Digital BW, The Print

----- Original Message -----
From: "Austin Franklin" <darkroom@...>
To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
Sent: Friday, October 11, 2002 10:15 AM
Subject: RE: [Digital BW] Re: 'combed' histograms in 16 bit ?


> Can anyone explain why it would even be any different?  It should be the
> exact same data...and as I said, the histogram is an 8 bit histogram, no
> matter whether the file is, 8 bits or 16 bits.  When displaying a 16 bit
> file, the low 8 bits are simply ignored...and when converting a 16 bit
file
> to 8 bits, simply the lower 8 bits are dropped...so they should be the
same?

Austin,

I am still trying to get a better grasp on what is happening with the data
during scanning and mode changes. So see if I am correct in the following.

As an example lets say the scan originally came from a 12-bit scanner and
the 12-bit data was mapped accurately into 16-bit space (each 12-bit value X
16) so that in 16-bit space there are pixels at intervals of 16 levels or
tones. If you did a strong levels or other adjustment on this 16-bit file
wouldn't the spacing change so that in some portions of the 16-bit range
data points are brought closer together, even overlapping giving the
"spikes" seen in histograms, and in others portions of the 16-bit range they
are spread farther apart than intervals of 16. To see a gap in the 16-bit
file, 65,535 levels, with an 8-bit histogram tool it would have to be 256
16-bit levels wide, which sounds like a lot, but I guess you could push the
data that far.

Assuming you had a "perfect", full range, 12-bit scan there would be 4096
tones in the original 16-bit scan but after manipulation it might be reduced
to a lower number. It is still going to be much more than 256, so when you
do a mode change to 8-bit wouldn't be likely that in mapping and throwing
out tones that the gaps get totally or partially filled in so that the
histogram of the 8-bit version of the file looks smoother?

Of course there is the question of whether the Histogram function in
PhotoShop reads 16-bit and 8-bit files the same way.

What I would love to see on the Histogram display, which does not seem like
it would be difficult, is first the bit depth of the file being analyzed and
a display of the number of discrete tones or levels in the file. This last
would be a great help in determining the amount of data loss caused by any
adjustment or action.

In the end though a lot of this may not matter for practical purposes since
you can often get good prints from 8-bit files with rather nasty looking
histograms. Degradation in the histogram of a 16-bit file may be even more
irrelevant.

I suspect that Frank has run into something Tyler Boley and I have
encountered with SilverFast software. There are certain workpaths that will
produce a 16-bit file with a nice looking histogram after scanning but that
immediately look degraded after the first adjustment.

Martin Wesley

>
> On the bottom of the histogram you are looking at the 16 bit file on,
isn't
> the scale 0-255?
>
> Austin
>
>
> > Andrew,
> > I duplicated the 16 bit file and converted to 8bit - then compared
> > the histograms - you are right there is less combing viewed in 88
> > bit - but it's still there albeit not quite as bad as in 16
> >
> > ?
> > Frank
> > > > after minor contrast adj in 16 bit scans (moving the end sliders
> > to
> > > > the edges of the histogram & the middle to fix the mid tones), I
> > > > still have 'drop out' or combed histograms - any ideas, tips ?
> > >
> > > Convert the file to 8 bits THEN look at the Histogram. You should
> > see NO
> > > combs.
>