(This material was originally posted as "supplementary material" on the web page of the journal Perception. Unfortunately, the link doesn't exist anymore: http://www.perceptionweb.com/perc0698/troje.html)
Demonstration of the illumination induced apparent orientation shift
(1) The first animation shows two images of the
same face in the same orientation (30 deg with respect to the frontal
view). As the position of the light source changes from 30 deg left to
30 deg right with respect to the line of sight, the perceived
orientation of the head changes, too.
(2) The illumination induced apparent orientation
shift can be quantified in terms of the physical orientation shift that
has to be introduced to compensate for the illumination induced effect.
The second animation shows a face in two different orientations 9 deg
apart (25.5 and 34.5 deg). The light source is at the same position in
both images. The orientation shift is clearly visible.
(3) This animation shows the face in the same two
orientations (25.5 and 34.5 deg) as in (2). However,
this time the position of the light source also changes (+30 and -30
deg). No orientation shift is perceived.
(4) The last animation shows the face in the same
two orientations and with the same two light source positions as in (3).
The only difference is the colour of the background. Now the
orientation change of the face is clearly visible.