Extracting temporally-coherent alpha mattes in video is an important but challenging problem in post-production. Previous video matting systems are highly sensitive to initial conditions and image noise, thus cannot reliably produce stable alpha mattes without temporal jitter. In this paper we propose an improved video matting system which contains two new components: (1) an accurate trimap propagation mechanism for setting up the initial matting conditions in a temporallycoherent way; and (2) a temporal matte filter which can improve the temporal coherence of the mattes while maintaining the matte structures on individual frames. Experimental results show that compared with previous methods, the two new components lead to high quality alpha mattes with better temporal coherence.