Ordered propagation with the source being the root of the tree. The environment consists of two faces F ₁ and F ₂ as well as of two half-edges E _a and E _b . Each node contains one mesh item which can be either a face or a half-edge. Faces can additionally be linked to further image sources or image edges depending on the position of the node in the tree. The receiver is connected to colored items that represent a valid path, grey-scale items are not forming valid paths.