estimation of subpixel values from images in Python
There are two common methods:

bilinear interpolation,
bicubic interpolation.

These evaluate an intermediate value, based on the values at four or sixteen neighboring pixels, using weighting functions based on the fractional parts of the coordinates.
Lookup these expressions.
From my experience, the bilinear quality is often sufficient.
I have an image and and am transforming it with a nonlinear spatial transformation. I have a written a function that, for every pixel (i, j) in the destination image array, returns a coordinate (y, x) in the source array. 
The returned coordinate is a floating point value, meaning that it corresponds to a point that lies between the pixels in the source image. 
Does anyone know if there an established method in PIL or opencv to interpolate the value of this subpixel, or should I roll my own? Thanks!