Dumping JPEG Tables

The command line tool djpeg can be used to dump the JPEG quantization tables. These tables affect the resulting quality of the image. Low values result in a better image quality with less artifacts and higher values result in lower image quality with more visible artifacts. For further information please refer to https://en.wikipedia.org/wiki/JPEG.

To check, if a poor image quality is caused by a high compression due to high quantization table values is can be useful to dump these tables and analyzing them.

Fast forward: using the pre-built 🐳 Docker image

Ready to use Docker image is available on DockerHub. https://hub.docker.com/repository/docker/ai2ys/djpeg-dump-qtables

To dump the quantization tables of a JPEG file run the following command.

Local JPEG file

cat <jpeg file path> | docker run --rm -i ai2ys/djpeg-dump-qtables

JPEG file from URL

curl -fsSL "<jpeg file url>" | docker run --rm -i ai2ys/djpeg-dump-qtables

Tested on Linux and Windows (with WSL2).

Installing `djpeg` locally on 🐧 Ubuntu/Debian

sudo apt-get update
sudo apt-get install -y libjpeg-progs

Dumping quantization tables using `djpeg`

Using the command line tool djpeg locally the quantization tables of a file can be dumped using the following command.

djpeg -verbose -verbose -fast -outfile /dev/null <jpeg file path or url>

Building the 🐳 Docker image and running the Docker container

This Docker image is based on "alpine" and therefore is small in size.

The batch script described below uses docker-compose for running the Docker container, which will also automatically build/pull the image, if not already available.

Batch processing

For batch processing multiple files at once use the bash script ./dump_jpeg_tables.sh as described below. As the bash script uses docker-compose having installed docker-compose is a prerequisite.

# processing single or multiple files
./dump_jpeg_tables <file paths or pattern, e.g. *.jpg>

Example usage & output, dumping the tables for a single file.

$ ./dump_jpep_tables.sh img2.jpg
input file: 'img2.jpg'
libjpeg-turbo version 2.0.6 (build 20210314)
Copyright (C) 2009-2020 D. R. Commander
Copyright (C) 2015, 2020 Google, Inc.
Copyright (C) 2019 Arm Limited
Copyright (C) 2015-2016, 2018 Matthieu Darbois
Copyright (C) 2011-2016 Siarhei Siamashka
Copyright (C) 2015 Intel Corporation
Copyright (C) 2013-2014 Linaro Limited
Copyright (C) 2013-2014 MIPS Technologies, Inc.
Copyright (C) 2009, 2012 Pierre Ossman for Cendio AB
Copyright (C) 2009-2011 Nokia Corporation and/or its subsidiary(-ies)
Copyright (C) 1999-2006 MIYASAKA Masaru
Copyright (C) 1991-2017 Thomas G. Lane, Guido Vollbeding

Emulating The Independent JPEG Group's software, version 6b  27-Mar-1998

Start of Image
JFIF APP0 marker: version 1.02, density 1x1  0
Define Quantization Table 0  precision 0
           5    3    3    5    7   12   15   18
           4    4    4    6    8   17   18   16
           4    4    5    7   12   17   21   17
           4    5    7    9   15   26   24   19
           5    7   11   17   20   33   31   23
           7   10   16   19   24   31   34   28
          15   19   23   26   31   36   36   30
          22   28   28   29   34   30   31   30
Define Quantization Table 1  precision 0
           5    5    7   14   30   30   30   30
           5    6    8   20   30   30   30   30
           7    8   17   30   30   30   30   30
          14   20   30   30   30   30   30   30
          30   30   30   30   30   30   30   30
          30   30   30   30   30   30   30   30
          30   30   30   30   30   30   30   30
          30   30   30   30   30   30   30   30
Start Of Frame 0xc0: width=30, height=20, components=3
    Component 1: 2hx2v q=0
    Component 2: 1hx1v q=1
    Component 3: 1hx1v q=1
Define Huffman Table 0x00
          0   1   5   1   1   1   1   1
          1   0   0   0   0   0   0   0
Define Huffman Table 0x10
          0   2   1   3   3   2   4   3
          5   5   4   4   0   0   1 125
Define Huffman Table 0x01
          0   3   1   1   1   1   1   1
          1   1   1   0   0   0   0   0
Define Huffman Table 0x11
          0   2   1   2   4   4   3   4
          7   5   4   4   0   1   2 119
Start Of Scan: 3 components
    Component 1: dc=0 ac=0
    Component 2: dc=1 ac=1
    Component 3: dc=1 ac=1
  Ss=0, Se=63, Ah=0, Al=0
End Of Image

ai2ys/djpeg-dump-qtables