While many representationalists take great care in defining the shapes and contours of their subjects—fewer tend to consider how the nature of those edges may impact values or colors that populate the territories they define. Take for example this representation of Lincoln. Does it appear that the lights and shadows consist of different values?
Other than the variations at the edges—they aren’t.
The base values are the same. We perceive them to be different mostly due to the significant contrast ramping found at the edges or boundaries of those areas.
Here’s an analysis of the image that was put forward by artist Michelle Rushworth:
The Lincoln image here is an example of the effect that is often demonstrated with the Cornsweet illusion or Craik–O’Brien–Cornsweet illusion. The demonstration of this effect shows just how impactful edge contrast can be on corresponding areas.
Here’s a version of the above Lincoln image that artist Mark Heng did to see if the effect would reverse with a Photoshop inversion. It definitely did!
Keep this in mind the next time you are trying to “turn” that edge or deploy some seemingly innocuous visual shorthand to tweak that drawing. The effects of those marks can be further reaching than you might think.
FURTHER CORNSWEET EDGE READING: https://www.jneurosci.org/content/19/19/8542.full