Re: ScanROM 4E

[P3D John Ohrt <johrt@xxxxxxx>]

P3D Peter Davis wrote:
> 
> At 06:38 PM 3/18/97 -0600, you wrote:
> >P3D Peter F Davis wrote:
> >>
> >> John Ohrt wrote:
> >Nope.  The way halftoning works is to vary the dotsize.  Ask any
> >printer.
 
<<<<< much deleted for brevity >>>>>>>



> But digital printers don't HAVE variable dot size.  What they do is use
> multiple physical device "dots" to make one halftone dot, and vary the
> "size" by controlling how many physical dots are on or off.
> Well, you just confirmed what I said above ... the scanning resolution is
> LOWER than the printer resolution, because it takes some number of printer
> dots to create the effect of one variable intensity halftone dot.  Also, 16
> levels of intensity is extremely small for any kind of photographic output.
> You probably want something like 256 or more, which means that the halftone
> screen resolution can only be 75 lines/inch for a a 600dpi printer.

Nope.  What counts is pixels per whatever.

pixel = picture element, the smallest unit containing all properties to
describe the intensity, hue and saturation of a single output or input
sample.

Dots per whatever is really meaningless.

Halftoning is an analogue process.  There are a number of digital
processes that try to simulate it.  Just as analogue halftoning is is
imperfect, so are the digital emulations.  Any magnifying glass will
demonstrate this.  You will resolve 3 or 4 high contrasting colours
without difficulty.  Epson tries harder but they still have yet to build
a printer that has an invisible "microweave".  I can easily detect it at
normal reading distance.  A good dyesub or photo just won't do that.

In fact a very experienced (selfclaimed) photographer just stated on
rec.photo.digital that there is no difference between a 150 pixel per
inch output and a 300 pixel per inch output from Photoshop.  I find this
hard to believe.  But then I don't use photoshop.  But if it is true
something truly fishy is going on.  I think confusion abounds on how to
print using digital technology.
> 
> >The Nyquist theorem relates solely to spatial resolution and roughly
> >states that if you do not sample the input at an optical spatial
> >resolution in excess of twice the final spatial resolution, then
> >aliasing will occur.  The only way to prevent aliasing when you violate
> >this theorem is to destroy information by filtering.  Sometimes though,
> >the aliasing isn't noticable/objectionable, but that is a judgement
> >call.
> 
> But, as we determined, the spatial resolution of the output device is
> determined by the "screen density" rather than the device's physical
> resolution, as we determined above.  For a 600dpi printer, using a 60lpi
> (lines/inch) screen density will allow you 100 gray levels.
> 
> Nyquist doesn't really enter into it here, since we're talking about a
> digital signal being output digitally.  Yes, the Nyquist theorem would help
> you determine scanning resolution relative to the frequency in the original
> photograph, but not relative to the output.  If you outputting at, say
> 75x75x256, you would sample at 75dpi.

Nope.  Doesn't work that way unless the scanned image has a resolution
below 37.5 pixels per inch.  Forget dots.  And yes it is relative to the
output in pixels per inch.


> 
> >> Also, when resolution is specified for a color printer, are they
> >> counting each color as a separate dot?
> >
> >For an inkjet or colour electrostatic, or a press yes!
> 
> Wrong!  A printing press doesn't have separate spaces for the different
> colors.  The screens are rotated and placed on top of each other, independently.
> 
> >Because they are using a CYMK process requiring separate dots.
> 
> Yes, but it's not a grid with separate spaces for cyan, magenta, yellow and
> black dots.  They all overlap.

Any magnifying glass will show you that it just doesn't work that way. 
The film lacks the stability to retain that registration and dots end up
all over the place.  So does paper.  That is the very reason you get
moire effects when scanning a halftone by any name and why the effects
are not consistant over the image.


> I've worked in prepress as well.  LPI refers only to the density of the
> halftone screen, which is then approximated on digital printers by some
> combination of physical device pixels.

Not pixels, dots.  Dots are used to build pixels both in the halftoning
and digital emulations of halftoning.  Trying to equate halftoning dots
to emulation dots is very tricky as you pointed out.


In the digital world, continuous tone implies that a pixel is a dot
which diffuses into its neighbours to essentially provide continuous
tone.

What everybody wants, is continuous tone.  The quality over any variant
of halftoning is noticeable, and with correct sampling and manipulation
there are no artifacts.  All halftoning processes have artifacts and
many are "sort of grainy" in appearance.  In 3d you want to avoid grain
as much as possible.


The only reason halftoning exists is because it is cheap.  It never will
match a continuous tone device for quality.

The forgoing doesn't mean that halftoning can't be "good enough" just as
there is no guarantee that continous tone of the desired resolution is
affordable.

I'm not sure that consumer technology in printers is viable below 4x6
inches for a stereo photo.  But I'm going to do my best to help find
out.  What the heck, I think a holmes card ( do I have that right ???)
is a neat format and larger anaglyphs have their niche.  As a final
resort, you can always shoot them down with a camera :-)

Regards,
John
--
John Ohrt,  Regina, SK, Canada
johrt@xxxxxxx



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