bash script to download an animation and create video with variable speeds

make_vids_variable_speeds.sh

# the goal:
## download the animation
## use ffmpeg to make it playable
## change the speed of the animation
## randomally change the speed of the animation
## concatenate multiple speeds together

# make a directory to keep things tidy
mkdir create_video
cd create_video

# grab the animation from the web, renaming it to original_video.mp4
wget https://ndownloader.figshare.com/files/8804191 -O original_video

# make the video actually about to be viewed, but only take the first 10 seconds
ffmpeg -t 00:00:10 -i original_video -vcodec libx264 original_video_corrected.mp4

# a quick google search reveals how to use ffmpeg to change the speed of a video:
## https://trac.ffmpeg.org/wiki/How%20to%20speed%20up%20/%20slow%20down%20a%20video
## values < 1 speed it up; > 1 slow it down

## possible speeds:
SPEEDS=(0.5 0.75 1.0 1.33 2.0 3.0 0.1 0.2)

echo $SPEEDS

# make a video for each possible speed
for i in ${SPEEDS[@]};
do
  echo "making video for speed $i"
  ffmpeg -i original_video_corrected.mp4 -filter:v "setpts=$i*PTS" -y "speed_$i.mp4"
done


# for this example, say we want to stitch together 10 videos
# this code block creates a list of randomized speeds
# the available speeds are defined from the SPEEDS variable
# at the end of this we have an array (a list of numbers) called RANDOM_LIST
# that contains the list of randomized speeds
for i in `seq 1 10`;
do
  echo randomization: $i # print loop # to the screen
  rand=$[ $RANDOM % ${#SPEEDS[@]} ] # generates random number from 1-5
  RANDOM_SPEED=${SPEEDS[$rand]} # chooses the rand'th speed from SPEED
  RANDOM_LIST[$i]=$RANDOM_SPEED # add random speed to RANDOM_LIST
done

# this create a plain text file, list, that lists the videos we want to stitch together
# based on out randomized list of speeds
for i in ${RANDOM_LIST[@]};
do
  printf "file '%s'\n" speed_$i.mp4 >> list
done

# this concatencates all the videos together
ffmpeg -f concat -i list result.mp4

# this opens the final resulting video
open result.mp4