Visual slant underestimation: a general model.

A general model of visual slant underestimation is presented. It is based on the idea that two specific types of perceptual error occur in the evaluation of the slant angle by the observer. The reason for these errors occurring is postulated to be that reduced viewing conditions result in the deviation of the observer's perceived straight-ahead direction from the true direction. Specifically this deviation is postulated to be in the direction of the nearest part of the surface in accord with conditions that exist in our everyday environment. In the case of a slanted rectangle correct registration of the projected length of half of the surface and the correct registration of the appropriate angle of convergence will result in perception being veridical. A mechanism is outlined which indicates how both of these factors can be in error, and an equation is developed which enables the predicted slant estimates to be calculated given the dimensions of the rectangle and its distance from the eye. Equations for the case of slanted surfaces viewed through apertures are also developed. The model is assessed in relation to past slant-perception experiments and is found to be a good predictor of the large amount of previously unexplained underestimation that occurred in these studies.