[Digikam-users] jpeg compression

[Gilles Caulier]

2007/6/30, Thorsten Schnebeck <thorsten.schnebeck at gmx.net>:
>
> Hi,
>
> > To have studied it indeep, i can said than :
> >
> > DNG == TIFF + new tag.
> > DNG is now an ISO standard (i have seen a message about it into libtiff
> ML)
> > DNG is limited to store image data in 2 way : the first is a pseudo
> > lossless JPEG compression supporting 16 bits color depth (in fact JPEG
> > algorith with compression level set to 100, but supporting 16
> > bits/color/pixels). The compression ratio is good, but it still JPEG
> > stuff...
>
> This is IMHO wrong. You mix up an image format with the Joint Photographic
> Experts Group. DNG uses JPEG lossless huffman:
> http://en.wikipedia.org/wiki/Lossless_JPEG
> or deeper:
> http://www.hpl.hp.com/loco/HPL-98-193R1.pdf
>
> > This way is used by all camera which support DNG as well, and not
> > the second way...
> > DNG support only a _real_ lossless image data storage, which is in fact
> the
> > linear 16 raw image data. DNG do not support the famous Adobe Deflate
> > compression algorithm provide by TIFF file format. It really stupid...
> This
> > one give the equivalent compression ratio results of PNG. If you try to
> use
> > DNG converter from Adobe, and you use the RAW linear storage of image
> data,
> > the DNG file will be huge !
>
> The linear 16bit raw converted data is not recommended. I found this
> thread
> which helps to understand some DNG (fallback) concepts:
> (from: http://www.adobeforums.com/cgi-bin/webx/.3bc0d8de)
> > > goodlux  - 2:31am Feb 26, 07 PST (#14 of 15)
> > > =============================
> > > I wish this information was presented by Adobe in a clearer fashion,
> > > perhaps in the help menu for the Converter itself.
> > >
> > > Users should be given a little more information so they can understand
> > > some of the implications of using the various options that the
> converter
> > > presents.
> > >
> > > For instance: it is helpful for a user to know that if they create a
> .dng
> > > file and embed the raw file, and create a full-sized preview, that
> they
> > > will end up with a file that is double the size of the original raw
> file.
> > > So if they convert all their raw files this way, they will need double
> > > the space.
> > >
> > > Also, I'm not clear at all about the whole "preserve raw image" vs
> > > "convert to linear image" option at all. Why would a use want to do
> one
> > > or the other? What are the pros and cons?
> > >
> > > Same thing about Lossless compression...why wouldn't someone want
> > > lossless compression? Is there a performance hit? How much of a
> > > difference does this compression usually make?
> > >
> > > I came across this post because I have the same question as the
> original
> > > poster.
> > >
> > > How is it possible that a .dng file is smaller than the original raw
> > > (.cr2) file? A .cr2 file is already compressed with lossless
> > > compression...and that compression is fairly state of the art. How is
> it
> > > possible that .dng can do it better? There must be some data getting
> > > thrown out in the conversion...or else you wouldn't have the option of
> > > embedding the raw file. So what is getting thrown out? How important
> is
> > > it? I'm particularly concerned with Canon files.
> > >
> > > Does anyone have some solid, knowledgeable information?
> > >
> > >
> **************************************************************************
> > >
> > > Barry Clive Pearson  - 8:10am Feb 26, 07 PST (#15 of 15)
> > > ====================================
> > > "Also, I'm not clear at all about the whole "preserve raw image" vs
> > > "convert to linear image" option at all. Why would a use want to do
> one
> > > or the other? What are the pros and cons?" By default, this is "don't
> > > convert", and that is right. Converting it means doing a raw
> conversion,
> > > which means that later products don't get their own chance to do it.
> (It
> > > also results in a bigger file).
> > >
> > > Sometimes products can handle "linear DNGs" but not the unconverted
> DNGs.
> > > For example, I've known products that can't handle the unconverted
> data
> > > from a Fujifilm camera, but could handle the Linear version, so in
> that
> > > case converting was the only way to get the file processed by that
> > > product.
> > >
> > > "Same thing about Lossless compression...why wouldn't someone want
> > > lossless compression? Is there a performance hit? How much of a
> > > difference does this compression usually make?" I think there have
> been
> > > products that couldn't uncompress DNG files, but that doesn't appear
> to
> > > be common. So normally compressed DNG is good.
> > >
> > > "How is it possible that a .dng file is smaller than the original raw
> > > (.cr2) file? A .cr2 file is already compressed with lossless
> > > compression...and that compression is fairly state of the art. How is
> it
> > > possible that .dng can do it better?" DNG just does it a little
> better!
> > > (In fact, CR2 and DNG both use the same type of lossless compression,
> but
> > > there appears to be different levels of compression, perhaps because
> DNG
> > > tiles it in a way that optimises compression?)
> > >
> > > DNG files from CR2s should hold a superset, not a subset, of what is
> CR2
> > > files.
>
>
> > For me DNG do not give any advantages against PNG...
> >
> > Gilles
>
> The main difference is that DNG can keep the mosaiced representation of
> the
> sensor in the image data. Together with new standard and open metatags you
> can use future raw converter techniques with your original sensor data.
> In PNG the image representation is already RGB ordered.
>
> This handles (in http://www.adobe.com/products/dng/pdfs/dng_spec.pdf )
> the "PhotometricInterpretation" metatag.
> When the spec speaks from CFA they mean Color Filter Array - this is the
> sensor pattern.
>
> DNG is not that evil ;-)



Perhaps, but in DNG, if you records original CFA data from original RAW file
+ the decode image in JPEG format, you have a huge file !

DNG It's not a revolution for me...

Gilles

HTH
>
>   Thorsten
>
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