Lightness, brightness and transparency: A quarter century of new ideas, captivating demonstrations and unrelenting controversy

Abstract

The past quarter century has witnessed considerable advances in our understanding of Lightness (perceived reflectance), Brightness (perceived luminance) and perceived Transparency (LBT). This review poses eight major conceptual questions that have engaged researchers during this period, and considers to what extent they have been answered. The questions concern 1. the relationship between lightness, brightness and perceived non-uniform illumination, 2. the brain site for lightness and brightness perception, 3 the effects of context on lightness and brightness, 4. the relationship between brightness and contrast for simple patch-background stimuli, 5. brightness "filling-in", 6. lightness anchoring, 7. the conditions for perceptual transparency, and 8. the perceptual representation of transparency. The discussion of progress on major conceptual questions inevitably requires an evaluation of which approaches to LBT are likely and which are unlikely to bear fruit in the long term, and which issues remain unresolved. It is concluded that the most promising developments in LBT are (a) models of brightness coding based on multi-scale filtering combined with contrast normalization, (b) the idea that the visual system decomposes the image into "layers" of reflectance, illumination and transparency, (c) that an understanding of image statistics is important to an understanding of lightness errors, (d) Whittle's logW metric for contrast-brightness, (e) the idea that "filling-in" is mediated by low spatial frequencies rather than neural spreading, and (f) that there exist multiple cues for identifying non-uniform illumination and transparency. Unresolved issues include how relative lightness values are anchored to produce absolute lightness values, and the perceptual representation of transparency. Bridging the gap between multi-scale filtering and layer decomposition approaches to LBT is a major task for future research.

Experimental Observations of the Visibility Threshold of Illusory effects in Hermann Grid, Sinusoidal and Square Gratings and their Possible Implications

“…Such linear scaling behavior has also been observed previously for Mach Bands [17]. Several researchers are also attempting to solve the problem of scale integration from other perspectives [18,19]. Experiment 1, on the other hand, is quite a remarkable and novel one that shows us that even small changes to the shape of the grid squares are sufficient to destroy the illusory effect even without distorting the straightness of the larger squares themselves.…”

Bioplausible Multiscale Filtering in Retinal to Cortical Processing as a Model of Computer Vision

“…This nonlinearity results in flattening the illumination component and makes it possible for the eye to see under poor light condition (Smith, 2003). Both nonlinearity and processing done in layer2 of the retina seems to be important clues for addressing brightness lightness illusions (Kingdom, 2011), as well as tilt illusions which the latter haven't been broadly studied like the former.…”

“…As an example, the ODoG model presented by Blakeslee and McCourt (2004) claimed to be a parsimonious model for brightness induction illusions (Kingdom, 2011) with the ability to predict both the illusion magnitude as well as its orientation.…”

Vision as a Fundamentally Statistical Machine

“…Thus, the same amount of light arising from a given region in a scene can elicit dramatically different brightness percepts when presented in different contexts ( Fig. 1) (Kingdom, 2011). For example, in Fig.1 (a), the central square (T) in the left panel appears brighter than the same target in the right panel.…”

“…A variety of explanations have been suggested since the basis for such phenomena was first debated by Helmholtz, Hering, Mach, and others (Gichrist et al, 1999;Purves et al, 2004;Kingdom, 2011). Although lateral inhibition in the early visual processing has often been proposed to account for these "illusions", this mechanism cannot explain instances in which similar overall contexts produce different brightness effects (compare Fig.…”