Convert multiple PNG images to JPGs using the guetzli encoder. Threaded.

convert.sh

#!/bin/bash

# This requires the guetzli jpg encoder: 
# https://github.com/google/guetzli/

# Set the desired max quality
# Quality setting of 84 is the lowest google/guetzli allows :(
# Use my fork of guetzli if you would like to use lower quality settings
# https://github.com/rectangular/guetzli 
QUALITY=84

# set max number of threads your processor can handle
MAX_THREADS=8

command -v guetzli >/dev/null 2>&1 || { echo >&2 "Error: guetzli not found. Please install. (https://github.com/google/guetzli/).  Aborting."; exit 1; }

# enable job control
set -m

echo "Converting all .png images in this folder to .jpg using guetzli encoder"

# create output directory if needed
if [ ! -d out ]; then
    mkdir out
fi

convert() 
{
    f=$1
    filename=$2
    q=$3
    guetzli --quality $q $f out/$filename.jpg
    echo "converted: $f -> out/$filename.jpg at $q quality"
}

for f in *.png; do
    q=$QUALITY
    filename="${f%.*}"

    # Adjust export quality based on filename
    # containing: @1x, @1.5x or @2x.
    # Guetzli doesn't do as well with really
    # low quality settings. Consider adding
    # 20-45% higher quality than usual for
    # standard JPG compression. Fortunately,
    # the filesize will likely still be
    # smaller than a typical JPEG export/convert.
    if [[ $f == *"@1x"* ]]; then
        q=$QUALITY
    elif [[ $f == *"@1.5x"* ]]; then
        q=80
    elif [[ $f == *"@2x"* ]]; then
        q=65
    fi

    # Limit concurrent threads
    while [ `jobs | wc -l` -ge $MAX_THREADS ]; do
        sleep 5
    done

    echo "starting task for: $f"
    convert "$f" "$filename" "$q" &
done

# wait for tasks to complete
while [ 1 ]; do fg 2> /dev/null; [ $? == 1 ] && break; done