Paint a triangle (◢, ◣, ◤, ◥) in some white cells. Some white cells may remain empty. A number in a grey cell indicates how many sides of triangles in orthogonally adjacent cells touch this cell. The remaining white space must form rectangular areas, oriented either horizontally, vertically or diagonally. Two white rectangles must not be orthogonally adjacent.
(Rules and example from WPC 2018 IB)
History of the puzzle
First appeared on Nikoli volume 123 (2008). Invented by a reader under the alias of Guten. The name is an onomatopoeia in Japanese.