You, the user of the RT3D application, looking at the 3D scene through your computer screen. The viewer looks into the RT3D world from a vantage point, which acts something like a camera, to frame your view. Most of the engine's calculations are tailored to make the world look great from that particular view. See also frustum.
Viewer may also refer to a version of the RT3D engine that is used to preview artwork while it is being created.