The Watercolor Illusion

*Note: From time to time old posts will be resurfaced on the blog. This one is from Sept. 2010. The watercolor illusion is similar to, but not exactly the same as, the vignette concept. A vignette would create a subtle illusion as well. A simple example would be this banding effect created in TileMill:

I was digging around the cartography literature yesterday and came across something called the watercolor illusion.*

The illusion is thus: if you have a dark line (the more squiggly the better) next to a light colored line, your eyes will fill in the missing white space with the lighter color, albeit in a lighter tone, and give a washed-out effect. In this example your eye may think that the outside rectangle is green but in fact it is white. It is just the light green fine line next to the purple that is causing that illusion:

Perhaps you all have some better examples of how this plays out on a map, but here is my attempt. You can see I am trying to create a major definition between the water and the land (these are geology polygons). The geology polygons are in dark purple and I’ve used a bright, light blue next to the outer edge by first creating a buffer of the polygon and then symbolizing the buffer:

There are other ways to produce a gradual fading of a line in your GIS and in your graphics programs, of course. In ArcGIS, for example,  you can use a gradient fill, though sometimes, especially with large datasets, this taxes the renderer quite a bit. However, this is a fairly easy and handy way to create the effect within the GIS and without exporting to graphics software.

*Pinna, Baingio and Gavino Mariotti, “Old Maps and the Watercolor Illusion: Cartography, Vision Science and Figure-Ground Segregation Principles,” Systemics of Emergence: Research and Development. 2006, 3, 261-278.

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