Lightness, illumination, and gradients

Abstract

The illumination interpretation approach claims that lightness illusions can be explained as misapplications of lightness constancy mechanisms, processes which usually enable veridical extraction of surface reflectance from luminance distributions by discounting illumination. In particular, luminance gradients are thought to provide cues about the interactions of light and surfaces. Several examples of strong lightness illusions are discussed for which explanations based on illumination interpretation can be proposed. In criticisms of this approach, a variety of demonstrations of similarly structured control displays are presented, which involve equivalent lightness effects that cannot readily be accounted for by illumination interpretation mechanisms. Furthermore, a number of known and novel displays are presented that demonstrate effects of gradients on the qualitative appearance of uniform regions. Finally, some simple simulations of neural effects of luminance distributions are discussed.

When is spatial filtering enough? Investigation of brightness and lightness perception in stimuli containing a visible illumination component

“…1, lower right) the test patches and near backgrounds are identical to those in the Control stimulus; however, a dark far surround has been added that causes the left half of the stimulus to appear to be in shadow. This makes it unclear whether the larger brightness/lightness difference perceived in this Experimental stimulus results from the added far surround exerting a non-local effect through spatial filtering (Kingdom, 2003; 2011; Blakeslee & McCourt, 2003; 2005; Todorovic, 2006) or whether additional mechanisms are required (Adelson, 2000; Purves et al, 2004; Gilchrist, 2006; Logvinenko & Ross, 2005). The situation in the snake illusion (Somers & Adelson, 1997; Adelson, 2000) is very similar.…”

“…The Experimental snake stimulus differs from the Control, however, in the luminances of more distant regions (the snake undulations) that cause the upper test patch to appear to lie beneath a transparent overlay. Again, it is unclear whether the larger brightness/lightness difference observed between the test patches in the Experimental stimulus is attributable to low-level spatial filtering (Todorovic, 2006; Kingdom, 2011) or to additional mid-level (Adelson, 2000) or high-level mechanisms (Logvinenko & Ross, 2006). The same confound occurs in each of the other illusory stimuli as well.…”

The Glare Effect Test and the Impact of Age on Luminosity Thresholds

“…Reports talk about a self-luminous mist spreading out from the central square that is perceived as a light source. The figure is rather interesting because it gives rise to the perception of self-luminosity or glow in absence of true light emission in the case of opaque reflecting surfaces, or with low light emission in the case of computer generated displays (Todorović, 2006). It has been hypothesized that this is possible because the GE partially simulates at a distal level what happens at a proximal level when the eye is invested by light of high intensity (Zavagno and Caputo, 2005).…”

The discoloration illusion

“…4, top, are replaced with achromatic contours having the same luminance, the discoloration is absent, and "the inner light red region appears dirtied by the gray coloration" spreading from the contours~not illustrated!. This condition leads to a second important point concerning chromatic vs. achromatic qualities of the discoloration illusion and, more specifically, the possible relations to other phenomena, like simultaneous contrast and overall surround contrast~see also Logvinenko, 1999;Bressan, 2001;Todorović, 2006!, in inducing the discoloration illusion. This point is fundamental~i!…”

“…2, the coloration effect does not disappear: "the inner region is perceived as a fuzzy bright gray, darker than the white of the background and like lighting"~see Pinna, 1990!. This is a lighting effect~see also Zavagno, 1999;Bressan, 2001;Todorović, 2006! ; however, unlike the chromatic condition of Fig.…”