OpenNI object is the starting point of everything in the framework. Using the OpenNI class we can access a list of connected devices as well as the version of OpenNI itself. Then using this information we can access a device object and read data.

This class uses the singleton pattern, which means there is only one instance of this class and all of its methods are static.

Also in OpenNI 2 we have the ability to register two callback functions by OpenNI object for capturing device connected and device disconnected events.

Device object is representing the actual physical device where each device supports a number of sensors (for example, depth, color, and IR) that can be accessed using Device object. We need to ask for access to a device before using its sensor's output. Also using this object, we can access some device-wide settings. Read more in Chapter 4, More about Low-level Outputs.

Using the VideoStream object we can access the output data of color, IR, and depth sensors. VideoStream in the new version of OpenNI supports event-based reading that gives us the ability to register a callback function to execute when a new frame of data becomes available.