  In the first course of general relativity, the Schwarzschild spacetime is the
most discussed analytic solution to Einstein's field equations. Unfortunately,
there is rarely enough time to study the optical consequences of the bending of
light for some advanced examples. In this paper, we present how the visual
appearance of a thin disc around a Schwarzschild black hole can be determined
interactively by means of an analytic solution to the geodesic equation
processed on current high performance graphical processing units. This approach
can, in principle, be customized for any other thin disc in a spacetime with
geodesics given in closed form. The interactive visualization discussed here
can be used either in a first course of general relativity for demonstration
purposes only or as a thesis for an enthusiastic student in an advanced course
with some basic knowledge of OpenGL and a programming language.
