 1” at the start of a row mean?

A block of three filled cells, then at least one empty cell, then a block of one cell, in that order. The exact positions remain to be deduced.

Do you have to guess at some point?

Never. A well-built grid can be solved entirely by deduction. If you are stuck, an overlap has slipped past you.

What are the crosses for?

To remember the cells you know are empty. They are as useful as filled cells and save you from redoing the same reasoning ten times.

Where should you start a grid?

With the rows and columns that have the biggest numbers: they give certain cells by overlap, without any other information.

More classics, free as well

• Game of the Goose — the 63-square spiral, its traps and its geese
• Othello — eight by eight, flip the discs, count at the end
• Pelotes — the flick, three colours, the mega bonus