2D UV Warp Modifier moves twice as far as desired

Note: This is my first post, so please give me gentle correction if I make an unintentional error.

I use blender for 2d animation, and am working to understand UV warp. I have attached a mostly-functional .blend with two planes to be UV-Warped, eyes on one plane and mouth on another. I have one “helper object” that is textured with the image containing the eyes and mouths, and two bones per UV warp modifier, a “base bone” and “target bone”. These are all part of one armature. The helper object is subdivided with vertexes falling on the center of each image, to aid with snapping the bone locations accurately.

The idea is to move the “target” bones in pose mode to change the UV, which will be animated with a constant interpolation. Everything is nearly working - the only catch is that the UV texture moves at 2 times the distance of the target and base bone. Said another way, if I visually move a target bone one “mouth” over, the corresponding UV texture warps by two “mouths” worth of distance.

It seems like there is a basic principle that I am missing. I’ve spent several hours reading the manual and searching the internet, but haven’t made any progress. I hope someone here can enlighten me.

Thank you for reading!


uv-warp-test.blend (512 KB)