Apply an audio lowpass on every frame of a gif

script.sh

#!/bin/sh

# Invocation : script.sh name-of-the.gif
# Find out the size
SIZE=$(identify $1 | egrep --only-matching '[0-9]+x[0-9]+' | head -n 1)

mkdir -p frames
rm -rf frames/*

# Extract individual frames
convert $1 frames/%05d.png
(
cd frames
for frame in *.png; do
  # Extract raw RGB data from the frames
  stream -map rgb -storage-type short $frame $frame.raw
  
  # Apply a lowpass filter
  sox -r 8k -e signed -b 16 $frame.raw $frame.raw.raw lowpass 25 
  
  # Back to png
  convert -size $SIZE -depth 16 rgb:$frame.raw.raw $frame
done
)
# Back to gif (likely we could have saved the PNG reconversion step but... lazy)
convert frames/*.png $1-out.gif