Re: NEWS: HD Photo to become JPEG XR

Barry Pearson schrieb:

I have published my own tests at the page below. My conclusion from
these (admittedly limited) tests was:

"For any given set of quality values, the HD Photo and JPEG 2000 files
were about the same size, and significantly smaller than the JPEG
file". ("Half the size" would be a fair generalisation, and at that
level probably fewer nasty artefacts). The basis for these conclusions
is at the following page - I won't repeat them here:
http://www.barrypearson.co.uk/articles/hdp/analysis_lossy.htm


How do you define "quality"? What is a "quality value"? You have to be
*very* careful defining terms, or you're likely comparing apples to
oranges. Sorry to be picky, but one has to be pretty careful when
testing.

Some typical mistakes (not saying that you did it, just things to watch out for):

The "quality" value of HDPhoto has nothing to do with the "quality" value of
JPEG. JPEG2000 doesn't even define something like that, but lets you choose the
target file size. Vendors might prefer to offer a "quality" setting, but that's
then entirely vendor defined.

Others measure "quality" in terms of PSNR (peak signal to noise ratio), then
often logarithmic in dB. This "quality" relates pretty badly to "visual impression",
it is easy to generate two images with the same dB as "quality", one of them
looking great, the other looking unacceptable. Better metrics exist, none of them
is perfect, but most of them are notably better than PSNR.

This said, our tests do the following:

i) Pick a test image set, for all images in the set:
ii) compress the image to quality level x of HDPhoto
iii) measure the file size returned by HDPhoto
iv) compress the same image to the same file size with JPEG2000 (yes, JPEG2000 can
do that)
v) Use either an objective visual quality metric (we've used M-SSIM, VDP and PQS)
or run subjective tests (with people actually looking at the images), and by that
define a quality.

vi) compile results by plotting "perceived" or "objectively measured" quality over
output rate.

When doing this test (and not measuring PSNR and not adjusting obscure quality
settings), you really compare apples to apples. Unfortunately, and that's what I
said, HDPhoto doesn't at all perform very well. And, as said, I hope that this is
fixable, but the current version is not.

Furthermore, please also note that the *current* test image set is still limited,
more tests are to be done.

The major drawback is its lack of standardization. The problem seems that camera
vendors prefer to bind their customers instead of making images interchangeable.
I don't see why JPEG-XR would change their position, but that's all my guesswork.
Otherwise, it would have been easy just to approach the JPEG to standardize *some*
type of raw format - it is IMHO just not desirable for the vendors.

There IS an ISO standard raw file format. ISO 12234-2 (TIFF/EP). (It
became an ISO standard in 2001, and some manufacturers such as Canon
and Nikon based their own raw file formats on it). The problem is that
it was never really fit for the purpose of standardised interchange,
and it has become out-of-date. (In effect, DNG, also based on that
standard, is ISO 12234-2 brought up-to-date and made fit for purpose).

ISO are revising ISO 12234-2, and Adobe have given them permission to
use the features of DNG in the revision. (Just as they gave ISO
permission to use TIFF in the original version of ISO 12234-2).

Given that ISO's TC42 WG18 has responsibility for TIFF/EP, it would be
diversionary for another working group to "compete" to standardise a
raw file format. Far better, surely, for all standards bodies and
working groups to concentrate on one standard, especially if it can
based on a format (DNG) that is supported in some way by nearly 200
products of various kinds. (I would like to see the revised ISO
12234-2 either BE DNG, or be compatible with DNG sufficiently for
products to work with a common subset).

Thanks for the pointers to TC42 (I'm SC29, so there you go), I'll ask
there. In my impression, TC42 doesn't really have too much support
or acceptance from the camera vendors, but I'm (currently) only watching
this from the outside, so I cannot really say for sure. I'd be personally
happy if any type of "raw format" standardization would arrive at a format
that is really accepted and that makes image data interchangeable, but
probably there are too many sensor types, or too many trade secrets - I don't
know.

So long,
Thomas
.

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