A big part of “realistic” painting for me is the deployment of the “right” visual cues so that the viewer’s visual completion yields the percept that I’m aiming for. One of the examples that I shared on social media to show how powerful a “color completion” scenario can be was this illusory color demonstration from Øyvind Kolås (twitter ID: Pippen.)
While at first glance this may look like a color photograph—it is actually an over-saturated colored grid or line pattern layed onto a grayscale image causing the grayscale cells to be perceived as having color.
Here is another from the same author:
For those interested in what is happening in these images:
Some people, including the author of these images, attribute the illusory colorization of these images to simultaneous color contrast, a form of chromatic induction. Simultaneous color contrast is a spatial context effect in which one color in a region influences the neighboring regions towards the complimentary hue.) In other words, with two colors placed in close proximity, we may perceive one color “push” a second towards the perceptual antagonist of the first (and vice versa). If the second is not “strong” (i.e., very low in chroma), it will take on a more salient “amount” of that antagonist.
However, by this definition, we can quickly observe that this is not the case. For example, in the second picture, the red lines crossing the face of the camera-wielding figure does not yield a face that appears to be mostly green. Rather, it appears to aggregate into a relatively appropriate flesh tone. Why? Because the chromatic induction at play here is chromatic Assimilation–not simultaneous color contrast.
Chromatic or color assimilation (also known as the Von Bezold spreading effect) can actually be described as having the opposite effect of simultaneous contrast. It is another spatial context effect, often encountered with interspersing stimuli, but where color contrast “pushes” a color towards a perceptual antagonist, assimilation “pulls” a color towards it’s interspersed counterpart(s). As with contrast, the “stronger” component of the interspersed colors will hold more sway, thus playing a dominant role in the perceived assimilation. Thus, the strong red lines in the face of the aforementioned figure pulls the surrounding grayscale regions towards it, resulting in a perception of relatively appropriate flesh-tone.
Some have been quick to link these illusory colors to a number of Munker Illusions that are making their rounds on social media at the time of this posting. The Munker Illusion (sometimes called Chromatic White’s illusion) is a demonstration of chromatic assimilation which often presents identical targets interacting with alternate components of an interspersed pattern to yield a perception of targets that vary significantly. Here’s a popular one that is making those rounds: “Confetti”, created by David Novick, Professor of Engineering Education and Leadership, the University of Texas at El Paso.
Each percieved circle is the same color, but when alternate colors from the interspersed pattern interact with each circle—chromatic assimilation ensues and different colored circles are perceived. It should also be noted that some have put forward the idea that visual/chromatic scission is at play here as well. (Chromatic scission is a type of figure-ground segregation in which we experience a perceptual separation of colors in an image by perceiving their belonging to “underlying” or “overlaying” surfaces.)