JPEG2000 is a fairly new standard which was meant as an update of the
wide-spread JPEG (Joint
Photographic Expert Group) image standard. JPEG2000 may share
its name with the older standard but under the skins the relationship
between the two is next to nil. The lossy image compression mode of
JPEG2000 is based on the so-called "discrete wavelet transformation"
which is supposed to be more efficient compared to the JPEG algorithm
based on "cosine transformation"..
Now where's the beef ... ? JPEG2000 offers ... (citing the JPEG2000 FAQ
at http://www.jpeg.org)
Better efficiency in compression (incl. 48 bit color depth support)
Possibility of lossless compression>
Decoding with different output resolutions>
A process to calculate the integrated bit rate (possibility of reaching an aimed bit rate)
Dividing the image into smaller parts to be coded independently from the others
Improvement in noise resilience
Access to the compressed bit rate at any point in order to access the image directly
Better performances in coding/decoding through many different cycles
More flexible file format
The current reality
Similar to JPEG there's no requirement to implement to complete
standard which can result in incompatibilities between different
applications (it was the same in the early days of JPEG). While
the complexity of the implementation didn't really help to boost the
acceptance in the field JPEG2000 is now slowly entering the mainstream.
The very latest imaging applications often offer a JPEG2000 file format
though it still tends to be limited to 24 bit color depth.
The relatively harsh computing requirements have probably prevented the
adaptation in digital cameras so far.
The Tool
Test Environment:
Photoshop 6 with JPEG2000 Plug-in from http://www.fnordware.com/
The plug-in is free and unlike many competitors it seems to be a usable
implementation incl. lossless mode and especially 48bit color support.
Some special features are not implemented but for normal image
archiving the Fnord implementation is just fine. The plug-in is
available both for Win as well as Mac for Photoshop 5.5 & up.
The JPEG2000 dialog box is pretty simple. You can select between lossy
and lossless compression. In lossy mode you can select whether to focus
either on a specific target file size or an abstract quality level. The
tool automatically switches between 24bit or 48bit storage dependent on
the color depth of the raw image in Photoshop.
Let's have a look at it ...
The analyzed image has a raw size
of about 2 MB (= 1024 x 669 pixel, 24 bit color depth).
At 100% quality (Photoshop JPEG quality = 12) JPEG gives us a
compression of 990 KB vs 810 KB with JPEG2000. So at 100% quality the
compression is a little better than 1:2 here. Right from the start
JPEG2000 has a 20% advantage here - not extreme but more than nothing.
Below you can find an image portion comparison of JPEG and JPEG2000
normalized to a specific compression level (except at 100% quality):
JPEG2000
set to 0.09MB
JPEG
Level 0
"low quality"
0.09MB
JPEG2000
set to 0.17MB
JPEG
Level 4
0.17MB
JPEG2000
set to 0.30MB
JPEG
PS Level 8
"high quality"
0.30MB
JPEG2000
100%
0.81 MB
JPEG
PS Level 8 (100%)
0.99 MB
As expected the difference in quality is next to zero when comparing
images with a quality level of 100% but there're some more or less
obvious differences at more bold compression rates.
Let's start having a look at the extreme end - at a compression rate of
1:20. See the left 2 pictures in the image row above as a reference
without magnification.
The following images are ENLARGED BY
200% for a better illustration of the differences:
ORIGINAL
IMAGE
JPEG2000
1:20
JPEG
1:20
A compression rate of 1:20 is quite extreme and naturally even JPEG2000
can't recreate the fine structures of the original image. Nonetheless
it does a much better job than JPEG here. You may notice that the
(in-)famous blocks (the 8x8 pixel blocks from the cosine
transformation) are no longer present in the JPEG2000 image. The halo
artifacts around contrast transitions (e.g. roof to sky) are also much
less pronounced and there're no extreme color defects. Overall the
JPEG2000 result looks soft, like sprinkled with water, but it isn't
completely unusable like the JPEG result.
Ok, let's have a look at a much more conservative compression rate -
Photoshop JPEG Level 8 ("High Quality") or a compression rate of about
1:7 plus a JPEG sample at 1:4. The images below are ENLARGED BY 400%.
ORIGINAL
IMAGE
JPEG2000
(compression 1:7)
JPEG -
PS Level 8 "high"
(compression 1:7)
JPEG -
PS Level 10 "maximum"
(compression 1:4)
This is getting more interesting now because compression levels of 1:6
or 1:7 are widely used in digital cameras. It is already quite hard to
spot any image degradation in the original-sized images. Nonetheless you
can easily notice some difference when enlarging some critical image
portions. Upon closer observation there's still a very slight
difference between the original and the compressed JPEG2000
variant. Looking at the sky the JPEG2000 compression has a
smoothening effect which is in fact positive in this context. All
critical details remain intact. On the other hand the compressed JPEG
picture is still introducing some quite strong halo artifacts again
though there're no color shifts anymore. The quality is probably less
than what can be considered acceptable as "near lossless". Even
at the further reduced compression level of 1:4 the image cannot
match the 1:7 compressed JPEG2000 variant.
JPEG2000 vs TIFF
Thanks to its 48bit color mode JPEG2000 is also serious competitor for
the ancient but still wide-spread TIFF format.
I did a short test with a large image file (5443x3636 pixel,
16bit color depth):
TIFF
116 MB
TIFF with LZW compression
150 MB (which proves that
LZW is ineffective for image compression)
JPEG2000 - lossless
61 MB
JPEG2000 - 100% quality
(lossy mode)
21 MB
PSD (native Photoshop)
116 MB
Assuming that the JPEG2000 implementations will stabilize soon it is
quite obvious that TIFF may not be the primary choice for image
archiving much longer. If needed you can convert from JPEG2000 back to
TIFF anyway.
References:
For technical background information about JPEG2000 you may have a look
at the JPEG2000 FAQ
at http://www.jpeg.org.
