Repository: opencv
Issue: Lens distortion model confusion in the docs
Labels: category: documentation,category: calib3d

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##### System information (version)
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##### Detailed description

Currently, OpenCV documentation does not provide any references or information on the employed lens model and its origins. My [question](https://forum.opencv.org/t/opencv-lens-distortion-model/3703) in the forum was left unanswered. Particularly, the `CALIB_RATIONAL_MODEL` flag causes confusion leading to statements (such as [this](https://opencv.yahoogroups.narkive.com/igfRkYot/opencv-2-2-new-rational-distortion-model) possibly by @vpisarev) claiming the lens model is derived from the [Rational Function Model](https://doi.org/10.1109/CVPR.2005.43) by D. Claus. This is however incorrect because the latter uses conics to represent the non-linear distortion of the lens which OpenCV does not.

I suggest amending the documentation for `initUndistortRectifyMap` and explicitly point out that OpenCV's *rational model* is not related to the *rational function model*. Instead, the model is a combination of the Brown-Conrady model and the [division model](https://doi.org/10.1109/CVPR.2001.990465).

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