Re: jpeg compression

Arnd,

I took an image out of the camera ( a Kodak C40, this time)
and saved it with 
different values for jpeg quality. The orginal file was 1.2
MB big, and here 
are the sizes with diffrerent levels of jpeg quality:

75 -> 1.3 MB
85 -> 1.8 MB
90 -> 1.9 MB
92 -> 2.3 MB
95 -> 2.6 MB
97 -> 3.2 MB
100 -> 4.5 MB

All of them bigger than the original file. I don't thimk I
understand this.
-Bjørn
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Arnd,
One more thing I discovered. The Gimp makes smaller jpeg
files than Digikam 
with the same levels of jpeg quality: 100: 2.9MB (Digikam
4.5), 75: 1.1 MB 
(Digikam 1.3)

 
Torsdag 28 juni 2007 16:16, skrev Bjørn Kvisli:
> Arnd,
>
> I took an image out of the camera ( a Kodak C40, this
time) and saved it
> with different values for jpeg quality. The orginal
file was 1.2 MB big,
> and here are the sizes with diffrerent levels of jpeg
quality:
>
> 75 -> 1.3 MB
> 85 -> 1.8 MB
> 90 -> 1.9 MB
> 92 -> 2.3 MB
> 95 -> 2.6 MB
> 97 -> 3.2 MB
> 100 -> 4.5 MB
>
> All of them bigger than the original file. I don't
thimk I understand this.
> -Bjørn
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> Digikam-userskde.org
> h
ttps://mail.kde.org/mailman/listinfo/digikam-users
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On Thu, 28 Jun 2007, [iso-8859-1] Bjørn Kvisli wrote:

> Arnd,
>
> I took an image out of the camera ( a Kodak C40, this
time) and saved it with
> different values for jpeg quality. The orginal file was
1.2 MB big, and here
> are the sizes with diffrerent levels of jpeg quality:
>
> 75 -> 1.3 MB
> 85 -> 1.8 MB
> 90 -> 1.9 MB
> 92 -> 2.3 MB
> 95 -> 2.6 MB
> 97 -> 3.2 MB
> 100 -> 4.5 MB
>
> All of them bigger than the original file. I don't
thimk I understand this.

This suggests, that maybe something like 70 was used inside
of the camera?

>From the number of pixels Nx * Ny, the number of bytes
needed
to store the image (without) compression should follow from
Nx * Ny * (8+8+8), i.e. one Byte per R, G, B.

What is the resolution of your camera?

Anyway, overall the above looks pretty reasonable to me.

Best, Arnd
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Torsdag 28 juni 2007 16:31, skrev Arnd Baecker:
> On Thu, 28 Jun 2007, [iso-8859-1] Bjørn Kvisli wrote:
> > Arnd,
> >
> > I took an image out of the camera ( a Kodak C40,
this time) and saved it
> > with different values for jpeg quality. The
orginal file was 1.2 MB big,
> > and here are the sizes with diffrerent levels of
jpeg quality:
> >
> > 75 -> 1.3 MB
> > 85 -> 1.8 MB
> > 90 -> 1.9 MB
> > 92 -> 2.3 MB
> > 95 -> 2.6 MB
> > 97 -> 3.2 MB
> > 100 -> 4.5 MB
> >
> > All of them bigger than the original file. I don't
thimk I understand
> > this.
>
> This suggests, that maybe something like 70 was used
inside of the camera?

If I understand this right, assuming that the Kodac C340
uses jpeg quality of 
70 internally, it makes no sense to set a higher value in
Digikam?

Hm.. I must find out what kind of compression my Nikon
uses.

>
> >From the number of pixels Nx * Ny, the number of
bytes needed
>
> to store the image (without) compression should follow
from
> Nx * Ny * (8+8+8), i.e. one Byte per R, G, B.
>
> What is the resolution of your camera?
The resolution is 2576x1932 (4.98Mpx)
>
> Anyway, overall the above looks pretty reasonable to
me.
>
> Best, Arnd
> _______________________________________________
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> Digikam-userskde.org
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ttps://mail.kde.org/mailman/listinfo/digikam-users
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On Donnerstag, 28. Juni 2007, Bjørn Kvisli wrote:

> If I understand this right, assuming that the Kodac
C340 uses jpeg quality
> of 70 internally, it makes no sense to set a higher
value in Digikam?

it still makes sense, because every new save looses some
more information. 
Although its not that dramatic in reality, you could think
of percents just 
to better understand: then if you save the 70% with 100% you
get 70% again, 
if you save it with 70% you get only 49% (70% of 70%) and so
on.

I found that a jpg value of 88 in digiKam gives approx. same
results as 80 in 
photoshop (in quality and size). That's the value I use for
my web pics.

However, as long as you work with the pictures better use a
lossless file 
format and only save the final pictures to .jpg (if at all).
And leave the 
originals untouched.

kind regards

Daniel
-- 
Daniel Bauer photographer Basel Switzerland
professional photography: http://www.daniel-bauer.c
om
erotic art photos: http://www.b
auer-nudes.com/en/linux.html
Madagascar special: http://www.sanic.ch
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Dnia czwartek 28 czerwiec 2007, Daniel Bauer napisał:
> On Donnerstag, 28. Juni 2007, Bj?rn Kvisli wrote:
> > If I understand this right, assuming that the
Kodac C340 uses jpeg
> > quality of 70 internally, it makes no sense to set
a higher value in
> > Digikam?
>
> it still makes sense, because every new save looses
some more
> information. Although its not that dramatic in reality,
you could think
> of percents just to better understand: then if you save
the 70% with
> 100% you get 70% again, if you save it with 70% you get
only 49% (70% of
> 70%) and so on.

AFAIU JPEG this is not exactly true. Quality is level of
losing
information. After saving to 70 you lost all information
above that and
next savings to 70 shouldn't cause losing of more detail. 

Note however that different implementations may differ and
70 level in
one program isn't exactly the same 70 in another.

m.

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Torsdag 28 juni 2007 18:13, skrev Gilles Caulier:
> 2007/6/28, Bjørn Kvisli <bjorn.kvislitele2.no>:
> > Arnd,
> > One more thing I discovered. The Gimp makes
smaller jpeg files than
> > Digikam
> > with the same levels of jpeg quality: 100: 2.9MB
(Digikam 4.5), 75: 1.1
> > MB (Digikam 1.3)
>
> Gimp and digiKam jpeg compression level range are
different. digikam is
> more like photoshop stuff.
>
> Also in digiKam i have set 2 options to optimize the
quality instead the
> size of files...
Giles, which 2 options do you mean? I use Digikam 0.9.0
>
> This is why you can see diff...
>
> Gilles
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Torsdag 28 juni 2007 19:01, skrev Mikolaj Machowski:
> Dnia czwartek 28 czerwiec 2007, Daniel Bauer napisał:
> > On Donnerstag, 28. Juni 2007, Bj?rn Kvisli wrote:
> > > If I understand this right, assuming that the
Kodac C340 uses jpeg
> > > quality of 70 internally, it makes no sense
to set a higher value in
> > > Digikam?
> >
> > it still makes sense, because every new save
looses some more
> > information. Although its not that dramatic in
reality, you could think
> > of percents just to better understand: then if you
save the 70% with
> > 100% you get 70% again, if you save it with 70%
you get only 49% (70% of
> > 70%) and so on.
>
> AFAIU JPEG this is not exactly true. Quality is level
of losing
> information. After saving to 70 you lost all
information above that and
> next savings to 70 shouldn't cause losing of more
detail.

Well, I really hope this is the case. Loosing detail with
each save would 
really be bad.
-Bjørn
>
> Note however that different implementations may differ
and 70 level in
> one program isn't exactly the same 70 in another.
>
> m.
>
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On Thu, 28 Jun 2007, [utf-8] Bjørn Kvisli wrote:

[...]

> Well, I really hope this is the case. Loosing detail
with each save would
> really be bad.

Have a look at
http://
www.cywarp.com/faq_jpg_degradation.htm
(Of course best is to try it out ... 

So if this is important for you, then
a) use png to save edited pictures
b) if you really aim at best quality, then start with raw
images
However, then you should not worry too much about
file sizes, they are just as big as they are .... 

Best, Arnd
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