Visual effects with a miniature Leonardo's window: photographs and real scenes fused stereoscopically

The drawing on a conventional Leonardo's window will be seen by both of an observer's eyes unless one of them is occluded. But, by substituting a photographic colour transparency (in a suitably compact viewer) for the drawing, one eye can view the transparency whilst the other eye gains an unobstructed direct view of the scene. The transparency can then be stereoscopically fused with the directly-viewed scene. As the perspective of the transparency depends not on the observer's viewpoint but only on the position of the camera at the time of photography, altering the viewpoint causes the effective eye-base to vary, thus enhancing, suppressing, or sometimes even reversing the stereoscopic depth. With very simple equipment several visual effects can easily be demonstrated. Practical applications of the technique have been found, particularly in those engineering and geological situations in which it is useful to detect changes since a photograph was taken.