By Joaquim Rocha. With the release of the Kinect device, there was finally an affordable camera capable of giving depth information. This, together with the Kinect's open USB connection, led to a lot of innovative projects. Still, the Kinect just gives raw signals and the only way to obtain more complex information, such as skeleton tracking was to use either the Microsoft SDK or the OpenNI framework. Both of these solutions are closed, proprietary and, in the case of Microsoft's, only for non-commercial work. To solve the issue above, Igalia developed Skeltrack, a Free and Open Source library published under LGPL that performs human skeleton tracking and identifies a number of skeleton joints. It is a more atomic solution than the other commercial counterparts because it does not connect directly to the Kinect nor to any other depth camera, instead it expects to be given the buffer corresponding to the depth buffer. In this talk Joaquim will present how Skeltrack was developed and show a demo of it working.

1. Skeltrack - Open Source Skeleton Tracking
Joaquim Rocha, Igalia
GUADEC 2012 - A Coruña

2. Igalia's Interactivity Team was on to investigate
immersive applications using FOSS

3. These applications represent
new/different ways of interaction for users

4. Those can go from entertainment to more serious applications

6. The Kinect

7. Microsoft's Kinect was the first camera
with a price affordable to the public

8. The USB connection is open and thus hackable

9. This originated Open Source projects like the libfreenect,
a library to control the Kinect device and get its information

10. We created a GLib wrapper for libfreenect called GFreenect

11. GFreenect offers asynchronous functions (and some synchronous as
well) and makes it easy to use with other GNOME technologies

12. GObject Introspection opens these technologies for users of
Python, Javascript, Vala, etc.

13. The Kinect has a structured light camera which gives depth information

14. But that's raw information... values from 0-2048

15. libfreenect/GFreenect can give those values in mm

17. Still...

18. It does NOT tell you there is a person in the picture

19. Or a cow

20. Or a monkey

21. Let alone a skeleton and where its joints are!

22. For this you need a skeleton tracking solution

23. Skeleton Tracking

24. Three proprietary/closed solutions exist:

25. Microsoft Kinect SDK: non-commercial only

26. OpenNI: commercial compatible

27. Kinect for Windows: commercial use allowed
but incompatible with the XBox's Kinect

29. Conclusion: There were no Free solutions to
perform skeleton tracking... :(

30. So Igalia built one!

31. Enter Skeltrack

32. What we wanted:
✩ A shared library, no fancy SDK
✩ Modular (do one thing)
✩ Device independent
✩ No pattern matching, no databases
✩ Easy to use (everybody wants that!)

33. Not as easy as it sounds!

34. The current version tracks up to 7 joints:
✩ Head;
✩ Shoulders;
✩ Elbows;
✩ Hands.

35. Smoothing is implemented using
Holt's Double Exponential formulas

36. Smoothing and heuristics used
can be tweaked using GObject properties

37. There are still some things missing...

38. Future work

39. Hands from elbows: If one of the extremas is an elbow, we
need to infer where the hand is

40. Robustness: Use restrictions to ignore objects that are not
the user

41. Multi-user: Track more than one person at a time

42. And of course, get the rest of the joints: hips, knees, etc.

43. How to use it?

44. First reduce the depth buffer's dimension
Reducing the buffer's dimension will make things faster

45. While at it, it's wise to also apply a depth threshold

46. Asynchronous API

47. SkeltrackSkeleton *skeleton = SKELTRACK_SKELETON (skeltrack_skeleton_new ());
skeltrack_skeleton_track_joints (skeleton,
depth_buffer,
buffer_width,
buffer_height,
NULL,
on_track_joints,
NULL);

49. Synchronous API

50. SkeltrackJointList list;
list = skeltrack_skeleton_track_joints_sync (skeleton,
depth_buffer,
buffer_width,
buffer_height,
NULL,
NULL);

51. Skeleton Joint:
ID: HEAD, LEFT_ELBOW, RIGHT_HAND, ...
x: X coordinate in real world (in mm)
y: Y coordinate in real world (in mm)
screen_x: X coordinate in the screen (in pixels)
screen_y: Y coordinate in the screen (in pixels)

52. Join us!
https://github.com/joaquimrocha/Skeltrack

53. Nifty Tools for Development:
GFreenect:
https://github.com/elima/GFreenect
GFreenect Utils:
https://github.com/joaquimrocha/gfreenect-utils

54. GFreenect Python Example

55. Tool: depth-file-viewer

56. Tool: record-depth-file

57. Demo?
Warning: Might go wrong!

58. Questions?

59. Creative Commons pictures from flickr:
Kinect: Auxo.co.kr
Monkey: nothingtosay
Kid Playing: Rob Welsh
Skeleton: Dark Botxy
Other non CC images:
Doctor: http://research.microsoft.com/en-us/projects/touchless/
Minority Report:
http://www.ovelf.com/2010/04/14/oblong-tan-g-speak-arttirilmis-gerceklikte-hareketlerin-3-boyutta-yorumlanmasi/