re JPEG lossiness, PNG

[Simon Oosthoek-6]

On 14/01/12 17:15, Jean-François Rabasse wrote:
>
> JPEG can compress in a lossless way, until the algorithm bumps against
> some limits. GIF does so, PNG does so. But JPEG can boost compression
> efficiency if the user accept some losses.
> That's what Marie-Noëlle Augendre said, on this thread :
> "I guess that to produce something smaller, you'll have to loose
> something."
> Definitely right, there's no magic at all, and Santa Claus doesn't
> exist:-)

Maybe I've missed a part of the discussion, but the main concern with
Jpeg is, AFAIK, that jpeg is 8-bits, so always loses something when
using RAW as the reference, since RAW formats usually have 10-16 bits
per colour available as bandwidth and most SLR sensors have the ability
to provide that dynamic range to a certain extend, converting from RAW
to JPEG will at least cost you the difference in expressibility of
colour and brightness (e.g. 12 bits in RAW to JPEG: 4096 to 256 shades).
Even if no loss was caused by the JPEG algorithm, JPEG loses something.

This is why PNG and JPG2000 are popular choices; they allow 16-bit
values to be preserved and allow lossless compression (meaning it is
reversible to "RAW" in theory)

/Simon
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[Remco Viëtor]

On Saturday 21 January 2012 19:58:22 Simon Oosthoek wrote:
> This is why PNG and JPG2000 are popular choices; they allow 16-bit
> values to be preserved and allow lossless compression (meaning it is
> reversible to "RAW" in theory)
No, it is not: RAW files have as many pixels as the resulting PNG files (in
theorie at least), but each represents only one colour channel out of three
(or four*). The different colours are arranged in a matrix (Bayer matrix), so
that a square of 4 pixels has the three colours, with green being present
twice.
To get the image on which we work (and which is stored as PNG or whatever),
those colours are interpolated, so the original values are replaced by
calculated values, and the missing colours are added for each pixel.Due to
this interpolation, there is no guarantee that you can recover the original
values.

Note that the term "lossless" as applied to a compression algorithm only
implies that the compression is reversible. RAW -> PNG is a bit more than just
a compression, and there is no guarantee that all steps are reversible.

*: certain systems use 2 different greens
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[Andrew Goodbody]

On 21/01/12 15:25, Peter Mc Donough wrote:

>> OK, not listed, but it may work anyway. Did you try it? If it did not
>> work did you provide sample files to Dave Coffin for him to implement
>> the support?
>
> No, I didn't. When I bought the camera, at that time I used jpeg - it
> was a special offer and has suited my idea of a DSLR since - there were
> at least three newer Olympus DSLR camera model generations.
> Later I gave RAW a try and browsing the web I couldn't find any demand
> for a RAW profile of my "new" camera, so asking for one especially for
> me seems to be a waste of the "resource" Dave Coffin.

It would not be especially for you. It would be for you and for all the
other people out there who also looked for support and did not find it
and then also did nothing. So your failure to send samples to Dave
Coffin is actually depriving other people. Just because you could find
no published demand for support does not mean the demand does not exist,
merely that those that want it have not published about it.
Why not let Dave Coffin be the judge of what is a waste of his time?

> In fact, Digikam could read the original raw file and I didn't notice
> any problems. On the other hand, I don't know enough about RAW files for
> deciding what, if anything, was missing or faulty.
>
> What I read in the web was an unhappiness about propriatary RAW formats.
> There may be a good reason for propriatary formats - the obvious one I
> see and don't like is that a user may stick to one brand because his
> valuable photos are of in a certain RAW type.

Sounds like a bogus reason to me. I would question if anyone thought
that way. Raw formats change even in the same brand from one generation
of camera to another. The solution is to have software support for both
old and new format. Lots of bigger reasons for sticking to one brand eg
investment in lenses and other accessories.

> What brough me to Adobe DNG were several discussions, among them:
>
> http://mansurovs.com/dng-vs-raw

Interesting article. I had not noticed the reduction in the size of DNG
files in comparison to raw files from the camera. I wonder why that is.
One possibility is that a PC has the computing resources to be able to
do a better compression than the camera processor. So that would not be
a feature of DNG per se, merely the recompression of the data with more
resources available to do it. I wonder how the file size of those
cameras that produce DNG natively compares.
Unfortunately the article is not clear about issues such as the reduced
size does not apply if you opt to embed the original raw file, quite the
reverse in fact. Also manufacturer developing programs can write to
their own raw file format so you do not necessarily have sidecar files.
Also it is likely that open source will, in time, gain the ability to
write to other raw formats (I think it can already write to some but not
all). Some of the advantages and disadvantages depend on the particular
software in use and do not really apply in practice.

> The version I have, DNGConverter 6.5, runs under standard Wine in
> Opensuse 11.4 64bit and of course in virtualized Windows XP.

digikam, darktable, rawtherapee, dcraw, ufraw etc all run natively in
Linux. BTW digikam can do the conversion from camera raw to DNG.

> I am very pro open standards and when I buy my next camera I will check
> before whether its RAW format is supported under Linux.
>
> Peter

I also am very pro open standards and would prefer all cameras to
produce raw files in a truly open standard format. Unfortunately even
DNG is not truly open. While Adobe have published the specification
there is no open process for developing it. DNG is owned and controlled
by Adobe.

Andrew
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[Peter Mc Donough]

Hi,

> >> OK, not listed, but it may work anyway. Did you try it? If it did
> >> not work did you provide sample files to Dave Coffin for him to
> >> implement the support?
> >
> > No, I didn't. . . .
> > . . . I gave RAW a try and browsing the web I
> > couldn't find any demand for a RAW profile of my "new" camera, so
> > asking for one especially for me seems to be a waste of the
> > "resource" Dave Coffin.
>
> It would not be especially for you. It would be for you and for all
> the other people out there who also looked for support and did not
> find it and then also did nothing. . .
>
> Why not let Dave Coffin be the judge of what is a waste of his time?

You are right, I didn't look at that from that point of view.

I checked his webside. There are instuctions on how to produce a
suitable RAW photo. I will send one to him.
http://www.cybercom.net/~dcoffin/dcraw/

Peter
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