Shape from shading with interreflections under proximal light source - 3D shape reconstruction of unfolded book surface from a scanner image

We address the problem to recover the 3D shape of an unfolded book surface from the shading information in a scanner image. From a technical point of view, this shape from shading problem in real world environments is characterized by (1) proximal light source, (2) interreflections, (3) moving light source, (4) specular reflection, and (5) nonuniform albedo distribution. Taking all these factors into account, we first formulate the problem based on an iterative nonlinear optimization scheme. Then we introduce piecewise polynomial models of the 3D shape. Image restoration experiments for a real book surface demonstrated that geometric and photometric distortions are almost completely removed by the proposed method</p

Equivalence of oblique and frontal illumination in perspective shape from shading

In this paper, it is shown that any oblique illumination shape-from-shading problem under perspective projection for Lambertian reflection and a single distant light source can be converted to an equivalent frontal illumination problem by a simple nonlinear intensity transformation which is equivalent to a rectification in stereo vision. Remarkably, it involves no approximation of depth. The method is evaluated on perspective shape-from-shading involving wide range of oblique angles. © 2007 IEEE.published_or_final_versio

The perception of shape from shading in a new light

How do humans see three-dimensional shape based on two-dimensional shading? Much research has assumed that a ‘light from above’ bias solves the ambiguity of shape from shading. Counter to the ‘light from above’ bias, studies of Bayesian priors have found that such a bias can be swayed by other light cues. Despite the persuasive power of the Bayesian models, many new studies and books cite the original ‘light from above’ findings. Here I present a version of the Bayesian result that can be experienced. The perception of shape-from-shading was found here to be influenced by an external light source, even when the light was obstructed and did not directly illuminate a two-dimensional stimulus. The results imply that this effect is robust and not low-level in nature. The perception of shape from shading is not necessarily based on a hard-wired internal representation of lighting direction, but rather assesses the direction of lighting in the scene adaptively. Here, for the first time, is an experiential opportunity to see what the Bayesian models have supported all along

Linear Differential Constraints for Photo-polarimetric Height Estimation

In this paper we present a differential approach to photo-polarimetric shape estimation. We propose several alternative differential constraints based on polarisation and photometric shading information and show how to express them in a unified partial differential system. Our method uses the image ratios technique to combine shading and polarisation information in order to directly reconstruct surface height, without first computing surface normal vectors. Moreover, we are able to remove the non-linearities so that the problem reduces to solving a linear differential problem. We also introduce a new method for estimating a polarisation image from multichannel data and, finally, we show it is possible to estimate the illumination directions in a two source setup, extending the method into an uncalibrated scenario. From a numerical point of view, we use a least-squares formulation of the discrete version of the problem. To the best of our knowledge, this is the first work to consider a unified differential approach to solve photo-polarimetric shape estimation directly for height. Numerical results on synthetic and real-world data confirm the effectiveness of our proposed method.Comment: To appear at International Conference on Computer Vision (ICCV), Venice, Italy, October 22-29, 201

Shape-from-shading and light source estimation in humans

Light source estimation is very important for the interpretation of shape-fromshading by humans. We used a range of methods to characterise the way in which the type, and position of the light source can influence observers’ performance in shape-from-shading tasks. Firstly, we used classification images to discover people’s priors for light source position using noise only stimuli. This cue-free approach uncovered the weakness of the light-from-above prior. We also examined the effect of varying the light source elevation on the perceived shape of isotropic and anisotropic surfaces, the impacts of lighting ambiguities on shape-from-shading and, finally, the interpretation of shadow regions. We found that lighting priors are weighted by the visual system in a way that is inversely proportional to the strength of lighting cues in the stimuli, revealing that knowledge about the light source position is critical to perceiving shapefrom- shading. Where ambiguous cues to lighting direction are present human vision seems to favour local cues over distal ones. We also showed that perceived surface shape varies with light elevation only in so far as elevation alters contrast. Finally we show that human vision does not treat shadows in the same way as objects

Sun and sky: Does human vision assume a mixture of point and diffuse illumination when interpreting shape-from-shading?

AbstractPeople readily perceive smooth luminance variations as being due to the shading produced by undulations of a 3-D surface (shape-from-shading). In doing so, the visual system must simultaneously estimate the shape of the surface and the nature of the illumination. Remarkably, shape-from-shading operates even when both these properties are unknown and neither can be estimated directly from the image. In such circumstances humans are thought to adopt a default illumination model. A widely held view is that the default illuminant is a point source located above the observer’s head. However, some have argued instead that the default illuminant is a diffuse source. We now present evidence that humans may adopt a flexible illumination model that includes both diffuse and point source elements. Our model estimates a direction for the point source and then weights the contribution of this source according to a bias function. For most people the preferred illuminant direction is overhead with a strong diffuse component

The light-from-above prior is intact in autistic children

Sensory information is inherently ambiguous. The brain disambiguates this information by anticipating or predicting the sensory environment based on prior knowledge. Pellicano and Burr (2012) proposed that this process may be atypical in autism and that internal assumptions, or “priors,” may be underweighted or less used than in typical individuals. A robust internal assumption used by adults is the “light-from-above” prior, a bias to interpret ambiguous shading patterns as if formed by a light source located above (and slightly to the left) of the scene. We investigated whether autistic children (n = 18) use this prior to the same degree as typical children of similar age and intellectual ability (n = 18). Children were asked to judge the shape (concave or convex) of a shaded hexagon stimulus presented in 24 rotations. We estimated the relation between the proportion of convex judgments and stimulus orientation for each child and calculated the light source location most consistent with those judgments. Children behaved similarly to adults in this task, preferring to assume that the light source was from above left, when other interpretations were compatible with the shading evidence. Autistic and typical children used prior assumptions to the same extent to make sense of shading patterns. Future research should examine whether this prior is as adaptable (i.e., modifiable with training) in autistic children as it is in typical adults

A cue-free method to probe human shape from shading

People readily perceive patterns of shading as 3-D shapes. Owing to the generalised bas-relief ambiguity when extracting shape from shading, people must simultaneously estimate the shape of the surface and the nature of the light source. In many cases cues in the image will be insufficient to resolve all of the ambiguities present, and in such cases the human visual system may employ one of a number of prior assumptions based on ecology and experience. One such assumption is the lighting-from-above prior. Here, in the absence of extrinsic cues to lighting direction, ambiguous shading patterns are interpreted as if lit by a light source that is above the observer’s head. Studies of this prior typically use ambiguous stimuli and observe perceptual biases. A degree of cueing is inherent to such methods. Participants see the shaded stimuli repeatedly and are asked to make shape judgments about them regardless of whether or not they actually perceive any 3-D shape. We wanted to access people’s lighting prior more directly by establishing the template they would employ to detect a shaded object in the absence of any visual cue to object shape. To this end, we adopted a classification image approach

A three-dimensional wire-feeding model for heat and metal transfer, fluid flow, and bead shape in wire plasma arc additive manufacturing

A three-dimensional wire-feeding model has been developed to study the transient coupling behaviour of heat and metal transfer, fluid flow, and solidified bead shape in wire plasma arc additive manufacturing (WPAAM). A novel surface heat source model considering the arc energy shading effect is proposed and adopted. An improved momentum source of the arc force considering the arc pressure shading effect is also developed and used. This model has been used to study the metal transfer dynamics, flow patterns, and bead shape of the WPAAM process with a wire-feeding speed (WFS) of 1–5 m/min. The simulated results agreed reasonably with the experimental data. As the WFS increased from 1 to 5 m/min, three different metal transfer modes were observed, which changed from globular droplet mode to droplet-liquid bridge mode and solid-liquid bridge mode. Detailed metal transfer information was analysed, including metal transfer position, shape, average temperature, and main driving force. The effects of the arc shading and metal transfer on the melt pool dynamics and bead shape were simulated and discussed. A periodic flow pattern of the melt pool produced by the metal transfer impact causes ripples or even humping defects. As the WFS increased, the melt pool depression gradually disappeared due to the arc pressure shading effect. When the WFS increased to 5 m/min, a temperature drop of about 140 K in the central melt pool, caused by the arc energy shading effect and cold metal transfer, weakened the lateral flow significantly, which explained the decrease of bead width at a large WFS. The results demonstrate that the developed wire-feeding model and findings could be used as a theoretical tool and basis to better understand the underlying physical mechanisms and achieve bead shape control in the WAAM process