At the Helmholtz Instituut
http://www.phys.uu.nl/~wwwpm/HumPerc/index.html#vacancy), department
Physics of Man, of Utrecht University (The Netherlands) we have a postdoc
position available for a period of three years, starting in September 2001.
The topic is "3D shape and material properties for recognition". The lab is
headed by Jan Koenderink and Astrid Kappers. Interested persons should
contact one of us by e-mail (a.m.l.kappers@phys.uu.nl,
j.j.koenderink@phys.uu.nl).
Short project description:
Shading and apparent contour as 3D cue
We will use the gauge figure method to sample local surface orientation and
the correspondence method to find corresponding points in different images
(e.g. due to different illumination or background) of the same object. The
stimuli will be shaded objects that contain no obvious landmark attached to
the object. In psychophysical experiments both photographs of real objects
and computer-generated images will be used. In physiological experiments
only the computer images will be used.
In a first experiment we will use pairs of images (lambertian surfaces) in
which either the contour is changed but the luminance gradients remain
identical (change of viewing direction) or the contour is the same but the
luminance gradients are changed (change in type or direction of
illumination). In the gauge experiments we will compare the pictorial
relief for the two images and ascertain whether or not the location of the
hyperbolic regions remain constant. In the correspondence experiments we
will verify whether or not correspondence is closest (scatter minimum) for
the parabolic lines (which can be inferred both from the apparent contour
and the luminance gradients).
In the second experiment the surfaces will be non-lambertian with a marked
highlight. Again pairs of images will be used and either direction of view
or illumination will be changed. Notice that the highlight will change in
either case. Both methods (gauge and correspondence) will be used as in
experiment one.
In a third experiment images of single objects will be tested using the
gauge method. For each point of the triangulation the depth is plotted
either for two different observers, repeated testing of the same subject,
or different illumination of the same object same observer. This
correlation plot has either single or multiple branches, which frequently
correspond to different parts of the object image. Thus multiple branches
in the correlation plots, if they are consistent across the multiple
replications (many pairs of subjects, many replications in a given subject,
many pairs of illumination conditions in a given subject), will provide
strong evidence for the presence of multiple, more or less independent
parts in the mental representation of the object. The expectation is that
these parts will be defined by the parabolic lines.
Material properties
In a first experiment we will use the patches of the Curet database to test
whether or not human observers can judge the direction of illumination from
texture. This is crucial: if the observer cannot abstract the direction of
illumination, he cannot use the micro-gradients (locally) as cues for
material. In the unlikely outcome that this proves true for all materials,
it would become unattractive to use the Curet patches in the physiological
experiments.
It is more likely that this will true for certain classes of material,
depending on the accompanying illumination. In this case one should compare
human judgements about direction of illumination, viewing direction and
material. The experiment can be repeated with histogram-transformed
textures and locally disordered textures to investigate the relative
importance of the two aspects of micro-gradients.
In a second experiment we will use spherical objects and test whether or
not humans can discriminate between pairs of materials. This can be tested
under a variety of illumination and viewing directions and with removal of
cues. First remove highlight only, then remove both highlight and
microgradients.
Helmholtz Instituut, Department Physics of Man
Princetonplein 5
3584 CC Utrecht
The Netherlands
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