Video chat becomes more and more popular in our daily life. However, how to provide a high-quality video chat with the limited bandwidth is a key challenging task. In this paper, beyond the state-of-the-art video compression system, we propose an encoder-decoder joint enhancement algorithm for the video chat. In particular, the sparse map of the original frame is extracted at the encoder side and signaled to the decoder, which is utilized together with the sparse map of the decoded frame to obtain the boundary transformation map. In this manner, the boundary transformation map represents the key difference between the original frame and the decoded frame and hence can be used to enhance the decoded frame. Experimental results show that the proposed algorithm brings clear subjective and objective quality improvements. At the same quality, the proposed algorithm can achieve 35% bitrate savings compared to the VVC.