1. A method of generating a two-dimensional output image of a volume data set from a selectable view point and view direction in a slab multi-planar reformatting (MPR) rendering process, the method comprising: providing a slab MPR volume data set comprising voxels arranged in rows along first, second and third directions; and projecting the volume data set along the view direction on to a plane to form an intermediate image having pixels arranged along first and second intermediate-image axes in which the projection of neighbouring voxel centres along two of the first, second and third directions are separated by respective integer numbers of pixels; and applying a warp mapping transform to transform the intermediate image to the output image.