Digital BW, The Print

Glen: What a great reply and exposition of the issues.  That is a keeper,
appreciate your taking the time to explain. Soon after I had posted I
noticed your previous postings explaining why one should not confuse bit
size with colour spaces.  In this your later posting the explanations are
deeper and I am delighted in your technical approach to the issues and awed
by your technical competence.  What a great resource for this group!
.
I should say also I was amussed about the apparent 'controversy' over Dan
Margulis' statements.  Guess that according to Margulis Abobe got it all
wrong putting so much effort in 16 bit capabilities in their latest
Photoshop CS.  For my part I hope to soon buy Photoshop CS from the money
saved not buying Margulis' book.
Thanks again
Julio
Glen's Posting:
<Message: 8
   Date: Sun, 04 Jan 2004 06:52:49 -0000
   From: "Glenn Mitchell" <gmitchel850@...>
Subject: Re: 16 bit vs 8 bit difference, Re Glen Mitchell\ufffds posting

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


Julio Fernandez