Contrast statistics for foveated visual systems: fixation selection by minimizing contrast entropy

Abstract

The human visual system combines a wide field of view with a high-resolution fovea and uses eye, head, and body movements to direct the fovea to potentially relevant locations in the visual scene. This strategy is sensible for a visual system with limited neural resources. However, for this strategy to be effective, the visual system needs sophisticated central mechanisms that efficiently exploit the varying spatial resolution of the retina. To gain insight into some of the design requirements of these central mechanisms, we have analyzed the effects of variable spatial resolution on local contrast in 300 calibrated natural images. Specifically, for each retinal eccentricity (which produces a certain effective level of blur), and for each value of local contrast observed at that eccentricity, we measured the probability distribution of the local contrast in the unblurred image. These conditional probability distributions can be regarded as posterior probability distributions for the "true" unblurred contrast, given an observed contrast at a given eccentricity. We find that these conditional probability distributions are adequately described by a few simple formulas. To explore how these statistics might be exploited by central perceptual mechanisms, we consider the task of selecting successive fixation points, where the goal on each fixation is to maximize total contrast information gained about the image (i.e., minimize total contrast uncertainty). We derive an entropy minimization algorithm and find that it performs optimally at reducing total contrast uncertainty and that it also works well at reducing the mean squared error between the original image and the image reconstructed from the multiple fixations. Our results show that measurements of local contrast alone could efficiently drive the scan paths of the eye when the goal is to gain as much information about the spatial structure of a scene as possible.

A robust perception based method for iris tracking

“…Several neurological studies proved that during the observation process few points attract human attention and those few points are necessary for understanding the scene content in the early vision [18,27,30]. Those fixation points depend on both the image subject and the luminance and contrast characteristics of the scene content [18,27,11].…”

“…In addition they might be not robust to occlusions or changes in illumination or scale. One of the most interesting neurological results is that human eyes are attracted by very few points (regions) of the observed scene [18,27,30]. In addition, human eye reacts to a moving stimulus which can be modeled as a combination of luminance modulated (first order motion) and contrast modulated (second order motion) sinusoidal stimulus [36,23,19,29].…”

Video quality assessment using a statistical model of human visual speed perception

“…First, we believe that the human visual system (HVS) is an efficient encoder or information extractor (subject to certain physical constraints such as power consumption), as widely hypothesized in computational vision science [16]. To achieve such efficiency, it is natural to assume that the areas in the visual scene that contain more information should be more likely to attract visual attention and fixations [17,18]. Such information content can be quantified using statistical information theory, provided that a statistical model about the information source is available.…”

Fixation selection by maximization of texure and contrast information

“…Specifically, we verify this hypothesis for the case of textural and contrast information. In continuation of our prior work on optimal contrast-based fixations [1], we develop, in Section 2, an optimum texturebased fixation strategy based on our recent theory of nonstationarity detection in natural images [2]. These two strands of work give us visual fixation patterns that optimally extract, respectively, contrast and textural information from natural scenes.…”