This is the second in a series of posts on visual processing and art.
Please review the post Line and Contour Enhancement for background.
Below, two identical but inverted Chevreul Illusions are placed on a luminance gradient.
The top right column and the bottom left column have identical luminance, however the top right column is clearly more brilliant. This helps me see the importance of context.
This is local sensitivity control.
The illusion above shows me how to create focal brilliance using regional shadow.
Note the brilliance of the window in this painting by one of Chevreul's contemporaries.
Berchtesgaden in Bavaria
Carl Triebel 1823-1885
Frye Art Museum, Seattle
In the drawing below I placed the model against a gradient background.
I used a gradient background to focus brilliance on her right torso, and to enhance the dark line traced by the left side of her body.
Local sensitivity control acts like a regional ISO adjustment. It allows me to to see detail simultaneously within regions of light and regions of shadow. In this drawing, understanding local sensitivity control made me comfortable darkening down the side of the model's neck, then bringing it back into view using the dark line of the sternocleidomastoid muscle and a few extra dark strokes of hair at the border with her shoulder.
Local sensitivity control is a pervasive feature of our vision. Relative brightness is strongly influenced by surroundings. Brilliance is best created with context. We can simultaneously appreciate detail in regions of brightness and regions of shadow.