Comparison of the construction of propagation paths without and with image edges. (a) By use of image source S ₁, a first order reflection path can be found. One second order diffraction path is composed of two diffractions at edges E _a and E _b of the left building. (b) Neither source S nor receiver R are directly illuminating the right building. Irrespective of the order of reflection, neither image sources nor mirrored receivers directly lead to valid reflection paths. (c) By introducing image edges, a valid propagation paths consisisting of reflections and diffractions is found.