The display step calculates a surface normal at every point on the surface to shade the 3D image using Lambert's law for a diffusely reflecting surface. The shading is calculated from the gradient of the surface at each point on the surface path in each voxel. Those surface points whose gradients are perpendicular to the plane are given full pixel intensity, while those points whose gradients are parallel to the plane are given minimum intensity. The gradient is estimated by central differences along the three coordinate directions. Also, we linearly interpolate the gradient vector in three dimensions to find the gradient vector at the surface point. Finally we get the unit surface normal by normalizing the interpolated gradient vector. Each surface point and normal vector is projected onto the view plane, and is shaded to form a three-dimensional image.