Inverting a real-valued index grid

Question

OpenCV\'s remap() uses a real-valued index grid to sample a grid of values from an image using bilinear interpolation, and returns the grid of samples as a new image.

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Answer 1

From what I understand you have an original image, and a transformed image, and you wish to recover the nature of the transform that has been applied without knowing it, but assuming it is something sensible, like a rotation or a fish-eye distort.

What I would try is thresholding the image to convert it to binary, in both the index image and the plain image. Then try to identify objects. Most mappings will at least retain connectivity and Euler number, mostly the largest object in the index will still be the largest object in the plain.

Then take moments for your matched image / indexed pairs and see if you can remove translation, rotation and scaling. That gives you several reverse maps, which you can then try to stitch together. (Hard if the transform is not simple, but the general problem of reconstituting just any transformation cannot be solved).