This stuff kind of blows my mind. I just kind of play with stuff but haven't gotten a full grasp of these parameters.
I think TIFFany makes more sense if you know a little about digital signal processing, especially the names/hierarchy of the actions.
I was playing with the Geometry actions last weekend, and came up with a couple of new ones.
I called this one "Arch" since that's the effect I was trying for, but it is really just a sine wave applied to the image. You can change d/2 to adjust the amplitude of the wave, and x*s will adjust the frequency; x*s + n (where n is some number) will apply a phase shift to the wave.
x' = x
y' = y - d/2 * sin(x * s)
Here's another, that I called "Perspective":
x' = x * ( 1 + x * tan(s/2) )
y' = y * ( 1 + x * sin(s/2) )
I came up with this more-or-less randomly, and it works best with small values of s in the slider. The image I'm attaching shows both of these effects applied to a text marquee copied from another image.[/code]