Appendix D: Coordinate Systems

The documentation may refer to different coordinate systems where virtual 3D objects are defined. This appendix defines the different coordinate spaces that can be interfaced with through the SDK.

NvAR World 3D Space

NvAR World 3D space, or simply world space, defines the main reference frame used in 3D applications. The reference frame is right handed, and the coordinate units are centimeters. The camera is usually, but not always, placed so that it is looking down the negative z-axis, with the x-axis pointing right, and the y-axis pointing up. The face model is usually, but not always placed so that the positive z-axis points out from the nose of the model, the x-axis points out from the left ear, and the y-axis points up from the head. This is so that no rotation of a face model or a camera model would correspond to the model being in view.

NvAR World 3D space

NvAR Model 3D Space

In NvAR Model 3D space, or simply the model space, a face model has its origin in the center of the skull where the first neck joint would be placed. The reference frame is right handed, and the coordinate units are centimeters. The positive z-axis points out from the nose of the model, the x-axis points out from the left ear, and the y-axis points up from the head.

NvAR Model 3D space

NvAR Camera 3D Space

In NvAR Camera 3D space, or simply the camera space, objects are placed in relation to a specific camera. The camera space can in many cases be the same as the world space unless explicit camera extrinsics are used. The reference frame is right handed, and the coordinate units are centimeters. The positive x-axis points to the right, the y-axis points up and the z-axis points back.

NvAR Camera 3D space

NvAR Image 2D Space

The NvAR Image 2D space, or screen space, is the main 2D space for screen coordinates. The origin of an image is the location of the uppermost, leftmost pixel; the x-axis points to the right and the y-axis points down. The unit of this coordinate system is pixels.

NvAR Image 2D space

This coordinate system has two flavors: on-pixel or inter-pixel.

In the on-pixel system, the origin point is located on the center of the pixel. In the inter-pixel system, the origin is offset from the center of the pixel by a distance of −0.5 pixels along the x-axis and the y-axis.

On-pixel should be used for integer-based coordinates; inter-pixel should be used for real-valued coordinates (usually defined as floating-point coordinates).

On-pixel integer coordinates	Inter-pixel real-valued coordinates