FieldWarpFilter
FieldWarpFilter warps an image using a field warp algorithm. You supply two arrays of lines: a source array and a destination array. The filter will warp the image so that pixels in the neighbourhood of each source line will move towards the corresponding destination line. It's as if each line were surrounded by a magnetic field which pulls pixels along as it moves.
FieldWarpFilter: Source lines in red, destination in green
In the Image Editor, the field warp is implemented as a tool which allows you to draw the source and destination lines on the image. You have three parameters to play with: amount which specifies the amount of warping. This is useful for producing morphs - just vary the amount from zero to one through the morph. Power also goes from zero to one and specifies how much the lengths of the line affects their strength. If zero, all lines have the same strength. If one, long lines are stronger than short lines. Strength, also from zero to one, specifies how much the influence of lines drops off with distance.
Tips
Because each line contributes to every pixel, you need to add several lines for a field warp to work well - if you have just one line, then every pixel in the image will only be affected by that line and you need to add more to dilute its influence. If you're trying to warp details in the centre of the image, you will probably find it useful to add four lines around the edges of the image in order to anchor the edges and stop the whole image from moving when you move one of your detail lines.