timelapse.bash

#!/usr/bin/env bash

#
# Please note: Frame rate of 25 fps is hardcoded.
# Adapt or generalize according to what you need.
#

set -euxo pipefail

echo "$1"

while read input_fname ; do echo "file '$input_fname'" ; done \
\
| ffmpeg \
  `# We decode input video files and pipe raw video frames.` \
  -safe 0 -protocol_whitelist file,pipe -f concat -i - \
  -an -vcodec rawvideo -pix_fmt yuv420p -f nut - 2>/dev/null \
\
| ffmpeg \
  `# For every group of m frames, take the first n frames.` \
  -r 50 -i - \
  -r 25 -vf \
    select="lt(mod(n-2\,50)\,25)" \
    -vcodec rawvideo -pix_fmt yuv420p -f nut - 2>/dev/null \
\
| ffmpeg -y \
  `# Average frames and then drop all but every 25th frame from prev stage.` \
  -i - \
  -vf tmix=frames=8,setpts=PTS/25 "$1"