Pixel antenna is a promising technology to enhance the wireless communication data rate by adaptively reconfiguring each antenna's radiation pattern via a so-called antenna coding technique which controls the states of switches connected to multiple pixel ports. This paper studies a multiple-input multiple-output (MIMO) system where both the transmitter and the receiver are equipped with multiple pixel antennas. We aim to characterize the fundamental capacity limit of this MIMO system by jointly optimizing the transmit covariance matrix and the antenna coders at both the transmitter and the receiver. This problem is a mixed-integer non-linear program (MINLP) which is non-convex and particularly challenging to solve due to the binary-valued optimization variables corresponding to the antenna coders. We first propose an exhaustive search based method to obtain the optimal solution to this problem, which corresponds to the fundamental capacity limit. Then, we propose a branch-and-bound based iterative algorithm aiming to find a high-quality suboptimal solution with lower complexity than exhaustive search as the number of pixel ports becomes large. Finally, we devise an alternating optimization (AO) based algorithm with polynomial complexity. Numerical results show that our proposed algorithms achieve a flexible trade-off between performance and complexity. Moreover, equipping the transceivers with pixel antennas can enhance the achievable rate of MIMO communications.