16 bit vs 8 bit (one more time, sorry)

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Thu, 27 October 2005 13:43 Go to next message
Bill Hilton (Photoshop expert)
Someone posted this link on another NG from Deke McClelland ("Photoshop
Bible" fame) which shows an image edited with only two operations
(Levels and Auto Color) in both 8 and 16 bit, with a very noticeable
difference. I know the lack of concrete examples is Mike's main
complaint about 16 bit advocates so I thought I pass this along ...

http://www.graphics.com/modules.phpname=Sections&op=view article&artid=279

Just fyi as I think minds are already made up on both sides of the
argument, and admittedly his first move in Levels is pretty radical ...

Bill
Thu, 27 October 2005 14:24 Go to previous messageGo to next message
toby (Photoshop expert)
Bill Hilton wrote:
> Someone posted this link on another NG from Deke McClelland ("Photoshop
> Bible" fame) which shows an image edited with only two operations
> (Levels and Auto Color) in both 8 and 16 bit, with a very noticeable
> difference. I know the lack of concrete examples is Mike's main
> complaint about 16 bit advocates

There is no debate here. Photoshop provides 8 bit and 15 bit
operations; those who care to can use either.

The *real* problem, and the usual cause for most postings here and in
other forums, are that image processing newbies find the choice
confusing: They don't know when or why to use one or the other. THAT is
the problem.

There is no "answer" to the question "which is better? 8 or 16?" It's
the same kind of useless timewaster as all the other Usenet perennials
(vi or emacs? Linux or BSD? MySQL or PostgreSQL?) I can't believe busy
and intelligent people put energy into trying to answer unanswerables.
As always: "use what works for you" and "it depends what you're trying
to do".

--T

> so I thought I pass this along ...
>
> http://www.graphics.com/modules.phpname=Sections&op=view article&artid=279
>
> Just fyi as I think minds are already made up on both sides of the
> argument, and admittedly his first move in Levels is pretty radical ...
>
> Bill
Thu, 27 October 2005 14:24 Go to previous messageGo to next message
Timo Autiokari (Photoshop expert)
"Bill Hilton" <bhilton665@aol.com> wrote:

> http://www.graphics.com/modules.phpname=Sections&op=view article&artid=279
>
>admittedly his first move in Levels is pretty radical ...

E.g. with an underexposed digicamera shot you do need pretty radical
Levels adjustment. With scanned slide film you need pretty radical
Curves adjustment. With scanned negative you need horrible Curves and
Levels adjustments.

In addition, nearly all editing operations do add quantization noise
(half of the least significant bit). This noise does accumulate (not
directly additively, but it does accumulate) so after many editing
operations there will be plenty of quantization noise in the image,
easily up to two or even three lowermost bits.

In the 16-bit/c space (that in Photoshop only has 15 bits) this
quantization noise does not matter much, 3 lowermost bits full of
quantization noise still leave 12 bits for the image data.

In the 8-bit/c the 3 lowermost bits full of quantization noise leave
only 5 bits for the image data.

Btw , in photographic workflow one does not need CS or CS2 for 16bit/c
editing, I still use version 7 and fully 16-bit/c work is very easy
with it.

Timo Autiokari http://www.aim-dtp.net/
Sun, 30 October 2005 15:59 Go to previous messageGo to next message
Mike Russell (Photoshop expert)
"Bill Hilton" <bhilton665@aol.com> wrote in message
news:1130435023.301828.67600@g44g2000cwa.googlegroups.com...
> Someone posted this link on another NG from Deke McClelland ("Photoshop
> Bible" fame) which shows an image edited with only two operations
> (Levels and Auto Color) in both 8 and 16 bit, with a very noticeable
> difference. I know the lack of concrete examples is Mike's main
> complaint about 16 bit advocates so I thought I pass this along ...
>
> http://www.graphics.com/modules.phpname=Sections&op=view article&artid=279

This is an interesting example, but why didn't Deke McClelland start with a
normal photograph instead of one that has been heavily modified? I suggest
it is because no such photograph exists.

The article demonstrates that 16 bits can represent more brightness levels
than 8. It does not address the issue of whether any actual photograph
looks better after editing in 16 bits per channel than in 8 bits.

> Just fyi as I think minds are already made up on both sides of the
> argument,

Many minds are made up, and I respect those who have determined that 16 bits
per channel, or 32 bits, is needed for them to achieve the desired result.
I believe this issue is worth revisiting every once in a while, particularly
when it is discussed constructively as has been the case here.

> and admittedly his first move in Levels is pretty radical .

Yes, it is a radical levels move that, in effect, closes the image like an
accordion enough to cause banding in 8 bit, but not 16 bit. The same sort
of move, just a more extreme, could be used to show that 24 bits per channel
is better for editing than 16, or that HDR (32 bits per channel) is better
than 24. In each case, nothing practical is being demonstrated except that
more bits equals more levels.

Once again, a qualification. I'm referring to color images in one of the
customary Photoshop color spaces, specifically not a wide gamut space such
as ProPhoto RGB. For those sorts of images, 8 bits per channel has plenty
of head room even for extreme edits, and I have not yet seen such an image
that shows otherwise.
--
Mike Russell
www.curvemeister.com
Sun, 30 October 2005 16:11 Go to previous messageGo to next message
toby (Photoshop expert)
Mike Russell wrote:
> ...
> > and admittedly his first move in Levels is pretty radical .
>
> Yes, it is a radical levels move that, in effect, closes the image like an
> accordion enough to cause banding in 8 bit, but not 16 bit.

Dithering the output of the first adjustment would help a lot here, in
8 bit. Does Photoshop not do this?

But this is a silly example, since anyone with half a clue would
understand that they had thrown away most of the significant bits by
collapsing Levels in the first place (that can't be retrieved by
expanding it again).

--T

> The same sort
> of move, just a more extreme, could be used to show that 24 bits per channel
> is better for editing than 16, or that HDR (32 bits per channel) is better
> than 24. In each case, nothing practical is being demonstrated except that
> more bits equals more levels.
> ...
> --
> Mike Russell
> www.curvemeister.com
Sun, 30 October 2005 21:55 Go to previous messageGo to next message
Mike Russell (Photoshop expert)
"toby" <toby@telegraphics.com.au> wrote in message
news:1130706685.389704.108340@g44g2000cwa.googlegroups.com...
[re banding showing up after a levels operation]
> Dithering the output of the first adjustment would help a lot here, in
> 8 bit. Does Photoshop not do this?

Afaik there is no the only cases where Photoshop dithers is in gradients,
profile conversions, and conversion from 16 to 8 bit mode. The first two
are optional, and the third is always done. BTW - "dithering" is the adding
of a typically small random value to break up banding patterns.

[re article discussing editing in 16 bits]
> But this is a silly example, since anyone with half a clue would
> understand that they had thrown away most of the significant bits by
> collapsing Levels in the first place (that can't be retrieved by
> expanding it again).

True enough. The scenario is unrealistic, and once again an artificial
example - in this case a highly compressed photograph - is used to
illustrate the superiority of editing in hibit.

It's worth mentioning that there are any number of situations, ones that do
not involve editing, where more than 8 bits per channel is useful. For
instance a raw file may be underexposed several stops and still yield a
relatively clean image. Raw files (which are linear gamma, BTW) can provide
a handy way to multiply your ISO number by 8. If the same image is captured
in 8 bits, such recovery is not possible.
--
Mike Russell
www.curvemeister.com
Sun, 30 October 2005 22:04 Go to previous message
toby (Photoshop expert)
Mike Russell wrote:
> "toby" <toby@telegraphics.com.au> wrote in message
> news:1130706685.389704.108340@g44g2000cwa.googlegroups.com...
> [re banding showing up after a levels operation]
> > Dithering the output of the first adjustment would help a lot here, in
> > 8 bit. Does Photoshop not do this?
>
> Afaik there is no the only cases where Photoshop dithers is in gradients,

Yes, a good feature. Before they did that, we used to have to do it by
hand: Make gradient over full range, add appropriate uniform noise,
then shrink levels to final range... The only way to avoid banding in
small-range gradients. There was no corresponding workaround for
banding in strong blurs (and I guess there still isn't).

> profile conversions, and conversion from 16 to 8 bit mode. The first two
> are optional, and the third is always done. BTW - "dithering" is the adding
> of a typically small random value to break up banding patterns.

....or error diffusion, which is arguably even more effective, since the
resulting noise is subjectively less obvious. Looks like Photoshop
could do with an option to dither adjustments, at least.

>
> [re article discussing editing in 16 bits]
> > But this is a silly example, since anyone with half a clue would
> > understand that they had thrown away most of the significant bits by
> > collapsing Levels in the first place (that can't be retrieved by
> > expanding it again).
>
> True enough. The scenario is unrealistic, and once again an artificial
> example - in this case a highly compressed photograph - is used to
> illustrate the superiority of editing in hibit.

Yes, I could add that anyone who did not realise they were throwing
away data in 8 bits, would not make the connection that "16"-bit mode
would help here.

>
> It's worth mentioning that there are any number of situations, ones that do
> not involve editing, where more than 8 bits per channel is useful. ...
> --
> Mike Russell
> www.curvemeister.com
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