Influenced by the French Impressionists' experimentation with colour, Georges-Pierre Seurat (1859 - 91) was the first Post-Impressionist painter. The photographic image of his Un Dimanche après-midi à l'Île de la Grande Jatte A Sunday Afternoon on the Island of La Grande Jatte has been decomposed to show changes in brightness.
Key is another word for value. Seurat provided a dark foreground and also placed darker figures prominently towards the right hand side, and also along the top of the frame. As such the dark colours encroach upon brighter colours to the centre and left of the painting. Division of a canvas between low and high key values can be used to convey meaning. The left side of the painting The Oxbow by Thomas Cole uses low key values to depict the harshness of the wilderness, with the approaching storm a reminder of the power of nature; the cultivated fields on the right of the canvas are bathed in high key values.
Seurat had a scientific theory to prove - that painting in dots, known as Pointillism (also Divisionism) would produce brighter colours, and he took great care in arranging this painting making 23 preparatory drawings and 38 oil studies.