We address the task of rendering by ray tracing the isosurface of a high-quality continuous spline model of volumetric discrete and regular data. By expressing the spline model as a sum of nonnegative B-splines, we are able to confine the potential location of the isosurface within a thin binary shell. We then show how to use the space-embedding property of splines to further shrink this shell to essentially a single-voxel width. We also propose a new illumination model that highlights the outline of the rendered isosurface, which provides for a sensitive test of the perceived quality of the rendering. We present experiments to support our claims, along with an efficient algorithm to compute simultaneously an array of B-splines and of its derivatives.