Apparatus for processing image data is provided, comprising first storage means, processing means and manual input means, wherein said first storage means is configured to store said image data and said image data includes a plurality of components defined by a hierarchy of data processing nodes, and said processing means is configured to process each of said nodes in turn. The nodes include standard nodes and switch nodes, each switch node having at least two child nodes one of which is the designated child node of said switch node. The processing means is configured to process a standard node only when each of its child nodes has been processed, and process a switch node when only its designated child node has been processed. Additionally, said processing means may include second storage means and stores, in either said first or said second storage means, information recording the states of said switch nodes that defines a version of said image data.