Cornsweet surfaces for selective contrast enhancement

A typical goal when enhancing the contrast of images is to increase the perceived contrast without altering the original feel of the image. Such contrast enhancement can be achieved by modelling Cornsweet profiles into the image. We demonstrate that previous methods aiming to model Cornsweet profiles for contrast enhancement, often employing the unsharp mask operator, are not robust to image content. To achieve robustness, we propose a fundamentally di erent vector-centric approach with Cornsweet surfaces. Cornsweet surfaces are parametrised 3D surfaces (2D in space, 1D in luminance enhancement) that are extruded or depressed in the luminance dimension to create countershading that respects image structure. In contrast to previous methods, our method is robust against the topology of the edges to be enhanced and the relative luminance across those edges. In user trials, our solution was significantly preferred over the most related contrast enhancement method.Kosinka was funded by EPSRC grant EP/H024816/1. Lieng was funded by a scholarship from the Norwegian Government.This is the accepted manuscript. The final version is available from Elsevier at http://www.sciencedirect.com/science/article/pii/S0097849314000405

Surface modelling for 2D imagery

Vector graphics provides powerful tools for drawing scalable 2D imagery. With the rise of mobile computers, of different types of displays and image resolutions, vector graphics is receiving an increasing amount of attention. However, vector graphics is not the leading framework for creating and manipulating 2D imagery. The reason for this reluctance of employing vector graphical frameworks is that it is difficult to handle complex behaviour of colour across the 2D domain. A challenging problem within vector graphics is to define smooth colour functions across the image. In previous work, two approaches exist. The first approach, known as diffusion curves, diffuses colours from a set of input curves and points. The second approach, known as gradient meshes, defines smooth colour functions from control meshes. These two approaches are incompatible: diffusion curves do not support the local behaviour provided by gradient meshes and gradient meshes do not support freeform curves as input. My research aims to narrow the gap between diffusion curves and gradient meshes. With this aim in mind, I propose solutions to create control meshes from freeform curves. I demonstrate that these control meshes can be used to render a vector primitive similar to diffusion curves using subdivision surfaces. With the use of subdivision surfaces, instead of a diffusion process, colour gradients can be locally controlled using colour-gradient curves associated with the input curves. The advantage of local control is further explored in the setting of vector-centric image processing. I demonstrate that a certain contrast enhancement profile, known as the Cornsweet profile, can be modelled via surfaces in images. This approach does not produce saturation artefacts related with previous filter-based methods. Additionally, I demonstrate various approaches to artistic filtering, where the artist locally models given artistic effects. Gradient meshes are restricted to rectangular topology of the control meshes. I argue that this restriction hinders the applicability of the approach and its potential to be used with control meshes extracted from freeform curves. To this end, I propose a mesh-based vector primitive that supports arbitrary manifold topology of the mesh

Color Perception in Natural Images

This review of color processing in natural image viewing – rather than artificial laboratory images – addresses the role of color edges. Much of the color variation in nature is a result of evolutionary processes in complex organisms that have developed eye-brain systems that use color signals for a variety of biological functions. One aspect of human color processing is the tendency to attribute the appearance of extended color fields to a process of filling-in from the differential color signals at color edges, where one color transitions to another. Does such a process account for the appearance of extended color fields in natural images? Some form of color filling-in must underlie the color filling-in percept known as the Watercolor Effect, but this effect is too weak to account for the appearance of extended color fields in natural images. Moreover, natural images do not look very colorful when their color is restricted to edge transitions. Conversely, purely chromatic images with maximally graded (‘edgeless’) transitions look fully colorful, leading to the conclusion that color filling-in makes no more than a minor contribution to the appearance of extended color regions in natural images. Other effects, such as the selective enhancement of perceived image color by luminance contours coordinated with the color contours and color image structure, also play a role the color perception of natural images

Microsaccade directions do not predict directionality of illusory brightness changes of overlapping transparent surfaces

AbstractTse (2005) recently introduced a new class of illusory brightness changes where shifts of attention lead to shifts in perceived brightness across overlapping, transparent figures, under conditions of visual fixation. In the absence of endogenous attentional shifts, illusory brightness changes appear to shift from figure to figure spontaneously, much as occurs in other multistable phenomena. The goal of the present research is to determine whether fixational microsaccades are correlated with perceived brightness changes. It has recently been demonstrated that microsaccades can reveal the direction of covert attentional shifts either toward (Engbert, R. & Kliegl, R. (2003). Microsaccades uncover the orientation of covert attention. Vision Research, 43, 1035–1045; Hafed, Z. M. & Clark, J. J. (2002). Microsaccades as an overt measure of covert attention shifts. Vision Research, 42(22), 2533–2545) or away from (Rolfs, M., Engbert, R., & Kliegl, R. (2004). Microsaccade orientation supports attentional enhancement opposite a peripheral cue: commentary on Tse, Sheinberg, and Logothetis (2003). Psychological Science, 15(10), 705–707) a peripheral cue under certain circumstances. Others (Horwitz, G. D. & Albright, T. D. (2003). Short-latency fixational saccades induced by luminance increments. Journal of Neurophysiology, 90(2), 1333–1339; Tse, P. U., Sheinberg, D. L., & Logothetis, N. K. (2002). Fixational eye movements are not affected by abrupt onsets that capture attention. Vision Research, 42, 1663–1669; Tse, P. U., Sheinberg, D. L., & Logothetis, N. K. (2004). The distribution of microsaccade directions need not reveal the location of attention. Psychological Science, 15(10), 708–710) found no change in the distribution of microsaccade directions as a function of where attention is allocated, although changes in the rate of microsaccades were observed in all of these studies in response to the onset of attentional reallocation. It is therefore possible that the distribution of microsaccade directions will change as a function of which figure is perceived to darken, or that changes in this distribution predict which figure will subsequently darken. We find no correlation between this distribution and which figure undergoes the effect, and therefore conclude that microsaccade directionality is not influenced by and does not influence which figure undergoes the effect. Moreover, the directions of microsaccades that occur immediately prior to a perceptual switch are not correlated with the perceived position of the figure that undergoes the effect. However, we do find that the rate of microsaccades decreases upon a perceptual switch, signifying an attentional shift coincident with the perceptual shift. We conclude that microsaccade directionality does not determine, predict, or cause which figure will subsequently be perceived to undergo an illusory brightness change

Vision Science and Technology at NASA: Results of a Workshop

A broad review is given of vision science and technology within NASA. The subject is defined and its applications in both NASA and the nation at large are noted. A survey of current NASA efforts is given, noting strengths and weaknesses of the NASA program

Precision modulation in predictive coding hierarchies: theoretical, behavioural and neuroimaging investigations

Estimation of uncertainty is an important aspect of perception and a prerequisite for effective action. This thesis explores the implementation of uncertainty estimation as precision modulation within a predictive coding hierarchy, optimised within a neurbiologically-plausible message-passing scheme via the minimisation of free-energy. This thesis consists of six chapters. The first presents a new model of a classic visual illusion, the Cornsweet illusion, which demonstrates that the Cornsweet illusion is a natural consequence of Bayes-optimal perception under the free-energy principle, and demonstrates that increasing contrast can be modelled by increasing signal-to-noise ratio. The second chapter describes dynamic causal modelling of EEG data collected from participants viewing the Cornsweet illusion, demonstrating that a reduction in precision, or superficial pyramidal cell gain, in lower visual hierarchical levels, is sufficient to explain contrast-dependent changes in ERPs. The third describes a model of a simple attentional paradigm – the Posner paradigm – recasting attention as the optimal modulation of precision in sensory channels. The fourth describes an MEG study of the Posner paradigm, using Bayesian model selection to explore the role of changes in backwards and modulatory connections and changes in local superficial pyramidal cell gain in producing the electrophysiological and behavioural correlates of the Posner paradigm. The fifth chapter recasts the Posner paradigm in the motor domain to investigate the level (intrinsic vs. extrinsic) of precision modulation by motor cues. The sixth describes a new model of sensory attenuation based on using precision modulation to balance the imperatives to act and perceive. I hope to demonstrate that precision modulation within predictive coding hierarchies, under the free-energy principle, is a flexible and powerful way of describing and explaining both behavioural and neuroimaging data

Multiresolution wavelet framework models brightness induction effects

A new multiresolution wavelet model is presented here, which accounts for brightness assimilation and contrast effects in a unified framework, and includes known psychophysical and physiological attributes of the primate visual system (such as spatial frequency channels, oriented receptive fields, contrast sensitivity function, contrast non-linearities, and a unified set of parameters). Like other low-level models, such as the ODOG model [Blakeslee, B., & McCourt, M. E. (1999). A multiscale spatial filtering account of the white effect, simultaneous brightness contrast and grating induction. Vision Research, 39, 4361-4377], this formulation reproduces visual effects such as simultaneous contrast, the White effect, grating induction, the Todorović effect, Mach bands, the Chevreul effect and the Adelson-Logvinenko tile effects, but it also reproduces other previously unexplained effects such as the dungeon illusion, all using a single set of parameters