Zavagno’s glare effect is a particular perceptual phenomenon of diffused brightness enhancement: this is an illusion in which a white region appears self-luminous (i.e. perceived as a light source) when surrounded by linearly decreasing luminance ramps away from it. It has been shown that the magnitude of the luminosity effect can be modulated by manipulating the luminance range of the gradients.

Using similar stimuli, it has been shown that the pupillary physiological response reflects the subjective perception of light and not simply the amount of physical light energy entering the eye. And pupil constrictions to the glare illusion have been found to be stronger for ‘blue’ than for other colors (when presented with the same physical luminance).

An unexpected difference between young and elderly people was also reported: luminosity thresholds of glare effect for elderly were higher than those for young, suggesting a non-direct relationship between luminosity perception and discomfort glare.

This specific instance of the glare effect is based on a version by Prof. Akiyoshi Kitaoka on his “Illusion of lightness” webpage.

References:

   Zavagno (1999) Some new luminance-gradient effects. Perception 28(7):835–838

   Zavagno & Caputo (2001) The Glare Effect and the Perception of Luminosity. Perception 30(2):209–222

   Zavagno & Bressanelli (2008) The animated glare effect: another case of perceptual causality? . Teorie Modelli 13:313–321

  Laeng & Endestad (2012)Bright illusions reduce the eye's pupil. PNAS 109(6):2162–2167

  Facchin, Daini & Zavagno (2017) The Glare Effect Test and the Impact of Age on Luminosity Thresholds. Front. Psychol. 8:1132

  Suzuki et al. (2019) Colorful glares: Effects of colors on brightness illusions measured with pupillometry. Acta Psychologica 198, 102882

Here is the math behind the basic element of the stimulus, a luminance ramp along a bar:
  
    deg = h+180
    b = 1; s = 1; hi = floor(deg/60); f = (deg-hi*60)/60
    p = b*(1-s); q = b*(1-f*s); t = b*(1-(1-f)*s)
    red = rhsb(hi,b,p,q,t); green = ghsb(hi,b,p,q,t); blue = bhsb(hi,b,p,q,t)
    xrot = x*cos(angle) - y*sin(angle)
    yrot = x*sin(angle) + y*cos(angle)
    envelop = abs(xrot)<th/2  & abs(yrot)<len/2
    gradient = (dir*yrot+len/2)/len
    bar = envelop*gradient
    zr = red*bar
    zg = green*bar
    zb = blue*bar
  

This element is then replicated at different positions (x_i, y_i) and modified according to:

  nb = 12
  for i = 0 : 11
    x_i = rad*cos(i*2*pi/nb)
    y_i = rad*sin(i*2*pi/nb)
    angle = pi/2-atan2(y_i ,x_i )

Where:

  'len' is the bar length (in deg)
  'th' is the bar thickness (in deg)
  'rad' is the mean radius (in deg)
  'dir' is the gradient direction (in In ↔ Out)
  'h' is the hue (in deg)

The whole stimulus is generated in real-time using a GLSL shader that runs right inside your WebGL-compatible browser. The plain Math behind the stimulus was converted to this optimized GLSL shader using the new Psykinematix Pro Edition. Translation to Matlab and Python code is also possible !

This whole widget was also fully generated using Psykinematix Pro Edition. The parameters that control the stimulus properties through the sliders are the same as the ones you would define as dependent or independent variables when using the stimulus in an actual psychophysical experiment run in Psykinematix. The widget creation is otherwise fully customizable with your own logo, copyright, links, etc.

To learn more about the widget creation, click on the above "Made With" button !