#/usr/bin/env sh
#check if argument for input file is set; fail if not
if [ ! -z ${1+x} ]
then
    INPUT_FILE="$1"

    #if second argument is given, use as output file name; use default if not
    if [ ! -z ${2+x} ]
    then
        OUTPUT_FILE="$2"
    else
        OUTPUT_FILE="output.webm"
    fi

    #make temporary directories
    TMP_DIR=$(mktemp -d)
    mkdir ${TMP_DIR}/{frames,scaled}

    #extract individual frames from inputfile
    ffmpeg -v error -i "${INPUT_FILE}" -qmin 1 -q:v 1 "${TMP_DIR}/frames/%04d.jpeg"

    #count frames
    FRAMES=$(ffprobe -v error -select_streams v -of default=noprint_wrappers=1:nokey=1 -count_frames -show_entries stream=nb_read_frames -i "${INPUT_FILE}")
    printf 'Frame count: %s\n' ${FRAMES}

    #get framerate from input file; used for intermediary files, can't be changed during concat
    FPS=$(ffprobe -v error -select_streams v -of default=noprint_wrappers=1:nokey=1 -show_entries stream=r_frame_rate -i "${INPUT_FILE}")
    printf 'Frame rate: %s\n' ${FPS}

    #scale each frame according to its index and transcode to webm again
    #this is EXTREMELY inefficient; the overhead of restarting ffmpeg possibly thousands of times means that even on a powerful machine this script will take ages to run
    for f in $(find ${TMP_DIR}/frames -name '*.jpeg' | sed -En 's#^.*/?frames/([0-9]+).jpeg$#\1#p' | sort); do ffmpeg -v error -i "${TMP_DIR}/frames/${f}.jpeg" -frames:v 1 -filter:v "scale=$(printf '%s + 10\n' ${f} | bc):-1" -c:v libvpx -crf 31 -r "${FPS}" "${TMP_DIR}/scaled/${f}.webm"; printf 'Progress: %s/%s [%s%%]     \r' ${f} ${FRAMES} $(printf 'scale=2;%s*100/%s\n' ${f} ${FRAMES} | bc); done; printf '\n'

    #concatenate all intermediary files; try to map audio from input file; if input file has no audio rerun without mapping it
    ffmpeg -thread_queue_size ${FRAMES} -f concat -safe 0 -i <(find ${TMP_DIR}/scaled/ -name '*.webm' -printf "file '%p'\n" | sort) -i "${INPUT_FILE}" -c:v copy -c:a libopus -map "0:v" -map "1:a" ${OUTPUT_FILE} || ffmpeg -thread_queue_size ${FRAMES} -f concat -safe 0 -i <(find ${TMP_DIR}/scaled/ -name '*.webm' -printf "file '%p'\n" | sort) -c:v copy  ${OUTPUT_FILE}

    #remove intermediary files
    rm -r "${TMP_DIR}"

    #exit normaly
    exit 0
else
    #exit with error
    printf 'Usage: %s [INPUT FILE] {OUTPUT FILE}\n' $0
    exit 1
fi