Re: ScanROM 4E

[P3D Peter F Davis <pd@xxxxxxxxxxxxx>]

> pixel = picture element,

This much is pretty much industry standard.

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

This is your definition, and is by no means universal.  However, if
you like, we can use "pixel" to talk about image samples, and dots to
speak of physical output device dots.  We also need to consider screen
density, the number of variable sized halftone dots used to
approximate intensity levels on a digital device.

> 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.

Halftoning can be either analog or digital.  In current practice, both
in professional printing and in desktop printing, it is digital.

> > 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.

I don't know what you're saying here.  If you use a 75 line/inch
screen, you can output an image scanned at 75 samples/inch.  Assuming
you're talking about classical halftoning (as opposed to FM screens
and other newers technologies), each halftone dot will represent
exactly one image sample, and the size of the dot will represent the
intensity of that sample.

> > 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.

You've just confirmed my point.  On a color printer, 600dpi means that
each color can be printed at 600dpi.  So, if you're using a 75 lpi
screen, each color can have 256 intensity levels.

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

Actually, in the printing industry, "continuous tone" refers to images
where the tone can vary from pixel to pixel, unlike "line art", which
consists of areas of solid color.  "Continuous tone" images are
reproduced by halftoning.

> 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.

I don't know how dye sublimation works, but I'm not aware of any true
continuous tone printing technologies.  TV might be considered
continuous tone, since an analog signal drives the intensity levels of
the phosphors on the screen.  I don't know of any printing technology
that does this.

Can you give some examples of continue tone output devices?

Thanks,
-pd
--------
                                Peter Davis
                        http://world.std.com/~pd

            "If you can't behave yourself, who can you behave?"


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