What are the algorithms that ENVI uses for RST and first degree polynomial warping?

When performing any type of image warping in ENVI, one can choose an RST, Polynomial, or Triangulation warping algorithm. Many ENVI users wonder what the difference is between an RST warp and a first degree polynomial warp.

RST stands for rotation, scaling and translation. The RST warping algorithm uses an affine transformation:

x = a1 + a2*X + a3*Y
y = b1 + b2*X + b3*Y

This algorithm does not allow for shearing in the image warp. To include the possibility for shearing, one would use a first order polynomial warp, which includes an XY term:

x = a1 + a2*X + a3*Y + a4*XY
y = b1 + b2*X + b3*Y + b4*XY

The RST warp is familiar to many who work in the field of remote sensing because it has been used for such a long time. It is also very fast to implement, although in almost every case more accurate results can be achieved by choosing a first order polynomial warp.