This texture was generated in Filter Forge, a Photoshop plugin that allows you to create your own filters.
This snippet demonstrates three additional gain curves. One based on 'Super Circular", and two based on 'Gamma'. (Note that the default gain curve is based on 'Bias')
One possible use for these curves is to increase an image's contrast without blowing out highlights or shadows.
==v1.1===
Simplified internals and sped things up a bit
*Thanks to byRo for gamma-to-circle logic
gain curve gamma circular arc wave snippet uber zev