1. A computer system for permitting interaction between a user of the computer system and a three-dimensional computer model, comprising: a processor for storing the model, a mapping defining a geometrical correspondence between portions of the model and respective portions of a real world workspace, and data defining an editing box of the workspace; display means for generating an image having an apparent location within the editing volume, the image being an image of the section of the model within the editing volume according to the mapping; at least one first input device for operation by the first hand of a user to input first signals to the processor, the processor being arranged to change both the editing volume and the geometrical correspondence between the model and the workspace, to change the section of the model within the editing volume, and at least one second input device for operation by the second hand of the user to input second signals to the processor, the processor being arranged to modify the model based on the second signals.