No replies, bump this up. Maybe one of the clever guys sees it.
As you know I am not into coding, most of what I need has aleady been done. Why re-invent the wheel. (unless it is a better wheel)
Looking at the handle transform tool as displayed, there is a 3x3 matrix of values. Just guessing but there is a procedure that uses a 3x3 matrix.
gimp-item-transform-matrix.
I am more used to IM, good reading here:
http://www.imagemagick.org/Usage/distor ... fine_otherThere is one of 'Freds' IM scripts that does a great page curl. Very complicated, but looking it seems to use regular trig. functions