[FFmpeg-user] v360 / perspective

Michael Koch astroelectronic at t-online.de
Sat Dec 5 22:00:05 EET 2020

Am 05.12.2020 um 13:03 schrieb Paul B Mahol:
> On Sat, Dec 5, 2020 at 12:30 PM Michael Koch <astroelectronic at t-online.de>
> wrote:
>> Am 05.12.2020 um 11:38 schrieb Paul B Mahol:
>>> On Sat, Dec 5, 2020 at 11:02 AM Michael Koch <
>> astroelectronic at t-online.de>
>>> wrote:
>>>> Am 04.12.2020 um 18:14 schrieb Paul B Mahol:
>>>>> On Wed, Dec 2, 2020 at 11:11 PM Michael Koch <
>>>> astroelectronic at t-online.de>
>>>>> wrote:
>>>>>> Am 01.12.2020 um 19:56 schrieb Michael Koch:
>>>>>>> Hello,
>>>>>>> there seem to be some problems in the v360 filter with "perspective"
>>>>>>> output.
>>>>>>> Tested with the latest Windows built, 2 days old. You can reproduce
>>>>>>> this with any equirectangular input image.
>>>>>>> ffmpeg -i equirectangular_test.png -lavfi v360=e:perspective -y
>>>> out1.png
>>>>>>> The output of the above command has two problems:
>>>>>>> -- The center of the input image is not mapped to the center of the
>>>>>>> output image. For most other projections the image center is
>> preserved.
>>>>>>> -- The output is mirror reversed (which means you cannot read text).
>>>>>>> Both problems can be corrected with this workaround:
>>>>>>> ffmpeg -i equirectangular_test.png -lavfi
>>>>>>> v360=e:perspective:pitch=90:v_flip=1 -y out2.png
>>>>>>> Now I want to add a yaw rotation after the pitch rotation:
>>>>>>> ffmpeg -i equirectangular_test.png -lavfi
>>>>>>> v360=e:perspective:rorder=pyr:pitch=90:yaw=20:v_flip=1 -y out3.png
>>>>>>> But in the output you can see that a roll rotation was done.
>>>>>> Today I did for the very first time figure out how to compile ffmpeg.
>>>>>> The above problem can be solved by changing two lines in vf_v360.c
>>>>>> Sorry, I haven't yet figured out how to use git.One thing after the
>>>>>> other...
>>>>>> Old version:
>>>>>> line 3136:   vec[1] = sin_theta;
>>>>>> line 3137: vec[2] = cos_theta * cos_phi;
>>>>>> New version:
>>>>>> line 3136: vec[1] = cos_theta * cos_phi;
>>>>>> line 3137: vec[2] = sin_theta;
>>>>>> I'm not sure if lines 3139 to 3141 must also be changed.
>>>>> Good job!
>>>>> Please write a patch with correct authorship and send it to the devel
>>>> list.
>>>>> Fixing this for you was much more work than writing & sending patch
>> will
>>>> be.
>>>> Can you please explain what's the exact meaning of "v_fov" with
>>>> perspective output?
>>> What's written in documentation. It just scales differently.
>> "Set perspective parameter" or "It just scales differently" are too
>> vague explanations. Especially since the v_fov parameter here seems to
>> have a completely different meaning than in other projections. One thing
>> is sure: It's not an angle in degrees.
> Look again at the formula of projection, it explains everything.

The input image is mapped on a sphere and this sphere is viewed with a 
camera from outside. The distance between camera and sphere is an 
unknown function of v_fov. It's sure that v_fov can't be the normalized 
distance, because for the smallest possible v_fov value (1e-5) there is 
roughly a 60° segment visible in the output image. It must be a more 
complicated function.

Too many unknowns:
-- The _exact_ meaning of 'v_fov' is unknown.
-- It's unknown what type of lens is used in the camera. Fisheye or 
rectilinear or something else?


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