1. A computer-implemented 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 volume data set comprising voxels arranged in rows along first, second and third directions; 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 the separation between pixels; and applying a warp mapping transform to transform the intermediate image to the output image; and wherein the plane of the intermediate image is parallel to the plane of the output image, and the step of projecting the volume data set along the view direction comprises: determining an intermediate mapping transform for transforming the volume data set to an intermediate volume data set in which the voxels are arranged in rows running parallel to the first intermediate-image axis, the second intermediate-image axis and the view direction; applying the intermediate mapping transform to the volume data set; and projecting the resultant intermediate volume data set along the view direction.