What type of algorithm does imwarp use for displacement fields?

I would like to evaluate the performance of different non-rigid image registration algorithms. The ouptut is always a densely sampled displacement field that I can plug into imwarp and get the resulting image. In this sense, imwarp is like a black box in the code, and although it works very well, I would like to learn what it is doing and try to implement something similar myself. One way I could make something similar to imwarp is to use scatteredinterpolant (that works by triangulation) to resample the warped image on a regular grid. However this is much much slower, so I guess imwarp uses a different approach for displacement fields?