N shades of gray - video generator (using ffmpeg)

n_shades_of_gray_video_generator.sh

#!/bin/sh

set -euo pipefail

# Check if FFmpeg is installed and display an error message if not
if ! [ -x "$(command -v ffmpeg)" ]; then
  echo 'Error: FFmpeg is not installed.' >&2
  exit 1
fi

############### User defined params ###################
# Set the duration of each video in seconds, float values allowed e.g. 0.1 
duration=0.5
# Video resolution
resolution="1366x768"
# Set font color of text label
fontcolor="white"
# Set the font color of label to white 
fontfile="/Library/Fonts/Arial Unicode.ttf"
# Set the x and y position of the text label
x="(w-text_w)/4"
y="(h-text_h)/4"
# How many steps and min,max color value
min=0; max=255; steps=50 
output_file_name="${steps}_shades_of_gray_${duration}_${resolution}.mp4"

echo "Resolution: ${resolution}"
echo "Each step duration: ${duration} seconds"
echo "Step count: ${steps}" 

[ ! -f "$fontfile" ] && { echo "Font file ${fontfile} does not exist!" >&2; exit 1; }

# Remove videos generated before
rm -rf ./tmp; mkdir ./tmp
# Remove the files.txt file if it exists, will be used to join all videos to one
tmp_file_list="tmp/files.txt"
[ -f "$tmp_file_list" ] && rm "$tmp_file_list"

interval=$(echo "$max / ${steps}" | bc -l)
step_num=0
echo "Going to generate $((steps+1)) videos with ffmpeg"
for i in $(seq $min $interval $max); do
    color_int=$(echo $i|cut -d. -f1) #sh's printf requires integers 
    color=$(printf "0x%02x%02x%02x" $color_int $color_int $color_int)
    # Make fontcolor of text lable black if step number > steps/2
    (( $(echo "$step_num > ($steps / 2)" |bc -l) )) && fontcolor="black"
    # Save file_name
    fname="v${step_num}_${color}_${duration}sec_${resolution}.mp4"; fpath="tmp/$fname" 
    label="${step_num}/${steps}"
    echo "file '${fname}'" >> "$tmp_file_list"
    # Use FFmpeg to create a video with a solid color background,
    # with a text overlay that displays the number and its value (e.g. "3/10")
    ffmpeg -loglevel error -f lavfi -i color=c=${color}:s=${resolution} -vf \
    "drawtext=fontfile=${fontfile}: text='${label}': \
    fontsize=24: fontcolor=${fontcolor}: \
    x=${x}: y=${y}" \
    -t ${duration} "${fpath}"
    echo "Generating ${fpath}"
    ((step_num++))
done

[ -f "${output_file_name}" ] && rm ${output_file_name}
# Use FFmpeg to concatenate all the videos and save the result to output.mp4
echo "Joining videos into single file: ${output_file_name}"
ffmpeg -f concat -i "$tmp_file_list" -c copy ${output_file_name}
echo "Done. File saved as \"${output_file_name}\""