A see-through, near-eye mixed reality head mounted display, HMD, device, comprising: left and right see-through display regions (112L, 112R) each having a transmittance that is less than one hundred percent, the transmittance defining an amount of incident ambient visible light that passes through, one or more virtual images being displayable within the left and right see-through display regions (112L, 112R), the left and right see-through display regions (112L, 112R) being surrounded by portions of a see-through transmittance compensation mask and not being coplanar with the see-through transmittance compensation mask, and the left and right see-through display regions (112L, 112R) and the portions of the see-through transmittance compensation mask being within a field-of-view of a user wearing the HMD device; the see-through transmittance compensation mask having a left window through which the left see-through display region is visible and a right window through which the right see-through display region is visible, having a transmittance that is substantially the same as the transmittance of the left and right see-through display regions (112L, 112R), and having a plurality of features (544, 644) that are individually selectively activated to adjust boundaries of the left and right windows (118L, 118R) and thereby adjust positions of the left and right windows (118L, 118R); one or more eye tracking cameras used to detect locations of left and right eyes of a user wearing the HMD device; and a controller that selectively activates individual ones of the features, in dependence on the detected locations of the left and right eyes of the user wearing the HMD device, to thereby position the left and right windows (118L, 118R) such that the user's left eye is centered relative to the left window and the user's right eye is centered relative to the right window. left and right see-through display regions (112L, 112R) each having a transmittance that is less than one hundred percent, the transmittance defining an amount of incident ambient visible light that passes through, one or more virtual images being displayable within the left and right see-through display regions (112L, 112R), the left and right see-through display regions (112L, 112R) being surrounded by portions of a see-through transmittance compensation mask and not being coplanar with the see-through transmittance compensation mask, and the left and right see-through display regions (112L, 112R) and the portions of the see-through transmittance compensation mask being within a field-of-view of a user wearing the HMD device; the see-through transmittance compensation mask having a left window through which the left see-through display region is visible and a right window through which the right see-through display region is visible, having a transmittance that is substantially the same as the transmittance of the left and right see-through display regions (112L, 112R), and having a plurality of features (544, 644) that are individually selectively activated to adjust boundaries of the left and right windows (118L, 118R) and thereby adjust positions of the left and right windows (118L, 118R); one or more eye tracking cameras used to detect locations of left and right eyes of a user wearing the HMD device; and a controller that selectively activates individual ones of the features, in dependence on the detected locations of the left and right eyes of the user wearing the HMD device, to thereby position the left and right windows (118L, 118R) such that the user's left eye is centered relative to the left window and the user's right eye is centered relative to the right window.