3D halftoning is a new technique that allows the approximation of digital volumetric objects of varying material density e.g. porous media for example, by an ensemble of binary material volume elements called vels. In theory, 3D halftoning is basically an extension of the well known 2D halftoning process, as widely used in binary printing applications. In practice, however, the development of 3D halftoning algorithms is strongly related to hardware specific boundary conditions, such as particular characteristics of additive volumetric object manufacturing procedures. This paper addresses theoretical as well as practical aspects of 3D halftoning that allow the rendition of digital volumetric objects of varying density using the stereolithographic additive fabrication technique. An ultimate application of 3D halftoning is the reproduction of volumetric objects in medicine that consist of a mixture of bone, cartilage and soft-tissues, for example.