Originating in Japan circa 1987, nonograms are one of the most popular and most published puzzle types in recent years, and are known by many names such as Hanji, Paint Logic, and Griddlers. However, they are most well known among the gaming community as Picross (a contraction of Picture Crossword), the name of a Nintendo series based on nonograms.
A nonogram involves a grid, with number clues along the side. The aim being to fill in certain squares while avoiding others, typically forming a small picture. Numbers indicate the number of filled squares in a line or column, and how they are grouped.
There are numerous strategies for solving a nonogram, although a common opening strategy is to look for any lines that are completely filled in or left completely empty. Beyond this, most lines typically lack sufficient clues to directly identify blocks, although blocks can often be identified due to them being filled in all possible solutions (overlap). Once the player has revealed enough of the picture to identify it, some assumptions such as whether the picture is symmetrical, can help the player progress.
Harder puzzles often require 'proof by contradiction' where one of a number of viable solutions is tried, until is is found to contradict the clues in another row or column. This trial and error aspect makes the game type particularly suitable for electronic media over pen and paper.
Over time, variations on the original nonogram formula have gained some popularity, such as the three-dimensional grid system first popularized by Picross 3D, or the multi-colored grid system seen in games like Color Cross, which adds multiple colors to account for when filling in the grid.