1. A computer-implemented walkthrough process for providing images of a surrounding scene to a viewer, said process comprising using a computer to perform the following process actions: defining a virtual walkthrough space in which the viewer can select a viewing position on a viewing plane therein for viewing the scene; dividing the viewing plane of the walkthrough space into a sectioned grid comprising a plurality of cells; assigning to each cell of the grid at least one source of image data from which an image of a part or all of the surrounding scene as viewed from that cell can be rendered, wherein each of said image data sources assigned to a cell is associated with at least one point within the cell; and whenever the viewer selects a viewing position, providing an image to the viewer depicting at least a part of the scene as would be viewed from the cell containing the selected viewing position.