Digital BW, The Print

--- In DigitalBlackandWhiteThePrint@y..., "Martin Wesley" <
mwesley250@e...> wrote:
> ----- Original Message -----
> From: "Austin Franklin" <darkroom@i...>
> To: <DigitalBlackandWhiteThePrint@y...>
> Sent: Saturday, August 03, 2002 8:15 PM
> Subject: RE: [Digital BW] Number of tones was Re: Do inkjets dither or not?
> 
> 
> > Tyler,
> >
> > Before responding to your post, I have one simple question for you.
> >
> > If the file only contains 256 levels of gray, and IF (BIG IF) what you
> claim
> > is true, that standard Piezo is capable of 1000 tones, where do the extra
> > 744 levels of gray come from?  How, exactly, are they derived?
> >

Hi Austin,

Sorry for joining late, but I've been away.
Your arguments seem so self-evident that it hard to see what else is
going on.  Say you were to create a 8 bit grayscale file with 256
separate one inch squares such that each square contained a different
gray value -- i.e. 0,1,2,... 255.  Each square has a different
gray value and at least theoretically a different gray tone on the
print --> thus there are 256 gray tones possible.  This is your
argument, right?

This isn't the whole situation though.  Say there are two adjacent
gray tones, values 102 and 103.  Let's make a new square that is a
checkerboard of thes two values.  What gray tone does this produce?
What are the possibilities:  first is that the pixels are big
enough that we see the checkboard pattern, however if the resolution
is high enough or our detector resolution gets lower, the two gray
values 102 and 103 merge together and average to 102.5.  This is
a new and additional gray tone.  You could do this between each
original pair.  Also you could do 75% 102's and 25% 103's giving
102.25.   You can also change the pattern to be more random making
it even more likely to be undetected.  This is dithering at the
pixel level.  Photoshop does this for gradients.  If you create
a gradient from 0 to 255, how many tones of gray depends heavily
on what resolution you look at it.  Same thing with the printer,
how many grays depends heavily on how close (resolution) you look.
In fact in the halftone printing they have this simple formula:

Gray levels = (output resolution/screen freq)^2 + 1

   where output resolution = the image setter dpi
   and screen freq = lines per inch  or
          pixel resolution on output.

With dithering or stochastic screening there is no fixed resolution
but the general idea still holds.  There is always a tradeoff
of more gray levels versus more resolution.  (this is one reason I
strongly object to the notion of counting the number of grays in
a print -- it depends on how you look not just the print).

The tradeoff of gray levels versus resolution is really the entire
basis of what's going on in printing.  Previously in this discussion
there was distinction of pixels versus dots, but I think this is
an unnecessary and misleading distinction.  Everything we have as
far as Epson printers these days are pixels. The very smallest point
on a print can contain any of 4 or 6 different gray/black inks drops,
plus with variable droplet size and overprint of multiple drops,
there are many possible gray values.  So I would call this a
pixel not a dot.  The printer 1440 and 2880 dpi specs relate
to high resolution, a few grays pixels.  Through the dithering
process we tradeoff those high resolution pixels to get pixels 
with lower resolution but more gray values.

Roy

---------------
> 
> Austin,
> 
> If the same 256 levels of gray pixels are fed to the printer and it has four
> channels each with a different shade of gray ink, it would seem to follow
> that each channel will print those 256 levels differently resulting in
> overall tonal interpolation. Lower than 1024 but greater than 256.
> 
....
> 
> I think the analogy to the pixel discussion is very apt. While we input
> pixel information to the printer is does not print pixels. The same with
> levels of gray. We may input levels but the driver and the nature of the
> inks produces an interpretation of those levels in a manner designed to
> convince our eyes that they are observing a continuous tone image.
> 
> Martin