Setting JPG density (dpi) with no increase in file size

Question

I use (under Windows) the following command

magick convert -units pixelsperinch file_in -density 600 file_out

to set the dpi (no resampling, as dpi basically is, as far as I understand, just a tag that specifies pixel size) of a JPG image. It works, but I don't understand why it increases file size by several kiloBytes (an image of mine originally of 1658 kB got to 1717 kB, which is a 59 kB increase), whereas I would expect an increase, if any, of just a few bytes.

Did I get something wrong? Is it possible to change by command line (tools other than ImageMagick are welcomed too) density/dpi of a JPG without increase in file size?

Thanks in advance for any clue.

Answer 1

As far as If I understand, there are several flavors of JPEG files. The details needed to change DPI metadata of JFIF flavor are explained here. Based on that I wrote my own Python script which allows to change DPI setting of a JFIF flavor JPEG without reencoding:

import sys,os

filename = sys.argv[1]
x_density = int(sys.argv[2])
y_density = int(sys.argv[3])

echo = True
if len(sys.argv) > 4:
    if sys.argv[4] == 'quiet':
        echo = False

assert x_density > 0 and x_density < 65536
assert y_density > 0 and y_density < 65536

# JPEG markers
APP0 = bytes.fromhex('FFD8FFE0') # JFIF
APP1 = bytes.fromhex('FFD8FFE1') # EXIF

with open(filename, 'rb+') as f: # 'w+b'
    chunk = f.read(4)
    if chunk == APP0:
        f.seek(2,1) # relative seek
        chunk = f.read(5)
        if chunk == bytes.fromhex('4A46494600'): # JFIF indentfier
            f.seek(2,1) # relative seek
            print('Setting density of ' + os.path.split(filename)[1] + ' to ' + str(x_density) + 'x' + str(y_density) + ' dpi.') if echo else None
            f.write(bytes.fromhex('01'))
            f.write(x_density.to_bytes(2,'big'))
            f.write(y_density.to_bytes(2,'big'))
        else:
            print('File hasn''t got the JFIF indentifier, nothing was done.')
    elif chunk == APP1:
        f.close() # needed otherwise exiftool cannot operate on file
        print('This is an EXIF-JPEG, using exiftool to set DPI...')
        os.system('exiftool -P -overwrite_original -XResolution={} -YResolution={} "{}"'.format(x_density,y_density,filename))
    else:
        print('File is not JFIF nor EXIF, cannot set DPI, nothing was done.')

print('Done.') if echo else None

Usage:

python this_script.py some-image.jpg Xdpi Ydpi [quiet]

The script does not read the full image nor change the file length, it just modifies a few bytes directly on the JPEG file. Moreover, no temporary/backup copy is made, because I wanted the script to work on hard-linked files, so the overall process is quite fast for JFIF JPEGs.

The script is able to identify EXIF JPEGs and use exiftool to change DPI. If you don't have exiftool installed in your computer remember to adjust the script accordingly. A reason to use this script even if you have exiftool installed is speed; in my test this script is much faster than exiftool.

JFIF and EXIF are the most common flavors of JPEG files, but I hope someone is able to improve this script or report a way to set DPI (without reencoding) also for Adobe JPEGs with APP14 marker, which are not so rare.

Answer 2

Trying to reproduce your problem:

• Gimp's interlaced/progressive JPEG is smaller than the non-interlaced version (also produced by Gimp)
• The magick convert output JPEG is not interlaced, and is in fact a bit smaller than Gimp's own non-interlaced. That file is the same size whether it is produced from an interlaced or non-interlaced version.

So, I would think that you are converting an interlaced/progressive JPEG. Note that this also demonstrates that IM is re-encoding the file, and comparing the original and the re-encoded in Gimp shows significant differences.

In the JPEG format, the H/V definitions are encoded in 4 bytes in the header, patching that should be a SMOP in about any programming language.

Answer 3

You can change/set the density without re-encoding the file (and thereby possibly changing its size or quality) with the much smaller, lighter weight and easier-to-install exiftool which "just" a Perl script:

exiftool -jfif:Xresolution=300 -jfif:Yresolution=300 YourImage.jpg

Different people call different things the density/resolution, so if the above command doesn't do what you want/need/hope/expect, maybe try:

exiftool -XResolution=300 -YResolution=300 YourImage.jpg

Answer 4

Try using the online tool https://convert.town/image-dpi. On my JFIF jpg this only changed the DPI and did not resample the image. Other software suggestions can be found on this Super User question, but this was the only one that worked for me without changing anything else in the image.

However I only have a JFIF jpg and you appear to have an Adobe jpg from your other comments, so your mileage may vary.

I verified the DPI change and no resampling using the Windows hex editor HxD, just looking at the 4 bytes detailed in the Wikipedia article linked by xenoid. And once you know what bytes are changing, you can skip the website and use a hex editor directly to set the X and Y DPIs.