ffmpeg - Zoompan IN and Out alternately

Question

I'd like to apply a Zoompan effect but instead of zoom in or zoom out, I'd like to mix both of them: the zoom will start with zoom in, and, after some period of time, the zoom continue with zoom out, and so on, alternatively.

While I'm thinking how to archieve this, has someone some tips?

Thanks !

Answer 1

You need to use state e.g.

ffmpeg -i src.png
 -vf zoompan=d=300:z='if(gte(zoom,2)+eq(ld(1),1)*gt(zoom,1),zoom-0.04*st(1,1),zoom+0.04+0*st(1,0))'
o.mp4

This expression zooms from zoom=1 to zoom=2 in steps of 0.04. Remember, zoom is initialized to 1. The above will create 6 loops.

In st(store) and ld(load) functions, the first argument is variable id (0 to 9), and the second is the value.

Answer 2

Based on Gyan's answer, here are more details.

The following creates a video from an image and an mp3 with an infinite zoom-in-out on the image:

ffmpeg -loop 1 -i img.png  -i adio.mp3 -vf "[0:v]scale=7680x4320,zoompan=z='if(gte(zoom,1.5)+eq(ld(1),1)*gt(zoom,1),zoom-0.00005*st(1,1),zoom+0.00005+st(1,0))':d=20000" -c:a copy -c:v libx264 -shortest out3.mp4

[0:v]scale=7680x4320 upscales 4 times the original 1920x1080 image for a smooth transition.

d=20000 specifies the duration of the zoom-in-out. In this case, I perform a zoom from 1 to 1.5 with increments or decrements of 0.00005. That's (1.5-1) / 0.00005 steps for the zoom, which means I need two times this for the zoom + dezoom, which is 10'000*2 = 20'000.

if(gte(zoom,1.5)+eq(ld(1),1)*gt(zoom,1),zoom-0.00005*st(1,1),zoom+0.00005+st(1,0)) can be translated to the following pseudo-code:

// '||' corresponds to a `+` and '&&' to a '*' 
if zoom >= 1.5 || (isDezooming && zoom > 1) {
  zoom-0.00005
  isDezooming = true // corresponds to '*st(1,1)'
} else {
  zoom+0.00005
  isDezooming = false // corresponds to '+st(1,0)'
}

where isDezooming corresponds to the variable 1 handled with the ld(1) and st(1,0) instructions. Here is the full list of instruction.