An apparatus for processing image data 603, 604 is provided, which comprises processing means 201, 202, memory means 205, 206, 207, display means 104 and manually operable input means 102, 103, 105, 106, wherein said memory means stores said image data and instructions 602, said image data and said instructions configure said processing means to perform the steps of outputting said image data to said display means as a hierarchical structure 603 of image data processing nodes 303, 401 to 413 having respective hierarchical properties 701, 702, 703, said structure being configured with a dimension. In response to selecting a plurality of said nodes by way of said input means, a group node 1301 is generated having hierarchical properties 701, 702, 703 processed (1103) from said respective properties. Said group node 1301 is output in said hierarchical structure 603. In response to selecting (1106) said group node 1301, said dimension is reconfigured (1107, 1501).