A simplified simulation method to calculate the transmission and diffraction of oblique incident light through a thin two-dimensional layer is presented. The algorithm, based on a ray-tracing technique, uses the Extended Jones-calculus and neglects diffraction within the layer. The algorithm is applied to an in-plane switching liquid crystal display. The transmission at the display surface and the diffracted field at a large distance from the display are compared with the more accurate reduced grating method. When the thickness of the layer is sufficiently small and the variation in liquid crystal orientation is slow, our method gives accurate results.