Mechanisms for image processing are provided. A computing device comprises a memory, an input device, a display, and a processor. The processor is configured to: acquire a three-dimensional image of an anatomical structure and store it in the memory. The processor renders on the display (i) an initial volume of the three-dimensional image corresponding to an initial portion of the anatomical structure, and (ii) a moveable control element. The initial volume has an outer surface defined by a position of the control element. The processor receives input data updating the position of the control element relative to the initial volume； and renders on the display, in place of the initial volume, a further volume of the three-dimensional image, corresponding to a further portion of the anatomical structure and having a further outer surface defined by the updated position of the control element.
