1. In a wearable head-mounted display (HMD) having a processor and a magnetometer device with three orthogonal measurement axes, a computer-implemented method comprising: operating in a background detection state; while operating in the background detection state, carrying out functions of the background state including, measuring three orthogonal components of a background magnetic field with the magnetometer device, and determining a field magnitude of the background magnetic field from the three measured orthogonal components, determining an occurrence of a trigger from a hand-wearable magnet (HWM) at a time T start upon detecting a perturbation by the HWM of the determined field magnitude at least as large as a perturbation threshold, and upon determining the occurrence of the trigger, transitioning to operating in a gesture detection state; and while operating in the gesture detection state, carrying out functions of the gesture detection state including, for the duration of the gesture detection state, tracking motion of the HWM by determining time derivatives of magnetic field strength measured by the magnetometer device along each of the three orthogonal measurement axes, making a comparison of the determined time derivatives with one or more sets of pre-determined time derivatives of magnetic field strength, each of the one or more sets being stored at the wearable HMD and each being associated with a respective known gesture, upon matching the determined time derivatives with a particular set of the one or more sets based on the comparison, identifying the respective known gesture associated with the particular set, and transitioning to operating in the background detection state upon both of, measuring the magnitude perturbation by the HWM of the determined field magnitude to be less than the perturbation threshold, and determining an expiration of a time interval W that begins at T start .
1. In a wearable head-mounted display (HMD) having a processor and a magnetometer device with three orthogonal measurement axes, a computer-implemented method comprising: operating in a background detection state; while operating in the background detection state, carrying out functions of the background state including, measuring three orthogonal components of a background magnetic field with the magnetometer device, and determining a field magnitude of the background magnetic field from the three measured orthogonal components, determining an occurrence of a trigger from a hand-wearable magnet (HWM) at a time T start upon detecting a perturbation by the HWM of the determined field magnitude at least as large as a perturbation threshold, and upon determining the occurrence of the trigger, transitioning to operating in a gesture detection state; and while operating in the gesture detection state, carrying out functions of the gesture detection state including, for the duration of the gesture detection state, tracking motion of the HWM by determining time derivatives of magnetic field strength measured by the magnetometer device along each of the three orthogonal measurement axes, making a comparison of the determined time derivatives with one or more sets of pre-determined time derivatives of magnetic field strength, each of the one or more sets being stored at the wearable HMD and each being associated with a respective known gesture, upon matching the determined time derivatives with a particular set of the one or more sets based on the comparison, identifying the respective known gesture associated with the particular set, and transitioning to operating in the background detection state upon both of, measuring the magnitude perturbation by the HWM of the determined field magnitude to be less than the perturbation threshold, and determining an expiration of a time interval W that begins at T start . 12. The method of claim 1 , wherein determining the time derivatives of magnetic field strength measured by the magnetometer device along each of the three orthogonal measurement axes comprises: during at least a portion of the time interval W, measuring orthogonal magnetic field components H x , H y , and H z of a magnetic field vector {right arrow over (H)}=[H x , H y , H z ] at consecutive discrete times t i , i=1, . . . , N, wherein N≧2, to obtain samples of the of H x , H y , and H z at the consecutive discrete times t i , i=1, . . . , N; and computing discrete time derivatives of the measured orthogonal magnetic field components H x , H y , and H z from discrete differences between successive samples.