Head rotation tracking from depth-based center of mass

1. A method for using a depth image to determine a rotation of a user's head, the method comprising:

receiving depth image data the depth image data including depth values for a plurality of pixels;

determining a background depth value;

identifying from the depth image data a head region within the depth image;

identifying a subset of pixels in the head region that have a depth value that does not exceed the background depth value;

calculating a center of depth-mass for the user's head based on depth values of the subset of pixels in the head region, wherein calculating the center of depth-mass comprises computing a center of mass for a solid of uniform density determined from the depth values for the subset of pixels and the background depth value; and

determining the rotation of the user's head based on the location of the center of depth-mass.

2. The method of claim 1 , further comprising:

determining the head region by finding an object with a silhouette that closely corresponds with the size and shape of a human body; and

determining a portion of the object that corresponds with the size and shape of a human head.

3. The method of claim 2 , wherein the image is captured using a camera other than a red blue green camera.

4. The method of claim 1 , wherein determining the rotation of the user's head based on the center of depth-mass comprises:

approximating a position in the head region corresponding with a center of the user's head; and

determining the rotation of the user's head based on a difference between the center of depth-mass and the position in the head region corresponding with the center of the user's head.

5. The method of claim 1 , wherein the method further comprises using the rotation of the user's head to control a camera viewpoint for a virtual environment.

6. A computing device comprising:

a processor configured to:

receive depth values for a plurality of pixels in a depth image;

determine a background depth value;

identify from the depth image data a head region within the depth image;

identify a subset of pixels in the head region that have a depth value that does not exceed the background depth value;

calculate a center of depth-mass for the user's head based on depth values of the subset of pixels in the head region, wherein calculating the center of depth-mass comprises computing a center of mass for a solid of uniform density determined from the depth values for the subset of pixels and the background depth value; and

determine the rotation of the user's head based on the location of the center of depth-mass.

7. The computing device of claim 6 , wherein the device is configured to determine the background depth value by:

identifying a maximum depth value from pixels within the head region; and

setting the background depth value as a function of the maximum depth value.

8. The computing device of claim 6 , wherein the device is configured to determine the background depth value by:

calculating an average depth value based on depth values for at least a portion of pixels in the head region; and

setting the background depth value as a function of the average depth value.

9. The computing device of claim 6 , wherein the device is configured to identify the head region by:

identifying objects in the depth image based on depth values; and

analyzing the objects to identify a first object having a silhouette that corresponds with the size and shape of a human head.

10. The computing device of claim 6 , wherein the device is configured to identify the head region by:

providing a user interface that allows the user to view an image of the user's head and to control a location of a boundary for the head region; and

receiving user input setting the location of the boundary for the head region.

11. The computing device of claim 6 , wherein the device is configured to identify the head region by:

finding an object with a silhouette that closely corresponds with the size and shape of a human body; and

determining a portion of the object that corresponds with the size and shape of a human head.

12. The computing device of claim 6 , wherein the device is configured to determine the rotation of the user's head based on the center of depth-mass by:

approximating a position in the head region corresponding with a center of the user's head; and

determining the rotation of the user's head based on a difference between the center of depth-mass and the position in the head region corresponding with the center of the user's head.

13. The computing device of claim 6 , wherein the rotation of the user's head is used to control a camera viewpoint for a virtual environment.

14. The computing device of claim 6 , wherein the device is configured to determine a depth of the user's head at each pixel based on the difference between the depth value for each pixel and the background depth value.

15. The computing device of claim 6 , wherein the image is captured using a camera other than a red blue green camera.

16. The computing device of claim 6 , wherein the image is captured using an infrared camera.

17. One or more computer-storage devices storing computer-useable instructions that, when used by one or more computing devices, cause the one or more computing devices to perform a method, the method comprising:

receiving depth image data the depth image data including depth values for a plurality of pixels;

determining a background depth value;

identifying from the depth image data a head region within the depth image;

identifying a subset of pixels in the head region that have a depth value that does not exceed the background depth value;

calculating a center of depth-mass for the user's head based on depth values of the subset of pixels in the head region, wherein calculating the center of depth-mass comprises computing a center of mass for a solid of uniform density determined from the depth values for the subset of pixels and the background depth value; and

determining the rotation of the user's head based on the location of the center of depth-mass.

18. The one or more computer storage devices of claim 17 , wherein the center of depth-mass for the user's head is calculated using pixels in the head region with a depth value that does not exceed the background depth value.

19. The one or more computer storage devices of claim 17 , wherein calculating the center of depth-mass for the user's head comprises:

identifying a group of pixels from the head region;

determining a thickness of the user's head for each pixel from the group of pixels based on a difference between the depth value for each pixel from the group of pixels and the background depth value;

calculating a first value by summing results from multiplying the thickness of the user's head for each pixel from the group of pixels by a position for each pixel from the group of pixels;

calculating a second value by summing the depth values for pixels in the group of pixels; and

calculating the center of depth-mass for the user's head by dividing the first value by the second value.

20. The one or more computer storage devices of claim 17 , wherein calculating the center of depth-mass for the user's head comprises applying weighting as a function of pixel position within the head region.