Magnetometer-based gesture sensing with a wearable device

• US 9,141,194 B1
• Filed: 01/04/2012
• Issued: 09/22/2015
• Est. Priority Date: 01/04/2012
• Status: Active Grant

• Current Assignee: Google LLC (Alphabet Inc.)
• Original Assignee: Google Inc. (Alphabet Inc.)
• Inventors: Keyes, Edward, Johnson, Michael Patrick, Starner, Thad
• Primary Examiner(s): Feliciano, Eliseo Ramos
• Assistant Examiner(s): Liu, I-Hui E
• Application Number:
  US13/343,652
• Time in Patent Office: 1,357 Days
• Field of Search: 702/150, 702/152, 702/153
• US Class Current: 1/1
• CPC Class Codes: G01R 33/0029 Treating the measured signals, e.g. removing offset or noise
• CPC Class Codes: G01R 33/0206 Three-component magnetometers
• CPC Class Codes: G02B 2027/0138 comprising image capture systems, e.g. camera
• CPC Class Codes: G02B 2027/014 comprising information/image processing systems
• CPC Class Codes: G02B 2027/0178 Eyeglass type eyeglass details G02C
• CPC Class Codes: G02B 27/017 Head mounted
• CPC Class Codes: G06F 1/163 Wearable computers, e.g. on a belt
• CPC Class Codes: G06F 2203/0331 Finger worn pointing device
• CPC Class Codes: G06F 3/011 Arrangements for interaction with the human body, e.g. for user immersion in virtual reality blind teaching G09B21/00
• CPC Class Codes: G06F 3/014 Hand-worn input/output arrangements, e.g. data gloves
• CPC Class Codes: G06F 3/017 Gesture based interaction, e.g. based on a set of recognized hand gestures interaction based on gestures traced on a digitiser G06F3/04883
• CPC Class Codes: G06F 3/0346 with detection of the device orientation or free movement in a 3D space, e.g. 3D mice, 6-DOF [six degrees of freedom] pointers using gyroscopes, accelerometers or tilt-sensors
• CPC Class Codes: G06F 3/038 Control and interface arrangements therefor, e.g. drivers or device-embedded control circuitry
• CPC Class Codes: G06T 19/006 Mixed reality object pose determination, tracking or camera calibration for mixed reality G06T7/00

Abstract

A wearable computing device such as a head-mounted display (HMD) may be equipped with a magnetometer for detecting presence and motion of a hand-wearable magnet (HWM). The HMD may analyze magnetic field measurements of the magnetometer to determine when the HWM moves within a threshold distance of the magnetometer, and may thereafter determine one or more patterns of motion of the HWM based the magnetic field measurements. The HMD may operate in a background detection state in order to determine a background magnetic field strength and to monitor for magnetic disturbances from the HWM. Upon occurrence of a trigger event corresponding to magnetic disturbance above a threshold level, the HMD may transition to operating in a gesture detection state in which it analyzes magnetometer measurements for correspondence with known gestures. Upon recognizing a known gesture, the HMD may carry out one or more actions based on the recognized known gesture.

Claims

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.

2. The method of claim 1, further comprising:

responsive to identifying the respective known gesture associated with the particular set, identifying a pre-determined computer-executable action associated with the identified respective known gesture; and

carrying out the identified pre-determined computer-executable action with the processor.

3. The method of claim 1, wherein detecting the perturbation by the HWM of the determined field magnitude at least as large as the perturbation threshold comprises determining that the perturbation by the HWM of the determined field magnitude remains at least as large as the perturbation threshold for at least a threshold length of time.

4. The method of claim 3, wherein determining that the perturbation by the HWM of the determined field magnitude remains at least as large as the perturbation threshold for at least the threshold length of time comprises determining that a time-average value of the perturbation of the determined field magnitude remains at least as large as the perturbation threshold for at least the threshold length of time.

5. The method of claim 1, wherein measuring the magnitude perturbation by the HWM of the determined field magnitude to be less than the perturbation threshold comprises determining that the perturbation by the HWM of the determined field magnitude remains less than the perturbation threshold for at least a threshold length of time.

6. The method of claim 5, wherein determining that the perturbation by the HWM of the determined field magnitude remains less than the perturbation threshold for at least the threshold length of time comprises determining that a time-average value of the perturbation of the determined field magnitude remains less than the perturbation threshold for at least the threshold length of time.

7. The method of claim 1, wherein measuring the three orthogonal components of the background magnetic field with the magnetometer device comprises measuring 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=N₁, . . . , N₂, over a sliding time window W_N1,N2 from N₁ to N₂, wherein N₂=N₁+N−

1, and N≧

1,

and wherein determining the field magnitude of the background magnetic field from the three measured orthogonal components comprises;

computing magnetic field magnitude samples h_i=∥

H∥

=√

{square root over (H_x²+H_y²+H_z²)} at the consecutive discrete times t_i, i=N₁, . . . , N₂ over W_N1,N2; and

computing a sample mean field strength μ

_N1,N2=1/NΣ

_i=N1^N2 h_i over W_N1,N2.

8. The method of claim 7, wherein detecting the perturbation by the HWM of the determined field magnitude at least as large as the perturbation threshold comprises:

making a determination that at least one of the field magnitude samples h_j deviates from μ

_N1,N2 by an amount at least as large as the perturbation threshold, wherein N₁≦

j≦

N₂.

9. The method of claim 8, wherein N≧

2,

and wherein making the determination that the at least one of the field magnitude samples h_j deviates from μ

_N1,N2 by an amount at least as large as the perturbation threshold comprises;

computing a standard deviation

10. The method of claim 9, wherein determining that d_j is at least as large as θ

for the at least one of the field magnitude samples h_j comprises;

determining that d_j≧

θ

for at least q values of j over W_N1,N2 as W_N1,N2 slides across p time increments, wherein q≦

p.

11. The method of claim 8, wherein the wearable HMD further includes a motion detector,

and wherein making the determination that the at least one of the field magnitude samples h_j deviates from μ

_N1,N2 by an amount at least as large as the perturbation threshold comprises both of;

determining that the at least one of the field magnitude samples h_j deviates from μ

_N1,N2 by an amount at least as large as the perturbation threshold; and

concurrently determining that the motion detector is detecting motion at no greater than a threshold level of motion.

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.

13. The method of claim 12, wherein each of the one or more sets of pre-determined time derivatives of magnetic field strength comprises a respective triplet of pre-determined sequences of discrete time derivatives of a magnetic field along the three orthogonal measurement axes,

and wherein matching the determined time derivatives with the particular set of the one or more sets based on the comparison comprises:

determining a closest match between the computed discrete time derivatives and the respective triplet of pre-determined sequences of discrete time derivatives of one of the one or more sets.

14. The method of claim 12, wherein t_i for i=1 is one of t₁<

T_start, t₁=T_start, and t_i>

T_start.

15. The method of claim 1, wherein each of the one or more sets of pre-determined time derivatives of magnetic field strength comprises a respective triplet of pre-determined sequences of discrete time derivatives of a magnetic field along the three orthogonal measurement axes,

and wherein the method further comprises:

receiving a recording command, and responsively transitioning to operating in a gesture recording state; and

while operating in the gesture recording state, carrying out functions of the gesture recording state including,

determining an occurrence of a recording trigger from the HWM by detecting a perturbation by the HWM of the determined field magnitude at least as large as a recording perturbation threshold,

upon determining the occurrence of the recording trigger, 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,

computing discrete time derivatives of the measured orthogonal magnetic field components H_x, H_y, and H_z from discrete differences between successive samples to obtain a recorded triplet of sequences of discrete time derivatives

16. The method of claim 1, further comprising:

while operating in the gesture detection state, carrying out further functions of the gesture detection state including,

upon matching the determined time derivatives with the particular set of the one or more sets based and identifying the respective known gesture associated with the particular set, transitioning to operating in the background detection state,

and upon failing to match the determined time derivatives with the set of any of the one or more sets, transitioning to operating in the background detection state.

17. The method of claim 1, further comprising:

while operating in the gesture detection state, carrying out further functions of the gesture detection state including,

upon matching the determined time derivatives with the particular set of the one or more sets based and identifying the respective known gesture associated with the particular set;

continuing to track motion of the HWM by determining subsequent time derivatives of magnetic field strength measured by the magnetometer device along each of the three orthogonal measurement axes,

making an additional comparison of the determined subsequent time derivatives with the one or more sets of pre-determined time derivatives of magnetic field strength,

and upon matching the determined subsequent time derivatives with a given set of the one or more sets based on the additional comparison, identifying the respective known gesture associated with the given set.

18. The method of claim 1, further comprising:

while operating in the background detection state, carrying out further functions of the background detection state including,

determining the occurrence of the trigger by detecting a directional perturbation of the determined field magnitude corresponding to motion of the wearable HMD with respect to the background magnetic field, wherein the directional perturbation is at least as large as a directional perturbation threshold.

19. The method of claim 1, wherein the three orthogonal measurement axes comprise a first triplet of orthogonal measurement axes of the magnetometer device,

wherein the magnetometer device comprises a second triplet of orthogonal measurement axes,

and wherein the method further comprises:

while operating in the background detection state, carrying out further functions of the background state including,

measuring three orthogonal components of the background magnetic field with the second triplet of orthogonal axes to determine a supplementary field magnitude of the background magnetic field,

determining an occurrence of a supplementary trigger from the HWM by detecting a supplementary perturbation by the HWM of the determined supplementary field magnitude at least as large as a supplementary perturbation threshold,

and upon determining the occurrence of at least one of the trigger and the supplementary trigger, transitioning to operating in the gesture detection state; and

while operating in the gesture detection state, carrying out further functions of the gesture detection state including,

tracking motion of the HWM by determining supplementary time derivatives of magnetic field strength measured with the second triplet of orthogonal axes,

making a joint comparison of the determined time derivatives and the determined supplementary time derivatives with one or more combined sets of pre-determined time derivatives and pre-determined supplementary time derivatives, each of the one or more combined sets being stored at the wearable HMD and each being associated with one of the respective known gestures,

and upon jointly matching both the determined time derivatives and the determined supplementary time derivatives with a particular combined set of the one or more combined sets based on the joint comparison, identifying the respective known gesture associated with the particular combined set.

20. The method of claim 1, wherein the HWM comprises a magnet with at least one of:

a dipole magnetic field and a quadrupole magnetic field.

21. 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,

analyzing the determined time derivatives to determine a known gesture,

identifying a pre-determined computer-executable action associated with the determined known gesture,

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.

22. A wearable head-mount display (HMD) comprising:

a processor;

memory accessible to the processor;

a magnetometer device with three orthogonal measurement axes; and

executable instructions stored in the memory that upon execution by the processor cause the HMD to carry out operations including;

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 by detecting a perturbation upon 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, wherein each of the one or more sets is stored at the wearable HMD and each is 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.

23. The wearable HMD of claim 22, wherein the operations further include:

responding to identifying the respective known gesture associated with the particular set by identifying a pre-determined computer-executable action associated with the identified respective known gesture; and

carrying out the identified pre-determined computer-executable action with the processor.

24. The wearable HMD of claim 22, wherein measuring the three orthogonal components of a background magnetic field with the magnetometer device comprises measuring 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, . . . , N₂, over a sliding time window W_N1,N2 from N₁ to N₂, wherein N₂=N₁+N−

1 and N≧

1,

wherein determining the field magnitude of the background magnetic field from the three measured orthogonal components comprises;

computing magnetic field magnitude samples h_i=∥

H∥

=√

{square root over (H_x²+H_y²+H_z²)} at the consecutive discrete times t_i, i=1, . . . , N₂ over W_N1,N2; and

computing a sample mean field strength μ

_N1,N2=1/NΣ

_i=N1^N2 h_i over W_N1,N2,

and wherein detecting the perturbation by the HWM of the determined field magnitude at least as large as the perturbation threshold comprises making a determination that at least one of the field magnitude samples h_j deviates from μ

_N1,N2 by an amount at least as large as the perturbation threshold, wherein N₁≦

j≦

N₂.

25. The wearable HMD of claim 24, wherein the wearable HMD further includes a motion detector,

wherein making the determination that the at least one of the field magnitude samples h_j deviates from μ

_N1,N2 by an amount at least as large as the perturbation threshold comprises both of;

determining that the at least one of the field magnitude samples h_j deviates from μ

_N1,N2 by an amount at least as large as the perturbation threshold; and

concurrently determining that the motion detector is detecting motion at no greater than a threshold level of motion.

26. The wearable HMD of claim 22, wherein each of the one or more sets of pre-determined time derivatives of magnetic field strength comprises a respective triplet of pre-determined sequences of discrete time derivatives of a magnetic field along the three orthogonal measurement axes,

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,

and wherein matching the determined time derivatives with the particular set of the one or more sets based on the comparison comprises determining a closest match between the computed discrete time derivatives and the respective triplet of pre-determined sequences of discrete time derivatives of one of the one or more sets.

27. The wearable HMD of claim 22, wherein each of the one or more sets of pre-determined time derivatives of magnetic field strength comprises a respective triplet of pre-determined sequences of discrete time derivatives of a magnetic field along the three orthogonal measurement axes,

and wherein the operations further include:

transitioning to operating in a gesture recording state in response to receiving a recording command; and

while operating in the gesture recording state, carrying out functions of the gesture recording state including,

determining an occurrence of a recording trigger from the HWM by detecting a perturbation by the HWM of the determined field magnitude at least as large as a recording perturbation threshold,

upon determining the occurrence of the recording trigger, 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,

computing discrete time derivatives of the measured orthogonal magnetic field components H_x, H_y, and H_z from discrete differences between successive samples to obtain a recorded triplet of sequences of discrete time derivatives

28. The wearable HMD of claim 22, wherein the three orthogonal measurement axes comprise a first triplet of orthogonal measurement axes of the magnetometer device,

wherein the magnetometer device comprises a second triplet of orthogonal measurement axes,

wherein operating in the background detection state further comprises carrying out further functions of the background detection state including:

measuring three orthogonal components of the background magnetic field with the second triplet of orthogonal axes to determine a supplementary field magnitude of the background magnetic field,

determining an occurrence of a supplementary trigger from the HWM by detecting a supplementary perturbation by the HWM of the determined supplementary field magnitude at least as large as a supplementary perturbation threshold,

and upon determining the occurrence of at least one of the trigger and the supplementary trigger, transitioning to operating in the gesture detection state;

and wherein operating in the gesture detection state further comprises carrying out further functions of the gesture detection state including;

tracking motion of the HWM by determining supplementary time derivatives of magnetic field strength measured with the second triplet of orthogonal axes,

making a joint comparison of the determined time derivatives and the determined supplementary time derivatives with one or more combined sets of pre-determined time derivatives and pre-determined supplementary time derivatives, each of the one or more combined sets being stored at the wearable HMD and each being associated with one of the respective known gestures,

upon jointly matching both the determined time derivatives and the determined supplementary time derivatives with a particular combined set of the one or more combined sets based on the joint comparison, identifying the respective known gesture associated with the particular combined set.

29. A nontransitory computer-readable medium having instructions stored thereon that, upon execution by one or more processors of a wearable head-mounted display (HMD), cause the wearable HMD to carry out operations 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 using three orthogonal measurement axes of magnetometer device of the wearable HMD, 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, wherein each of the one or more sets is configured to be stored at the wearable HMD and each is 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.

30. The nontransitory computer-readable medium of claim 29, wherein the instructions, upon execution by one or more processors of the wearable HMD, cause the wearable HMD to carry out further operations comprising:

responsive to identifying the respective known gesture associated with the particular set, identifying a pre-determined computer-executable action associated with the identified respective known gesture; and

carrying out the identified pre-determined computer-executable action.

31. The nontransitory computer-readable medium of claim 29, wherein detecting the perturbation by the HWM of the determined field magnitude at least as large as the perturbation threshold comprises determining that the perturbation by the HWM of the determined field magnitude remains at least as large as the perturbation threshold for at least a first threshold length of time,

and wherein measuring the magnitude perturbation by the HWM of the determined field magnitude to be less than the perturbation threshold comprises determining that the perturbation by the HWM of the determined field magnitude remains less than the perturbation threshold for at least a second threshold length of time.

32. The nontransitory computer-readable medium of claim 29, wherein measuring the three orthogonal components of a background magnetic field comprises measuring 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, . . . , N₂, over a sliding time window W_N1,N2 from N₁ to N₂, wherein N₂=N₁+N−

1 and N≧

1,

wherein determining the field magnitude of the background magnetic field from the three measured orthogonal components comprises;

computing magnetic field magnitude samples h_i=∥

H∥

=√

{square root over (H_x²+H_y²+H_z²)} at the consecutive discrete times t_i, i=N₁, . . . , N₂ over W_N1,N2; and

computing a sample mean field strength μ

_N1,N2=1/NΣ

_i=N1^N2 h_i over W_N1,N2,

and wherein detecting the perturbation by the HWM of the determined field magnitude at least as large as the perturbation threshold comprises making a determination that at least one of the field magnitude samples h_j deviates from μ

_N1,N2 by an amount at least as large as the perturbation threshold, wherein N₁≦

j≦

N₂.

33. The nontransitory computer-readable medium of claim 32, wherein

making the determination that the at least one of the field magnitude samples h_j deviates from μ

_N1,N2 by an amount at least as large as the perturbation threshold comprises both of;

determining that the at least one of the field magnitude samples h_j deviates from μ

_N1,N2 by an amount at least as large as the perturbation threshold; and

concurrently determining that a motion detector of the wearable HMD is detecting motion at no greater than a threshold level of motion.

34. The nontransitory computer-readable medium of claim 29, wherein each of the one or more sets of pre-determined time derivatives of magnetic field strength comprises a respective triplet of pre-determined sequences of discrete time derivatives of a magnetic field along the three orthogonal measurement axes,

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,

and wherein matching the determined time derivatives with the particular set of the one or more sets based on the comparison comprises determining a closest match between the computed discrete time derivatives and the respective triplet of pre-determined sequences of discrete time derivatives of one of the one or more sets.

35. The nontransitory computer-readable medium of claim 29, wherein each of the one or more sets of pre-determined time derivatives of magnetic field strength comprises a respective triplet of pre-determined sequences of discrete time derivatives of a magnetic field along the three orthogonal measurement axes,

and wherein the instructions, upon execution by one or more processors of the wearable HMD, cause the wearable HMD to carry out further operations comprising:

receiving a recording command, and responsively transitioning to operating in a gesture recording state; and

while operating in the gesture recording state, carrying out functions of the gesture recording state including,

determining an occurrence of a recording trigger from the HWM by detecting a perturbation by the HWM of the determined field magnitude at least as large as a recording perturbation threshold,

upon determining the occurrence of the recording trigger, 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,

computing discrete time derivatives of the measured orthogonal magnetic field components H_x, H_y, and H_z from discrete differences between successive samples to obtain a recorded triplet of sequences of discrete time derivatives