Example 4: (small lens effect)

This is a lens effect <PARAM name="mapping" value="lens|70|1.5">
The second value is the lens radius
The third value is the magnification. (optional default is 2)
Note negative values are also possible (The opposite of enlarging)
an optional fourth value can change the lens shape (default is 0) (optional)
Allthough I don't think it is a usefull option.

Other examples:
Some function
Parabolic bent on x axis
Parabolic bent on x and y axis
lens effect (small)
lens effect (large)
Posterize effect.
Very large lens.
Parabolic bent on y axis.