Impossible shadows and lightness constancy

The intersection between an illumination and a reflectance edge is characterised by the `ratio-invariant' property, that is the luminance ratio of the regions under different illumination remains the same. In a CRT experiment, we shaped two areas, one surrounding the other, and simulated an illumination edge dividing them in two frames of illumination. The portion of the illumina- tion edge standing on the surrounding area (labelled contextual background) was the contextual edge, while the portion standing on the enclosed area (labelled mediating background) was the mediating edge. On the mediating background, there were two patches, one per illumination frame. Observers were asked to adjust the luminance of the patch in bright illumination to equate the lightness of the other. We compared conditions in which the luminance ratio at the contextual edge could be (i) equal (possible shadow), or (ii) larger (impossible shadow) than that at the mediating edge. In addition, we manipulated the reflectance of the backgrounds. It could be higher for the contextual than for the mediating background; or, vice versa, lower for the contextual than for the mediating background. Results reveal that lightness constancy significantly increases when: (i) the luminance ratio at the contextual edge is larger than that at the mediating edge creating an impossible shadow, and (ii) the reflectance of the contextual background is lower than that of the mediating one. We interpret our results according to the albedo hypothesis, and suggest that the scission process is facilitated when the luminance ratio at the contextual edge is larger than that at the mediating edge and/or the reflectance of the including area is lower than that of the included one. This occurs even if the ratio-invariant property is violated

Molecular Line Profile Fitting with Analytic Radiative Transfer Models

We present a study of analytic models of starless cores whose line profiles have ``infall asymmetry,'' or blue-skewed shapes indicative of contracting motions. We compare the ability of two types of analytical radiative transfer models to reproduce the line profiles and infall speeds of centrally condensed starless cores whose infall speeds are spatially constant and range between 0 and 0.2 km s-1. The model line profiles of HCO+ (J=1-0) and HCO+ (J=3-2) are produced by a self-consistent Monte Carlo radiative transfer code. The analytic models assume that the excitation temperature in the front of the cloud is either constant (``two-layer'' model) or increases inward as a linear function of optical depth (``hill'' model). Each analytic model is matched to the line profile by rapid least-squares fitting. The blue-asymmetric line profiles with two peaks, or with a blue shifted peak and a red shifted shoulder, can be well fit by the ``HILL5'' model (a five parameter version of the hill model), with an RMS error of 0.02 km s-1. A peak signal to noise ratio of at least 30 in the molecular line observations is required for performing these analytic radiative transfer fits to the line profiles.Comment: 48 pages, 20 figures, accepted for publication in Ap

Tracking objects with point clouds from vision and touch

We present an object-tracking framework that fuses point cloud information from an RGB-D camera with tactile information from a GelSight contact sensor. GelSight can be treated as a source of dense local geometric information, which we incorporate directly into a conventional point-cloud-based articulated object tracker based on signed-distance functions. Our implementation runs at 12 Hz using an online depth reconstruction algorithm for GelSight and a modified second-order update for the tracking algorithm. We present data from hardware experiments demonstrating that the addition of contact-based geometric information significantly improves the pose accuracy during contact, and provides robustness to occlusions of small objects by the robot's end effector