Image Cropping Path and Manipulation Library

Question

I am looking to see if there is a library that would let me get a cropping path and then manipulate that path. Here is what I am thinking of:

1. Take a gray scale image and find the crop path to remove the white background. Have a threshold to help ignore artifacts.
2. Display the crop path.
3. Allow the user to manipulate the crop path by moving it, adding additional sub-paths, doing bezier curves, etc...

I am wondering if there are any libraries that I could use that would do this, or any portion of the requirements. I am trying to determine whether this is a doable feature, or if it would need to be written from scratch.