skeltrack: a free software library for skeleton tracking (linuxtag 2012)
DESCRIPTION
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 I will present how Skeltrack was developed and show a demo of it working.TRANSCRIPT
Skeltrack - Open Source Skeleton Tracking
Joaquim Rocha, IgaliaLinuxTag 2012 - Wunderbare Berlin
Guten Tag!
✩ I am a developer at Igalia✩ I like doing innovative stuff like OCRFeeder and SeriesFinale✩ and today I am presenting my latest project: Skeltrack
The Kinect
Microsoft's Kinect was the first camerawith a price affordable to the public
The USB connection is open and thus hackable
This originated Open Source projects like the libfreenect,a library to control the Kinect device and get its information
We created a GLib wrapper for libfreenect called GFreenect
GFreenect offers asynchronous functions (and some synchronous aswell) and makes it easy to use with other GNOME technologies
GObject Introspection = free bindings (Python, Javascript, Vala)
Kinect has a structured light camera which gives depth information
But that's raw information... values from 0-2048
libfreenect/GFreenect can give those values in mm
Still...
It does NOT tell you there is a person in the picture
Or a cow
Or an ampelmann
Let alone a skeleton and where its joints are
For this you need a skeleton tracking solution
Three proprietary/closed solutions exist:
Microsoft Kinect SDK: non-commercial only
OpenNI: commercial compatible
Kinect for Windows: commercial use allowedbut incompatible with the XBox's Kinect
Conclusion: There were no Free solutions toperform skeleton tracking... :(
So Igalia built one!
Enter Skeltrack
What we wanted:✩ A shared library, no fancy SDK✩ Device independent✩ No pattern matching, no databases✩ Easy to use (everybody wants that!)
Not as easy as it sounds!
After some investigation we found Andreas Baak'spaper "A Data-Driven Approach for Real-Time FullBody Pose Reconstruction from a Depth Camera"
However this paper uses a database ofposes to get what the user is doing
So we based only part of our work on it
How does it work?
First we need to find the extremas
Make a graph whose nodes are the depth pixels
Connect two nodes if the distance is less than acertain value
Connect the different graph's components by usingconnected-component labeling
Choose a starting point and calculate Dijkstra toeach point of the graph; choose the furthest point.There you got your extrema!
Then create an edge between the starting pointand the current extrema point with 0 cost andrepeat the same process now using the currentextrema as a starting point.
This comes from Baak's paper and the differencestarts here: choosing the starting point
Baak chooses a centroid as the starting point
We choose the bottom-most point starting from thecentroid (this showed better results for the upperbody extremas)
So we got ourselves some extremas!What to do with them?
What extrema is a hand, a head, a shoulder?
For that we use educated guesses...
We calculate 3 extremas
Then we check each of them hoping they are the head
How?
For each extrema we look for the points in placeswhere the shoulders should be, checking their distancesbetween the extrema and between each other.
If they obey those rules then we assume they arethe head'n'shoulders (tm)
With the remaining 2 extremas, we will try to see ifthey are elbows or hands
How to do it?
Calculate Dijkstra from the shoulders to each extrema
The closest extrema to any of the shoulders is either ahand of an elbow of that shoulder
How to check if it's a hand or an elbow?
If the distance between the extrema and the shoulder isless than a predefined value, then it is an elbow. Otherwiseit is a hand.
If it is a hand, we find the elbow by choosing the first point(in the path we created with Dijkstra before) whose distanceexceeds the elbow distance mentioned before
There is still some things missing...
Future work
Hands from elbows: If one of the extremas is an elbow, weneed to infer where the hand is
Smoothing: Smooth the jittering of the joints
Robustness: Use restrictions to ignore objects that are notthe user
Multi-user: Track more than one person at a time
And of course, get the rest of the joints: hips, knees, etc.
How to use it?
Asynchronous API
SkeltrackSkeleton *skeleton = SKELTRACK_SKELETON (skeltrack_skeleton_new ());skeltrack_skeleton_track_joints (skeleton, depth_buffer, buffer_width, buffer_height, NULL, on_track_joints, NULL);
Synchronous API
SkeltrackJointList list;list = skeltrack_skeleton_track_joints_sync (skeleton, depth_buffer, buffer_width, buffer_height, NULL, NULL);
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)
Code/Bugs: https://github.com/joaquimrocha/Skeltrack
Nifty Tools for Development:
GFreenect: https://github.com/elima/GFreenect
GFreenect Utils: https://github.com/joaquimrocha/gfreenect-utils
GFreenect Python Example
Tool: record-depth-file
Tool: depth-file-viewer
Questions?
Creative Commons pictures from flickr:Kinect: Auxo.co.krAmpelmann: echiner1Kid Playing: Rob WelshSkeleton: Dark Botxy