Re: Controlling the amount of an effect with gradients and/or masks?

this may help, Ian - can't remember where I recently found this solution but it is essentially the After Effects version of using a luminance-based layer mask in Photoshop. I just tried it out applying Turb Displacement selectively, and it seems to obey white, black & grey areas of the 'masking' layer. Try this:
1. in a new comp, place the video layer that will be receiving the turb. displace effect.
2. create an adjustment layer above the video layer & apply your effect to it (ie turb. displacement)
3. create a black solid & put it above the adjustment layer
4. apply some form of mask to the solid - a gradient, a feather mask, fractal noise, whatever. If desired, use a curves or levels effect next to tweak the appearance of this 'mask'
5. set the transfer mode of the adjustment layer to "Luma Inverted matte" and woohoo! - turbulent displace is applied selectively to your video layer.
I tried adjusting the curves effect (+ turned this effect on & off) in the 'mask' layer to determine whether shades of grey do in fact influence the degree to which the turb displacement effect was applied to my video layer. It seemed to - I used a radial ramp and watched the turb displacement effect shrink & grow as I manipulated the End Position of the Ramp. Everything in the video layer that lay in the zone between Start & End points of the Ramp was affected, which means that grey levels in the mask definitely influence the effect. Try it for yourself & see if this is doing what you need.
And of course, you can then also alter the opacity of the adjustment layer itself to further fine-tune the amount of turb. displacement that reaches your video layer. Keep in mind that you're using an adjustment layer which will pass the effect to all layers beneath it - if you only want to affect a limited number of layers, you'll have to quarantine them by precomping them with the 'mask' and adjustment layers.