The perceptual scale shows that relative motion enhanced both transparency and opacity, depending on which layers were moving relative to each other, acting more like a potentiating agent than a cue, similar to its role in distinguishing reflections from paint (
Doerschner et al., 2011). One possibility is that motion could be enhancing the effects of X-junctions and T-junctions by increasing the displayed number or their salience. Relative salience versus relative validity has been studied in associative learning and navigation (
Kahnt et al., 2014;
Leathers & Olson, 2012), but not yet in visual cue combination. Decreasing the sizes of ellipses in the displays would increase the number of X- or T-junctions, whereas increasing the sizes would increase the salience, but neither seems to have much effect on verbal ratings of transparency perception (
Falkenberg & Faul, 2019). Instead, the Dynamic T-junctions condition, which is a novel contribution of this study, demonstrates that the main role of relative motion is increasing the likelihood of perceiving separate layers that do not share a common fate. In pointing out the role of certain shape changes in promoting impressions of transparency, particularly for shadows,
Metzger (1936) mentions in a footnote that the law of common fate would add a contribution to transparency perception in a movie, but without presenting details of the conditions or evidence, and it is highly unlikely that he was imagining a condition where common fate conflicts with other cues that are voting against transparency. The role we demonstrate for motion defined common fate explains motion enhancing effects in previous studies (
D'Zmura et al., 2000;
Falkenberg & Faul, 2019;
Gerardin et al., 2006;
Khang & Zaidi, 2002a), but goes beyond them in showing that common fate can override the information provided by junctions, which are otherwise quite powerful factors (
Anderson, 1997). In fact, by replacing ellipses with rectangles in the overlaid surface (
Figure 9), we demonstrate that, because of common fate, transparency can still be seen despite junction, color, and pattern information all being otherwise incompatible with transparency.