Colour Relations in Form

The orthodox monadic determination thesis holds that we represent colour relations by virtue of representing colours. Against this orthodoxy, I argue that it is possible to represent colour relations without representing any colours. I present a model of iconic perceptual content that allows for such primitive relational colour representation, and provide four empirical arguments in its support. I close by surveying alternative views of the relationship between monadic and relational colour representation

The effects of illumination direction on the perception of 3D shape from shading

3rd Place at the Denman Undergraduate Research Forum 2016A fundamental problem for the perception of 3D shape from shading is to achieve some level of constancy over variations in the pattern of illumination. The present research was designed to investigate how changes in the direction of illumination influence the apparent shapes of surfaces. The stimuli included images of 3D surfaces with Lambertian reflectance functions that were illuminated by a rectangular area light source. The direction of illumination was systematically manipulated. Observers judged the 3D shapes of these surfaces by marking local depth minima and maxima along three designated scan lines using a hand-held mouse. The results revealed that the local depth maxima were shifted slightly toward the direction of illumination, while the local depth minima were shifted slightly away from the direction of illumination. However, these changes were much smaller than what would be expected based on differences in the pattern of luminance among the stimulus images. These findings demonstrate that there is a substantial amount of illumination constancy in the perception of 3D shape from shading, but that it is not perfect. Several hypotheses are considered about how this constancy could potentially be achieved.No embargoAcademic Major: Psycholog

Modelling the human perception of shape-from-shading

Shading conveys information on 3-D shape and the process of recovering this information is called shape-from-shading (SFS). This thesis divides the process of human SFS into two functional sub-units (luminance disambiguation and shape computation) and studies them individually. Based on results of a series of psychophysical experiments it is proposed that the interaction between first- and second-order channels plays an important role in disambiguating luminance. Based on this idea, two versions of a biologically plausible model are developed to explain the human performances observed here and elsewhere. An algorithm sharing the same idea is also developed as a solution to the problem of intrinsic image decomposition in the field of image processing. With regard to the shape computation unit, a link between luminance variations and estimated surface norms is identified by testing participants on simple gratings with several different luminance profiles. This methodology is unconventional but can be justified in the light of past studies of human SFS. Finally a computational algorithm for SFS containing two distinct operating modes is proposed. This algorithm is broadly consistent with the known psychophysics on human SFS

Stereo improves 3D shape discrimination even when rich monocular shape cues are available

We measured the ability to discriminate 3D shapes across changes in viewpoint and illumination based on rich monocular 3D information and tested whether the addition of stereo information improves shape constancy. Stimuli were images of smoothly curved, random 3D objects. Objects were presented in three viewing conditions that provided different 3D information: shading-only, stereo-only, and combined shading and stereo. Observers performed shape discrimination judgments for sequentially presented objects that differed in orientation by rotation of 0--60-in depth. We found that rotation in depth markedly impaired discrimination performance in all viewing conditions, as evidenced by reduced sensitivity (dV ) and increased bias toward judging same shapes as different. We also observed a consistent benefit from stereo, both in conditions with and without change in viewpoint. Results were similar for objects with purely Lambertian reflectance and shiny objects with a large specular component. Our results demonstrate that shape perception for random 3D objects is highly viewpoint-dependent and that stereo improves shape discrimination even when rich monocular shape cues are available