Viewport - A viewport is the meeting of a -camera and a rendering surface - the camera renders the scene from a viewpoint, and places its results into some subset of a rendering target, which may be the whole surface or just a part of the surface. Each viewport has a single camera as source and a single target as destination. A camera only has one viewport, but a render target may have several. A viewport also has a -Z-order, i.e. if there is more than one viewport on a single render target and they overlap, one must obscure the other in some predetermined way.