The viewer can use base layer groups to optimize image rendering times. Layers in a base layer group are rendered together to generate a single raster image that can be sent to the viewer. The image is divided into tiles so only the required tiles need to be rendered and sent, instead of the entire image. Tiles are cached on the server; if a tile already exists in the cache it does not need to be rendered before being sent.
Each base layer group has a series of pre-defined scales that are used for rendering. When a request is made to view a portion of the map at a given scale, the viewer renders the tiles at the pre-defined scale that is closest to the requested map view scale.