When colours leak out of their lines
Assimilation, the Munker illusion, watercolour and neon.
How this site works. Every fact is in our own words and linked to its source, beside it. Where good sources disagree, we show each version in a “Sources differ” box. Every picture is our own drawing: we don’t copy the famous originals, we make our own versions of the idea.
Assimilation
This is assimilation, first described by Bezold: each disc takes on the mixed colour of the lines crossing it. Michael Bach
In Bach’s assimilation demo three discs behind coloured grids look cyan, magenta and yellow, but all three are white (strictly, a light yellow). Michael Bach
Assimilation is the opposite of colour contrast. Michael Bach
One idea, from King in 1988: we see contrast when patches look like separate things, and assimilation when they look like one whole. Michael Bach
Bach admits he doesn’t understand the brain mechanism behind assimilation. Michael Bach
The Munker illusion
In the Munker illusion two stripes look green and blue, yet there are only three colours in the picture: both stripes are the same green. Michael Bach
It shows how much the colours around something change how we see it. Michael Bach
Hans Munker wrote about coloured grids in 1970. Michael Bach
The watercolour illusion
In the watercolour illusion, shapes edged with a dark wavy line and a lighter coloured line inside look faintly tinted, though the inside is plain white. Michael Bach
Baingio Pinna discovered it in 1987. Michael Bach
Even with pale lines the illusory colour is quite strong. Michael Bach
Bach suspects map colourers knew about it centuries ago. Michael Bach
Neon colour spreading
Neon colour spreading: a coloured star set inside black lines seems to glow, so it looks a little like a coloured disc. Michael Bach
Any gap between the coloured and black lines destroys the effect. Michael Bach
It works with greys too. Michael Bach
Who described it first? Sources differ
- Bach says Van Tuijl was the first to describe it, in 1975. Michael Bach
- Bach’s own reading list starts earlier, with D. Varin’s 1971 Italian paper on contrast and colour spreading. Michael Bach
Both come from the same page: Bach credits Van Tuijl, but his reading list includes an earlier paper on colour spreading.
Our view: the watercolour illusion is the most useful one here. Draw a dark line with a lighter line inside it and you have coloured in a shape without colouring it in.
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