Local contrast used by Paul Signac in Quay at Clichy 1887
46.4 x 65.4 cm Oil on canvas The Baltimore Museum of Art, Baltimore, Gift of Frederick H. Gottlieb
downloaded from https://www.pubhist.com/w15231
The point of this webpage is to show that Signac intentionally changed values of areas that we would consider uniform for the purpose of increasing contrast of neighboring areas.
Because painting materials have a limited dynamic range, artists may use contrast shifts locally within an image to make us perceive larger intensity dynamics.
In this example, the sky gets brighter as it approaches the dark edge of the building. Our visual perception (and the software in many cameras systems, such as the Samsung Galaxy S22 phone) tends to do this, so making it explicit with paint is an exaggeration.
Could be ambitious and say there is a mathematical fit, such as in the red channel this fit to exponential decay of intensity by distance as stepping away from the dark object.