Tracking arm movements to generate inputs for computer systems

A method, comprising:

• receiving, from a first sensor module attached to an upper arm of a user, first motion data identifying an orientation of the upper arm;
• receiving, from a second sensor module attached to a hand of the user, second motion data identifying an orientation of the hand;
• calculating, based on the orientation of the upper arm and the orientation of the hand, an orientation of a forearm connected to the hand by a wrist of the user and connected to the upper arm by an elbow joint of the user;
• wherein the orientation of the forearm is calculated to be equal to a first orientation resulting from:
• rotating, from a second orientation aligned with the orientation of the upper arm, about a lengthwise direction of the forearm, by a first angle to arrive at third orientation;
• and
• rotating, from the third orientation, about an axis parallel to a direction from a back side of the upper arm to a front side of the upper arm, by a second angle to arrive at the first orientation;
• and
• determining a position of the hand in a three dimensional space based on the orientation of the upper arm, and the orientation of the forearm, the orientation of the hand, and a skeleton model of the upper arm, the forearm, and the hand.

A non-transitory computer storage medium storing instructions which, when executed by a computing device, instructs the computing device to perform a method, the method comprising:

• receiving, by the computing device from a first sensor module attached to an upper arm of a user, first motion data identifying an orientation of the upper arm;
• receiving, by the computing device from a second sensor module attached to a hand of the user, second motion data identifying an orientation of the hand;
• and
• calculating, by the computing device based on the orientation of the upper arm and the orientation of the hand, an orientation of a forearm connected to the hand by a wrist of the user and connected to the upper arm by an elbow joint of the user, wherein the calculating comprises:
• calculating, by the computing device, an orientation of the hand relative to the upper arm;
• determining, by the computing device, a first rotation of a lengthwise direction of the hand from a lengthwise direction of the upper arm along a closest arc to the lengthwise direction of the hand in the orientation of the hand relative to the upper arm;
• subtracting, by the computing device, the first rotation from the orientation of the hand relative to the upper arm to obtain a second rotation of the hand along the lengthwise direction of the upper arm;
• projecting, on to a plane by the computing device, the lengthwise direction of the hand in the orientation of the hand relative to the upper arm to determine a lengthwise direction of the forearm relative to the upper arm;
• computing, by the computing device, a closest arc rotation from the lengthwise direction of the upper arm to the lengthwise direction of the forearm relative to the upper arm;
• and
• determining, by the computing device, the orientation of the forearm based on combination of a rotation from an orientation aligned with the upper arm, along the lengthwise direction of the upper arm according to the second rotation of the hand along the lengthwise direction of the upper arm and a rotation within the plane according to the closest arc rotation from the lengthwise direction of the upper arm to the lengthwise direction of the forearm relative to the upper arm.