[FFmpeg-devel] ffmpeg-filter: lenscorrection parameters converted from OpenCV

[Paul B Mahol]

On Fri, Feb 19, 2021 at 10:16 AM 王康(五达) <wangkang.wk at alibaba-inc.com> wrote:

> Hi Paul,
>
> I figured out that parameters (k1, k2) used in `vf_lenscorrection.c` is
> different from (k1, k2) calculated from OpenCV chessboard methods.
>
> In summary:
>
> The k1,k2 parameters in `vf_lenscorrection.c` should be converted in this
> way:
>
> corrected_k1 = (w**2 + h**2) / (4 * f**2) * opencv_calculated_k1
> corrected_k2 = ((w**2 + h**2) / (4 * f**2)) **2 * opencv_calculated_k2
>

And what is "f" parameter?


> Detail with equations can be found in the attachment html file for your
> consideration. Perhaps you can add a new option or update the document.
> Hopefully this will help other users.
>
>
> --
> Wang Kang
> Alibaba Group
>
>    -
>
>    OpenCV: Principle
>    <https://docs.opencv.org/2.4/modules/imgproc/doc/geometric_transformations.html#initundistortrectifymap>,
>    Code
>    <https://github.com/opencv/opencv/blob/199ddff13b73071529a42cff97ee8286cfc15d02/modules/calib3d/src/undistort.dispatch.cpp#L377>
>    -
>       -
>    -
>    -
>       -
>       -
>
>    ffmpeg/libavfilter/vf_lenscorrection.c: Code
>    <https://build.verhagen.dev/scverhagen/FFmpeg/blob/e3212bfd1493be2c04e78b3a2e998d90c49e3935/libavfilter/vf_lenscorrection.c#L92>,
>    Early version in github
>    <https://github.com/FFmpeg/FFmpeg/blob/9f617a14a00805ffd54dee4fce34e3e4099d21f3/libavfilter/vf_lenscorrection.c>
>    -
>       -
>    - From the document:
>    -
>       -
>       -
>
>    The parameters in lenscorrection should be:
>    -
>       -
>    -
>    -
>       -
>       -
>
>    Let's take a picture with the dimension of 1600x1200, focal length of
>    1280.5 as an example. The parameters in lenscorrection should be:
>    -
>       -
>    -
>
>
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