Multiple users, one background, here's how...


Today's project by John Elsbree is a cool sample of how to push the new background removal in the Kinect for Windows SDK v1.8 even farther...

Using Kinect Background Removal with Multiple Users

Introduction: Background Removal in Kinect for Windows

The 1.8 release of the Kinect for Windows Developer Toolkit includes a component for isolating a user from the background of the scene. The component is called the BackgroundRemovedColorStream. This capability has many possible uses, such as simulating chroma-key or “green-screen” replacement of the background – without needing to use an actual green screen; compositing a person’s image into a virtual environment; or simply blurring out the background, so that video conference participants can’t see how messy your office really is.

To use this feature in an application, you create the BackgroundRemovedColorStream, and then feed it each incoming color, depth, and skeleton frame when they are delivered by your Kinect for Windows sensor. You also specify which user you want to isolate, using their skeleton tracking ID. The BackgroundRemovedColorStream produces a sequence of color frames, in BGRA (blue/green/red/alpha) format. These frames are identical in content to the original color frames from the sensor, except that the alpha channel is used to distinguish foreground pixels from background pixels. Pixels that the background removal algorithm considers part of the background will have an alpha value of 0 (fully transparent), while foreground pixels will have their alpha at 255 (fully opaque). The foreground region is given a smoother edge by using intermediate alpha values (between 0 and 255) for a “feathering” effect. This image format makes it easy to combine the background-removed frames with other images in your application.

As a developer, you get the choice of which user you want in the foreground. The BackgroundRemovalBasics-WPF sample has some simple logic that selects the user nearest the sensor, and then continues to track the same user until they are no longer visible in the scene.


Wait, only one person?

If you wanted to select more than one person from the scene to appear in the foreground, it would seem that you’re out of luck, because the BackgroundRemovedColorStream’s SetTrackedPlayer method accepts only one tracking ID. But you can work around this limitation by running two separate instances of the stream, and sending each one a different tracking ID. Each of these streams will produce a separate color image, containing one of the users. These images can then be combined into a single image, or used separately, depending on your application’s needs.

Wait, only two people?

In the most straightforward implementation of the multiple stream approach, you’d be limited to tracking just two people, due to an inherent limitation in the skeleton tracking capability of Kinect for Windows. Only two skeletons at a time can be tracked with full joint-level fidelity. The joint positions are required by the background removal implementation in order to perform its job accurately.

However, there is an additional trick we can apply, to escape the two-skeleton limit. This trick relies on an assumption that the people in the scene will not be moving at extremely high velocities (generally a safe bet). If a particular skeleton is not fully tracked for a frame or two, we can instead reuse the most recent frame in which that skeleton actually was fully tracked. Since the skeleton tracking API lets us choose which two skeletons to track at full fidelity, we can choose a different pair of skeletons each frame, cycling through up to six skeletons we wish to track, over three successive frames.

Each additional instance of BackgroundRemovedColor stream will place increased demands on CPU and memory. Depending on your own application’s needs and your hardware configuration, you may need to dial back the number of simultaneous users you can process in this way.

Wait, only six people?

Demanding, aren’t we? Sorry, the Kinect for Windows skeleton stream can monitor at most six people simultaneously (two at full fidelity, and four at lower fidelity). This is a hard limit.

Introducing a multi-user background removal sample

We’ve created a new sample application, called BackgroundRemovalMultiUser-WPF, to demonstrate how to use the technique described above to perform background removal on up to six people. We started with the code from the BackgroundRemovalBasics-WPF sample, and changed it to support multiple streams, one per user. The output from each stream is then overlaid on the backdrop image.


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