[FFmpeg-user] v360 filter with h_offset option

[Paul B Mahol]

On Wed, Jan 12, 2022 at 5:00 PM Michael Koch <astroelectronic at t-online.de>
wrote:

> Hello,
>
> I have a projector with a 180° fisheye lens in a dome. Things are easy
> if the lens is at the center of the dome. Any input format can be
> transformed to fisheye output with the v360 filter. But things are
> getting complicated when the lens isn't at the dome's center. Let's
> assume the dome has 1m radius and the lens is shifted 0.5m to the side.
> In this case the +60° point from the input image must be mapped to the
> center of the output image (because acos(0.5) = 60°) ).
> The center of the input image must be mapped to 63.4° in the output
> image (because 90° - atan(0.5) = 63.4°).
>
> The v360 filter has h_offset and v_offset options. There isn't much
> documentation for these options, so let's just try out what they do.
> I'm using the test pattern from Paul Bourke:
> http://www.paulbourke.net/dome/testpattern/1200.png
>
> ffmpeg -i 1200.png -vf v360=fisheye:fisheye:h_offset=0.5 -y out.png
>
> The +60° point from the input image is not mapped to the center of the
> output image, that's wrong.
> The center of the input image is mapped to +60°, that's also wrong.
>
>
> Next I did try 0.577 (=tan(30°)) as offset:
>
> ffmpeg -i 1200.png -vf v360=fisheye:fisheye:h_offset=0.577 -y out.png
>
> Now the +60° point from the input image is mapped to the center of the
> output image, that's good.
> The center of the input image is mapped to about 54.5°, that's wrong.
>
>
> These results leave me with two questions:
> -- Obviously the offset options aren't doing what I was hoping for. What
> else are they supposed to do?
> -- How can a fisheye image be transformed to an off-center fisheye image?
>

http://paulbourke.net/panorama/sphere2persp/

I got idea from that one, and I believe its correctly implemented.


>
> Michael
>
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