The fusion of electromagnetic (EM) waves and information theory in wireless and waveguide communication technology has enjoyed a remarkable revival during the last few years. In particular, unlike traditional transceiver systems, the recently proposed information metasurface system directly links the controllable binary 2-D array sources with reradiated waves generated through electromagnetic scattering mechanisms, making the combination of electromagnetic and information theories highly desirable and natural. In this paper, EM in-formation characteristics of a direct digital modulation (DDM) system enabled by programmable information metasurface are analyzed. The information metasurface is used as a modulator of the illuminating field, while the scattered far-field complex amplitudes are measured, effectively treated as the received quantities. The posterior probability for a specific source coding pattern, conditioned over a given measured scattering fields, is obtained through Bayesian analysis technique, from which the average mutual information (AMI) is obtained in order to estimate the metasurface observation capability along any particular direction. The averaged receiver mutual information (ARMI) is then introduced to characterize generated field correlation structures along different observation directions. Based on ARMI, the joint observation capability is also analyzed. Furthermore, the channel capacity of such a system is derived, and the influencing factors are analyzed from four different perspectives, including the observation direction, the size of the information metasurface, potential joint observations in multiple directions, and the noise level. The proposed method, together with the various related performance measure metrics introduced therein, are expected to provide the research community with easy-to- implement quick tools for analyzing and designing current and future information metasurface-based communication systems, which can also be extended to other aspects in the now growing field of the electromagnetic theory of information.