[FFmpeg-user] How to create trapezoid videos with ffmpeg?

[Nicholas Robbins]

> On Thursday, October 16, 2014 11:59 AM, Moritz Barsnick <barsnick at gmx.net> wrote:

> > Hi Zenny,
> 
> On Wed, Oct 15, 2014 at 10:55:40 +0200, Zenny wrote:
>>  New to ffmpeg. How can one create a trapezoid shaped video as a
>>  overlay to the background as seen at
>>  http://www.youtube.com/watch?v=_380o5B9MrA (starts at 00:02:25) using
>>  ffmpeg command?
> 
> I can't seem to find a filter which does that directly, but the
> "perspective" does a similar transformation, in an inverse way. I
> believe it "virtually pulls the given viewpoints to the corners." 
> Using
>
> ....
>
> Note the reference points _outside_ of the frame to achieve a trapezoid
> size reduction. The filter you are looking for should probably accept
> actual corner values. That said, such a filter could probably be
> derived from the "perspective" filter's code. (I failed at finding 
> an
> easy way to do this though.)
> 
> Moritz

Ok, I'm not sure what you meant by an easy way, but the formula for what to put into the perspective filter options to essentially have it be the inverse (where to send the corners, not what to send to the corners) is huge mess, so it can't be done easily for any given picture (just change the x0 to a -x0 or whatever). However it might be able to be hardcoded into a different filter (vf_keystone?) I'll inquire on ffmpeg-devel if that is possible, to have one filter basically just call another filter with different options.

For example, if you wanted the original corners to go to x0:y0:x1:y1:x2:y2:x3:y3 the first option (x0 for that filter) you would want to give to vf_perspective is

(W (x2 y0 - x0 y2) (x3 (y0 - y1) + x0 (y1 - y3) + 
     x1 (-y0 + y3)) (x3 (y1 - y2) + x1 (y2 - y3) + 
     x2 (-y1 + y3)))/(-x0 (x3^2 (y1 - y2) (-2 y1 y2 + y0 (y1 + y2)) - 
      2 x2 x3 y1 (y1 - y2) (y0 - y3) + x2^2 y0 (y1 - y3)^2) + 
   x2 x3 (y0 - y1)^2 (-x3 y2 + x2 y3) + 
   x1 (x3^2 y1 (y0 - y2)^2 + 2 x0 x3 (y1 - y2) y2 (y0 - y3) - 
      x0^2 y1 (y2 - y3)^2 - 2 x2 (y1 - y2) (y0 - y3) (x3 y0 + x0 y3) +
       x2^2 (y0 - y3) (y0 (y1 - 2 y3) + y1 y3)) + 
   x0^2 (x2 y2 (y1 - y3)^2 - x3 (y1 - y2) (2 y1 y2 - y1 y3 - y2 y3)) +
    x1^2 (x0 y0 (y2 - y3)^2 - x3 (y0 - y2)^2 y3 - 
      x2 (y0 - y3) (y0 (y2 - 2 y3) + y2 y3)))

so, yeah, not easily computed, but totally something you could compute once when you initialize the filter.

-Nick