Introduction - Feature Extraction and Image Processing - page 9

(a) Zollner (b) Ebbinghaus

Figure 1.7 Static illusions

There are dynamic illusions too: you can always impress children with the 'see my
wobbly pencil' trick. Just hold the pencil loosely between your fingers then, to whoops of
childish glee, when the pencil is shaken up and down, the solid pencil will appear to bend.
Benham's disk , Figure 1.8 , shows how hard it is to model vision accurately. If you make
up a version of this disk into a spinner (push a matchstick through the centre) and spin it
anti-clockwise, you do not see three dark rings, you will see three coloured ones. The
outside one will appear to be red , the middle one a sort of green , and the inner one will
appear deep blue . (This can depend greatly on lighting - and contrast between the black
and white on the disk. If the colours are not clear, try it in a different place, with different
lighting.) You can appear to explain this when you notice that the red colours are associated
with the long lines, and the blue with short lines. But this is from physics, not psychology.
Now spin the disk clockwise. The order of the colours reverses: red is associated with the
short lines (inside), and blue with the long lines (outside). So the argument from physics
is clearly incorrect, since red is now associated with short lines not long ones, revealing the
need for psychological explanation of the eyes' function. This is not colour perception, see
Armstrong (1991) for an interesting (and interactive!) study of colour theory and perception.

Figure 1.8 Benham's disk

Naturally, there are many texts on human vision. Marr's seminal text (Marr, 1982) is a
computational investigation into human vision and visual perception, investigating it from