Digital BW, The Print

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, "Tyler Boley" 
<tyler@t...> wrote:

> snip...
> > I and others who work in 16-bits are not talking theory. Our 
very 
> > real experience is that there is more editing headroom with 
> > increased bit depth.
> 
> That's obvious and indisputable, providing the science (hardly 
theory)
> simply supports it.
> Many, including Fraser, have noted that in RGB some editing can be
> done in 8 bit without deterioration at output. There are 3 channels
> blending, and multiple inks dithering at output.

Who has said some editing cannot be done in 8-bits without 
deterioration?! That's not the point of contention at all.

The question is, is there a significant difference between 8-bit and 
16-bit images when it comes to editing. My experience says 
unequivocally "Yes."

You can make linear changes in an 8-bit image and a 16-bit image and 
the effect on both will be identical. A linear change is one that 
affects every pixel equally. Brightness is one example. It simply 
increments every pixel by the same value.

The trouble is nonlinear changes. Levels, curves, hue/saturation, 
etc. affect some pixels more than others. Hence, information in 
portions of the histogram are compressed and/or expanded and others 
are unaffected or affected more/less. It is nonlinear 
transformations that are likely to encounter visible posterization. 
The more changes and the more extreme those changes, the more likely 
an 8-bit image will show more evidence of posterization than a 16-
bit image.

Inks are not necessarily dithered at all. Not all inkjet printers 
use dithering and many use it only for specific modes, like BO 
printing with the Epson 2200.


> This is the digital black and white printing list. Most of the 
users
> here are editing in gray at some point in the workflow, that's one
> channel, no others will blend and possibly mask tonal gaps.

Unless you use BO ink, neutrals are printed using predominantly CMY 
ink (or CcMmY, etc. depending on how many colors are used in 
printing). B&W printing remains color printing unless you go to BO 
and suffer the disadvantages of dithering.

No black ink is used at all for neutrals brighter than midtone. Even 
0,0,0 -- black -- will be something like 75,68,67,90 in CMYK. Black 
ink is just 30% of the total ink. Go to 64,64,64 -- a three-quarter 
tone -- and you can expect something like 68,61,60,49. Blank ink 
drops to 21% of the total ink.

There is also the issue of where B&W images come from when the 
original digital image is a color image. Information from the color 
channels is not necessarily combined to make a single gray channel. 
There are B&W conversion techniques that retain three RGB channels.

Even if the information in the three channels (and perhaps other 
channels, such as the "l" channel from a Lab conversion of the K 
channel from a CMYK conversion) are combined in various proportions 
to make a single gray channel, there can be considerable editing 
going on prior to the conversion. Those can lead to posterization in 
the original color image *AND in the subsequent B&W image.

Cheers,

Mitch