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JPEG
Compression: Data Loss & Image Impact
Guest Article by Oskar Breuning | Additional Comments By Jim
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A friend of mine, Oskar Breuning, recently enlightened me regarding
an aspect of working
with JPEG format files that may drastically impact image quality.
At the core of Oskar's observation is that when working with a JPEG
file, in many programs (potentially even Photoshop which we're verifying),
each alteration made will prompt a save in the program before you
manually pursue a final save of your changes. The end result is not
just one round of data loss due to JPEG compression when you save
your final version of an image, but several rounds of data loss reflecting
the number revisions made before your final image is saved. I've always
known about the theoretical impact of JPEG compression on images,
but have never seen a definitive test showing the extent compression
can ruin an image until now thanks to Oskar. For those not sold on
working with a lossless format such as Photoshop (PSD) upon seeing
the results posted below you will likely become a believer.
I know a lot of photographers now use dSLRs saving photos in RAW format. (I do,
too). For those photographers capturing images in JPEG format only and others that edit JPEG images outside of the world of photography here's something useful to know:
Since JPG compression is lossy compression, with every save you lose
data. The loss is set up in a way that it tries to remove data the
eye cannot quite see (two pixel with almost same color and brightness).
So every time you open a JPG, do something with it and save it as
JPG again you lose data. That also means when you press the "rotate"
button in windows, you already lose data (the image is opened, rotated
and saved again).
I became curious and wanted to see what happens as numerous alterations
are made. I set up ImageMagick's command line "convert" program on
my home Linux box to do several rotations, and re-saving. Attached
is the result with (4) four different images and different numbers
of rotations.
What was interesting to me was, that I expected to see the typical
8x8 JPG blocks or something along that line to appear, but instead
the main change in the image was the desaturation of color .
Note: Images were always saved with highest JPG quality. Lower quality will
probably cause more severe image degradation.
To prevent this, if you do shoot JPEG, you will want to save your
image on your camera or computer as TIFF or Photoshop (PSD) and then
work with that format. You can save this as often as you like without
losing anything. Then, you save a copy as the final version as JPG
again, which you can email to your friends and family. In this way
you lose a minimal amount of data once that is not visible. It's the
saving again and again that is a problem. Otherwise JPEG is great
is a great file format.
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| JPEG
Compression TEST - Image Gallery |
click to enlarge |
| JPEG Compression - Sequence of Alteration Results |
click to enlarge |
| JPEG
Compression - Before & After Comparison |
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