patch of bright snow might contribute to the impression of a forehead or cheek, while a dark gap between branches could resemble an eye. A tree trunk might define the edge of a face without representing the face itself. Once those separate areas are mentally grouped together, an image that initially seemed absent can suddenly become recognizable.
A hidden face does not need to be outlined continuously. In fact, giving it a complete outline would usually make the puzzle much easier. Instead, the illusion can rely on scattered visual clues
