In their desire to create a unique artistic medium Russian-born French filmmaker Alexandre Alexeïeff and his collaborator Claire Parker conceived the pinscreen, a rectangular white screen into which hundreds of thousands of headless pins are inserted. By retracting or pushing out groups of pins and adjusting light sources, Alexeïeff discovered that all possible shades of gray could be achieved and that the resulting three-dimensional shapes created the effect of an animated engraving. The process is extraordinarily difficult and time-consuming; Canadian filmmaker Jacques Drouin is the only animator other than Alexeïeff to have mastered the medium.
The prologue scenes of Orson Welles film 'The Trial' were created by Alexandre Alexeïeff and Claire Parker using the pinscreen. It's a common misconception that these prologue scenes were charcoal sketches drawn by Orson Welles himself. This just shows how much depth of shade and engraving-like quality that could be achieved with the pinscreen.
I've included a video of the prologue to 'The Trial' and a video of Alexandre Alexeïeff demonstrating the complexity of the pinscreen process.