The origin of this puzzle is not known.
Puzzle and Goal
An unsolved puzzle is a rectangular grid. Some cells are black with number clues.
The goal is to color in some cells with triangles touching two sides, in order to satisfy the clues.
The solved grid must satisfy the following conditions:
Numbers indicate how many sides neighbor black sides.
All white areas must be rectangular (although possibly rotated 45°).
Proof of Quilt