1. A shading method for rendering volumetric data having a plurality of pixels, each of the plurality of pixels having a scalar value, comprising: direct rendering of the volumetric data, including the steps of: determining a partial derivative with respect to the x direction of a first of the plurality of pixels in a screen-space; determining a partial derivative with respect to the y direction of the first of the plurality of pixels in the screen-space; determining a partial derivative with respect to a z direction of the first of the plurality of pixels in an object-space; and determining a gradient based on the partial derivative with respect to the x direction, based on the partial derivative with respect to the y direction and based on the partial derivative with respect to the z direction.