A reversed -implode will do this, but only if you limit it to a value less than 2. A value more that 2 exhibits infinities into the formula that results in artificial effects.
It is not exactly a fish eye distortion, and you may need to enlarge the image (add a temporary border) to expand the outside radius, to get what you want.
Also the pixel lookup is just interpolative, so parts of an image that is compressed by more than 50% will start have aliasing effects, (unless a super-sampling tenchnique is used to improve results). That will remain the case until I improve the general -distort function and migrate circular distorts to it (no time line for this, due to other commitments and a china trip in April).
If you must have a better fish eye distortion, look at the DIY distostion methods using the -fx operator.
http://imagemagick.org/Usage/distorts/a ... affine_diy
And the use of pre-calculated distortion maps for handling multiple images with the same distortion parameters.
http://imagemagick.org/Usage/distorts/#lut
Please let us know how you get on, the method/path you finally take, and hopefully some examples and results of what you achieve.
