Coordinate Systems

OpenNI defines two types of coordinate systems:

• Projective Coordinates - a two-dimentional map of depth values
• Real World Coordinates - A collection of three-dimentional points.

Those two coordinate systems are defined as below:

Feature	Projective Coordinates	Real World Coordinates
X-Y Units	pixels	millimeters
Z Units	millimeters	millimeters
X-Y Origin	upper-left corner of the FOV	center of FOV
Z Origin	sensor's plane	sensor's plane
X Direction	left to right	left to right
Y Direction	top to bottom	bottom to top
Z Direction	away from sensor	away from sensor

A main difference between the two is that in projective coordinates, a real-world object (person, chair) gets bigger (in pixels) as it gets closer to the sensor, whereas its real-world size remains the same.

Note:

Some data in OpenNI is returned in projective coordinates (for example, depth maps), and some is returned in real-world coordinates (for example, skeleton joints). Pay attention when comparing the two.

A depth generator creates the depth value of each pixel. By knowing the distance of the object and some geometrical information about the sensor (like its FOV), translation can be made between the two systems. see xn::DepthGenerator::ConvertProjectiveToRealWorld() and xn::DepthGenerator::ConvertRealWorldToProjective().