The Witness Puzzle Trainer: Squares

An experimental game to test some new procedural generation algorithms.

Squares (sometimes called colored squares) are one of the simplest rule in The Witness (2016). The rules are simple:

1. Draw a line from the start to the end.
2. Different colors must be separated by the line.

With such a simple rule by itself, how hard could the puzzles be? (Spoiler: it can be pretty hard even for small boards)