Quality loss when saving as JPEG

root

So recently an article was linked on reddit about a bunch of photography myths. Number 7, called “Saving a JPEG multiple times degrades quality” which was apparently proven to be wrong disturbed me most and so I did a quick test on my own.

Further below are Three images. The Original, the one saved 30 times and an XOR of the first Two showing the differences. If you toggle between the first two you can see quite a difference in the color on the car and several other details. Running any more tests at lower than 100% quality is pretty much a waste of time once you see these results.

Here’s how I constructed these images. Feel free to run your own test, it’s really simple. All you need is a linux machine and ImageMagick. The code below will do the following:

  • save image0 to image1 at quality 100%
  • save image1 to image2 at quality 100%
  • …
  • save image29 to image30 at quality 100%
  • calculate the difference (XOR) between image0 and image30
# re-save the image 30 times
L=0
for x in {1..30}; do
    echo $x
    convert -quality 100 image_${L}.jpg image_${x}.jpg
    L=${x}
done

# calculate the diff between the 2 images
convert image_0.jpg image_30.jpg -fx "(((255*u)&(255*(1-v)))|((255*(1-u))&(255*v)))/255" diff.jpg