A contrast ratio constraint on stereo matching

Stereopsis employs differences in the location of features in the two eyes to reconstruct their relative depths. Computational models largely ignore the contrast of these features; they simply require them to be visible in each eye and to possess the same contrast polarity. With a competitive matching paradigm we show that only features with a certain ratio of contrasts in the two eyes match. Increasing the contrast in one eye requires proportionally more contrast in the other eye for matching. This contrast relation exactly parallels the relation found for dichoptic masking, which behaves unlike any other form of contrast masking. A strange consequence of this contrast ratio constraint is that a feature may be monocularly visible, yet unmatched because the contrast ratio has not been satisfied. In this case features are seen as faint unmatched 'ghosts' near the plane of fixation. In a competitive matching situation then, stereopsis acts as if it imposes a contrast threshold on matches; features failing to exceed the threshold remain unmatched. This is a simple and biologically plausible way for the system to eliminate false matches and to reduce matching ambiguity.