Is it possible to create with FFMPEG a series of pictures with continuously shorter duration?

Question

The most simple slideshow is just a sequence of pictures with each being displayed for the same amount of time.

I would also like to make a sequence, but more complicated with respect to the duration:

I want the first picture to have a duration of 1 second, the next a little bit less than a second, the next even a little shorter, so that the sequence starts slow and gets faster towards the end.

I do not know how to do it, the only thing I can guess is, that the duration for each picture could be calculated by a function like

duration = 1/picture_number
duration = 1 - (picture_number/picture_total)

or in a script:

duration=1
while [duration > 0]
do
duration=duration*0.99
done

Answer 1

This can be done in a single command. Basic method is to start with a slideshow where each image has the same duration and then use the select filter to trim out greater amounts from the display time of each successive image.

ffmpeg -framerate 1 -i img%d.jpg \
       -vf fps=25,select='lt(mod(t,1),1-floor(t)/25)',setpts=N/25/TB \
       out.mp4

-framerate 1 --> this controls how fast the images originally turn over.

fps=25 --> we set the final framerate. Also, need to do this to have frames to work with.

select='lt(mod(t,1),1-floor(t/1)/25)' --> From each second, keep one frame less.

setpts=N/25/TB --> make the timestamps of the selected frames continuous, else ffmpeg will duplicate frames to fill gaps and defeat our goal.

To start with an initial duration of 3 seconds, you would change framerate to 1/3 and change select to lt(mod(t,3),3-floor(t/3)/25). You would need to change the 25 to control the speed at which t he duration is reduced. Higher values will reduce duration at a slower speed.