Digital BW, The Print

Thanks for the thoughtful reply, Julio.

It is not the number of colors that is at issue. 256x256x256 is a 
large number of colors. More than 16 million colors.

The problem is that the entire range of a particular color channel 
are distributed over 256 values. There are 256 discrete values for 
red, for example. Even in a smaller color space like sRGB, each one 
unit difference can cover quite a range of red.

Now, as you work on the image, you move the information around. 
Adjacent pixels in a flower petal, for example, might start out one 
or two units different and then end up further apart. The result can 
quite easily be an image that goes from the appearance of continuous 
tones to one with visible banding.

You only need to get non-continuous tones in one channel out of the 
three, where previously there were continuous tones to result in 
visible banding.

With a 12 bit image, the same range of color for each channel is 
spread across 4096 values. That's 16 times finer available 
gradations. Same range for the information, but each unit increment 
covers less red, for example. This means you are less likely to see 
banding after making nonlinear changes.

Smaller color spaces are indeed less likely to posterize than larger 
color spaces, but a 16-bit image in any color space is less likely 
to posterize than an 8-bit image in the same color space. (If you 
have posterization problems with an image in AdobeRGB or a larger 
color space, conversion to a smaller space can sometimes 
reduce/eliminate the effects.)

The reason why smaller color spaces are less likely to posterize is 
that the range of colors is smaller. The 256 values in 8-bits and 
the 4096 values in 12-bits when spread across a smaller range means 
each unit change covers fewer tones.

The gamut of the Epson 2200 printer broader than you seem to give it 
credit. It is not a small gamut at all. The gamut varies, depending 
on the paper and ink you use, but it is wider than sRGB and 
ColorMatchRGB, smaller than AdobeRGB (with the exception of a peak 
in the Yellow-Oranges).

Don't get confused by the number of colors in 8-bits. That's really 
not the issue driving posterization. A change in an 8-bit image that 
results in just a one unit difference, say from 127 to 128, could be 
represented in a 16-bit image by a fraction of that same unit. The 
finer the gradations in color from one unit to the next, the less 
likely we are to see evidence of posterization.

Cheers,

Mitch

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, "Julio 
Fernandez" <gluemax@s...> wrote:
> Glen: Agree with your technical points and I am enlightened by 
your latin,
> now I know it is called the "Ipse dixit" fallacy otherwise known 
as name
> dropping in place of facts.
> 
> Your points about posterization are alone a perfect reason for 
working with
> 16 bit files. Beyond that there are the questions that follow:
> 
> Indeed the number of colours possible under 8 bits is mega smaller 
than in
> 16 bits and somehow the differences have to show.  The question is 
when?
> For one thing does the color space used not have something do with 
whether
> you see the difference or not?  I wonder then if using small color 
spaces
> like sRGB or Color Match will limit the number of colours and put 
many of
> those colors which actually reside in film and in 16 bit RGB files 
out of
> gamut anyway so that the small color space becomes the limiting 
factor?  If
> so, as far as colour is concerned, would 16 bit files in small 
colour spaces
> look much different from 8 bit files in the same small colour 
spaces?
> 
> By the same token, inkjet printers have a typically smaller color 
gamut than
> true photographic printers like a Lightjet.  As I understand it, 
inkjets
> like the Epson 2200 can only print to 8 bit colour so in fact, 16 
bit files
> are printed to 8 bit,  is that true?  If it were, the differences 
between 16
> and 8 bit files printed in ink jets may not be readily evident, 
that has
> been my experience.
> 
> Are there any other advantages in large bit files besides non-
banding ?  I
> think many of us wrestle with these questions and would appreciate 
your
> answer.  Thanks.
>