GSK is conceptually split into two scene graphs: * a simple rendering tree of operations * a complex set of logical layers The latter is built on the former, and adds convenience and high level API for application developers. The lower layer, though, is what gets transformed into the rendering pipeline, as it's simple and thus can be transformed into appropriate rendering commands with minimal state changes. The lower layer is also suitable for reuse from more complex higher layers, like the CSS machinery in GTK, without necessarily port those layers to the GSK high level API. This lower layer is based on GskRenderNode instances, which represent the tree of rendering operations; and a GskRenderer instance, which takes the render nodes and submits them (after potentially reordering and transforming them to a more appropriate representation) to the underlying graphic system.